Slide 1

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 2

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 3

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 4

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 5

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 6

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 7

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 8

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 9

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 10

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 11

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 12

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 13

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 14

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 15

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 16

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 17

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 18

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 19

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 20

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

11

Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21


Slide 21

Induction Motor Drives – Scalar Control
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

1

Outline
 Introduction
 Speed Control of Induction Motors
 Pole Changing
 Variable-Voltage, Constant Frequency
 Variable Frequency

 Constant Volts/Hz (V/f) Control
 References
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

2

Introduction
 Scalar Control - control of induction machine

based on steady-state model (per phase
steady-state equivalent circuit)
Rs

Is

Lls

Llr’

+

+
Vs
–

Dr. Ungku Anisa, July 2008

Ir ’

Lm
Im

E1

Rr’/s

–

EEEB283 - Electrical Machines & Drives

3

Introduction
Te
Pull out
Torque
(Tmax)

Intersection point
(Te=TL) determines the
steady –state speed

Te

TL

Trated

What if the load must
be operated here?

s

sm

rated
rotors
rotor’

1
Dr. Ungku Anisa, July 2008

r

0
EEEB283 - Electrical Machines & Drives

4

Speed Control of IM
 Given a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor
Te 

3R

s 

2

'
r

Vs

2

2

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


 

P

4

3

f

P

1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Varying voltage
(amplitude)

Varying line
frequency

Pole Changing
5

Speed Control of IM
Pole Changing
 Machines must be specially manufactured
 Only used with squirrel-cage motors
 Two methods:
 Multiple stator windings – simple, expensive
 Consequent poles – single winding divided into few coil groups
 Consequent poles:
 No. of poles changed by changing connections of coil groups
 Change in pole number by factor of 2:1 only
 Discrete step change in speed
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

6

Speed Control of IM
Variable-Voltage
(amplitude), Constant
Frequency
 Controlled using:
 AC Voltage Controllers (anti-

parallel thyristors)




voltage control by firing angle
control
also used for soft start of
motors

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

7

Speed Control of IM
Variable-Voltage (amplitude), Constant Frequency
 From torque equation, Te  Vs2
 When Vs , Te and speed reduces.
 If terminal voltage is reduced to bVs,:
Te 

3R

'
r

bV 

2

s

2
'
s s 

Rr 
2
   X ls  X lr  
  R s 
s 
 


Note: b  1
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

8

Speed Control of IM
Variable Voltage (amplitude),
Constant Frequency
 Disadvantages:
 limited speed range  when applied
to Class B (low-slip) motors
 Excessive stator currents at low
speeds  high copper losses
 Distorted phase current in machine
and line
 Poor line power factor
 Hence, only used on low-power,

appliance-type motors where
efficiency is not important

 e.g. small fan or pumps drives
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

9

Speed Control of IM
Variable Frequency
 Speed control above rated (base)

speed
 Frequency increased
 Stator voltage held constant at

rated value
 Airgap flux and rotor current
decreases
 Developed torque decreases

 For control below base speed

– use Constant Volts/Hz
method
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

10

Constant Volts/Hz (V/f) Control
 Airgap flux in the motor is related to the induced stator

voltage E1 :
 ag 

E1
f



Vs
f

Assuming small voltage drop
across Rs and Lls

 For below base speed operation:
 Frequency reduced at rated Vs - airgap flux saturates
(f  ,ag ):
- excessive stator currents flow
- distortion of flux wave
 Hence, keep ag = rated flux
 stator voltage must be reduced proportionally
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

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Constant Volts/Hz (V/f) Control
 Max. torque remains almost

constant
 For low speed operation:

T max 

Vs

2

s

 can’t ignore voltage drop

across Rs and Lls
 poor torque capability
 stator voltage must be
boosted – maintain constant

ag
 For above base speed
operation (f > frated):

 stator voltage maintained at

rated value

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

12

Constant Volts/Hz (V/f) Control
Vs

Vrated

Linear offset

Boost

Vs vs. f relation in
Constant
Volts/Hz drives

Non-linear offset – varies with Is
frated

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

f
13

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
 fs = fs,rated  s = s,rated

V
, when
 Stator voltage: V   s ,rated

s
 V s ,rated , when

(1)
f s  f s ,rated
f s  f s ,rated

(2)

Voltage-to-frequency ratio = d = constant:
d 

Dr. Ungku Anisa, July 2008

V s ,rated

 s ,rated

EEEB283 - Electrical Machines & Drives

(3)

14

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
Hence, the torque produced:
Te 

3R

'
r

Vs

2

s s 
R 
  
  R s 
s 
 
'
r

2

2

 X ls

 X lr 

2





(4)

where s and Vs are calculated from (1) and (2)
respectively.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

15

Constant Volts/Hz (V/f) Control
 For operation at frequency  times rated frequency:
The slip for maximum torque is:
'

s max 

Rr
Rs  
2

2

 X ls

 X lr 

(5)

2

 The maximum torque is then given by:
T max 

Vs

3
2 s  R 
 s

Rs  
2

2

2

 X ls

 X lr 

2




(6)

where s and Vs are calculated from (1) and (2)
respectively.
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

16

Constant Volts/Hz (V/f) Control
Constant
Torque Area

Field Weakening Mode (f > frated)
• Reduced flux
• Torque reduces

 Constant

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

Power Area

17

Constant Volts/Hz (V/f) Control
Constant Torque Area

Constant Power Area
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

18

Constant Volts/Hz (V/f) Control –
Open-loop Implementation

PWM
Voltage-Source
Inverter
(VSI)

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

19

Constant Volts/Hz (V/f) Control –
Open-loop Implementation
 Most popular speed control method
 Used in low-performance applications
 where precise speed control unnecessary
 Speed command s* - primary control variable
 Phase voltage command Vs* generated from V/f relation
 Boost voltage applied at low speeds
 Constant voltage applied above base speed
 Sinusoidal phase voltages (vabc*)generated from Vs* &

s*

 vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

20

References
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control,






Prentice-Hall, New Jersey, 2001.
Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,
New Jersey, 2002.
Trzynadlowski, A. M., Control of Induction Motors, Academic Press,
San Diego, 2001.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd
ed., Pearson, New-Jersey, 2004.
Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008

EEEB283 - Electrical Machines & Drives

21