Transcript EEEB283 Electrical Machines & Drives

Induction Motor Review
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering
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Outline
 Introduction
 Construction
 Concept
 Per-Phase Equivalent Circuit
 Power Flow
 Torque Equation
 T- Characteristics
 Starting and Braking
 References
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Introduction
 Induction motors (IM) most widely used
 IM (particularly squirrel-cage type) compared to
DC motors
 Rugged
 Lower maintenance
 More reliable
 Lower cost, weight, volume
 Higher efficiency
 Able to operate in dirty and explosive environments
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Introduction
 IM mainly used in applications requiring
constant speed
 Conventional speed control of IM expensive or
highly inefficient
 IM drives replacing DC drives in a number of
variable speed applications due to
 Improvement in power devices capabilities
 Reduction in cost of power devices
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Induction Motor – Construction
 Stator
 balanced 3-phase winding
 distributed winding – coils
distributed in several slots
 produces a rotating magnetic
field
 Rotor
 usually squirrel cage
 conductors shorted by end
rings
 Rotating magnetic field induces
voltages in the rotor
 Induced rotor voltages have
same number of phases and
poles as in stator winding
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a
120o
120o
c’
b’
c
b
a’
120o
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Induction Motor – Construction
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Induction Motor – Concept
 Stator supplied by balanced 3-phase AC source (frequency f Hz or 
rads/sec )
 field produced rotates at synchronous speed s rad/sec
(1)
120
2
4
ns 
f
s    f
P
P
P
where P = number of poles
 Rotor rotates at speed m rad/sec (electrical speed r = (P/2) m)
 Slip speed, sl – relative speed
(2)
sl  s  m
between rotating field and rotor
  m
 Slip, s – ratio between slip speed
s s
s
and synchronous speed
(3)
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Induction Motor – Concept
 Relative speed between stator rotating field and rotor induces:
 emf in stator winding (known as back emf), E1
 emf in rotor winding, Er
fr  sf
 Frequency of rotor voltages and currents:
(4)
 Torque produced due to interaction between induced rotor currents
and stator field
 Stator voltage equation: Vs  Rs I s  j 2πf Lls I s  E1
 Rotor voltage equation:
sEr  Rr I r  js2πf Llr I r
Er   Rr
 s
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 I  j 2πf L I
 r
lr r

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Induction Motor – Concept
 E1 and Er related by turns ratio aeff
Rs
Lls
Llr
Is
+
+
Vs
Im
+
Lm E1
–
Ir
–
Er
Rr/s
–
 Rotor parameters can be referred to the stator side :
E1  aeff Er
Rr'  aeff2 Rr
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I r'  I r
aeff
L'r  aeff2 Lr
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Induction Motor –
Per Phase Equivalent Circuit
Rs
Is
Lls
Llr’
Ir ’
+
+
Lm
Vs
Im
–
E1
Rr’/s
–
 Rs – stator winding resistance
 Rr’ – referred rotor winding resistance
 Lls – stator leakage inductance
 Llr’ – referred rotor leakage inductance
 Lm – mutual inductance
 Ir’ – referred rotor current
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Induction Motor – Power Flow
Airgap
Power
Pag
Converted
Power Pconv
Mechanical
Power
Rr' P  3I ' 2 R '  1  s 
conv
r
r
Pag  3I r
 s 
s
'2
Electrical
Power
Pout  TLm
Pin 
3VT I L cos 
Stator
Copper
Loss (SCL)
PSCL  3I Rs
2
s
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Rotor
Copper
Loss (RCL)
'2
r
PRCL  3I Rr'
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Rotational losses Prot
(Friction and windage, core and
stray losses)
Note:
Pconv  1  s Pag
PRCL  sPag
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Induction Motor – Torque Equation
 Motor induced torque is related to converted power by:
Te 
Pconv
m
(5)
 Since Pconv  1  s Pag and r  1  s s , hence
'2
r
3I Rr'
Te 

s
ss
Pag
(6)
 Substituting for Ir’ from the equivalent circuit:
Dr. Ungku Anisa, July 2008
3Rr'
Te 
ss 
Rr'
 Rs 
s

