Transcript EEEB283 Electrical Machines & Drives

Speed Control of DC Motors
By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
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DC Drives Outline
 Introduction to DC Drives
 Separately Excited DC Motor
 Speed Control Methods
 Speed Control Strategy
 Operating