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Vs
2


2
   X ls  X lr  


2
(7)
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Induction Motor –
T- Characteristic
 T-
characteristic of
IM during
generating,
motoring and
braking
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Induction Motor –
T- Characteristic
Te
 Maximum torque or pullout
torque occurs when slip is:
Rr'
smax  
2
2
Rs   X ls  X lr  (8)
Pull out
Torque
(Tmax)
Trated
 The pullout torque can be
r
0
s
1
smax
rated
s
0
calculated using:
Tmax 
3
Vs
2
2 s  R  R 2   X  X 2 
s
ls
lr
 s

(9)
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Induction Motor –
T- Characteristic
Linear region of operation
(small s)
 Te  s
 High efficiency
Te
Pull out
Torque
(Tmax)
 Pout = Pconv – Prot
 Pconv = (1- s )Pag
Trated
 Stable motor operation
s
0
smax
rated
r
s
1
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0
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Induction Motor –
NEMA Classification of IM
 NEMA = National Electrical
s
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Manufacturers Association
 Classification based on T-
characteristics
 Class A & B – general
purpose
 Class C – higher Tstart (eg:
driving compressor pumps)
 Class D – provide high Tstart
and wide stable speed range
but low efficiency
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Induction Motor – Starting
 Small motors can be started ‘direct-on-line’
 Large motors require assisted starting
 Starting arrangement chosen based on:
 Load requirements
 Nature of supply (weak or stiff)
 Some features of starting mechanism:
 Motor Tstart must overcome friction, load torque and inertia of motor-
load system within a prescribed time limit
 Istart magnitude ( 5-7 times I rated) must not cause


machine overheating
Dip in source voltage beyond permissible value
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Induction Motor – Starting
 Methods for starting:
 Stat-delta starter
 Autotransformer starter
 Reactor starter
 Soft Start
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Induction Motor – Starting
 Star-delta starter
 Special switch used
 Starting: connect as ‘star’ (Y)
 Stator voltages and currents
reduced by 1/√3
 Te  VT2  Te reduced by 1/3
 When reach steady state speed
 Operate with ‘delta’ ( )
connection
 Switch controlled manually or
automatically
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Induction Motor – Starting
 Autotransformer starter
 Controlled using time relays
 Autotransformer turns ratio aT
 Stator voltages and currents
reduced by aT
 Te  VT2  Te reduced by aT2
 Starting: contacts 1 & 2 closed
 After preset time (full speed
reached):



Contact 2 opened
Contact 3 closed
Then open contact 1
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Induction Motor – Starting
 Reactor starter
 Series impedance (reactor)
added between power line and
motor
 Limits starting current
 When full speed reached,
reactors shorted out in stages
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Induction Motor – Starting
 Soft Start
 For applications which require
stepless control of Tstart
 Semiconductor power switches
(e.g. thyristor voltage
controller scheme) employed


Part of voltage waveform
applied
Distorted voltage and current
waveforms (creates harmonics)
 When full speed reached,
motor connected directly to
line
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Induction Motor – Braking
 Regenerative Braking:
 Motor supplies power back to line
Provided enough loads connected to line to absorb power
 Normal IM equations can be used, except s is negative
 Only possible for  > s when fed from fixed frequency source

 Plugging:
 Occurs when phase sequence of supply voltage reversed
by interchanging any two supply leads
Magnetic field rotation reverses  s > 1
Developed torque tries to rotate motor in opposite direction
If only stopping is required, disconnect motor from line when  = 0
Can cause thermal damage to motor (large power dissipation in rotor)





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Induction Motor – Braking
 Dynamic Braking:
 Step-down transformer and





rectifier provides dc supply
Normal: contacts 1 closed, 2 & 3
opened
During braking: Contacts 1 opened,
contacts 2 & 3 closed
Two motor phases connected to dc
supply - produces stationary field
Rotor voltages induced
Energy dissipated in rotor
resistance – dynamic braking
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References
 Chapman, S. J., Electric Machinery Fundamentals, McGraw Hill,




New York, 2005.
Rashid, M.H, Power Electronics: Circuit, Devices and
Applictions, 3rd ed., Pearson, New-Jersey, 2004.
Trzynadlowski, Andrzej M. , Control of Induction Motors,
Academic Press, 2001.
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.
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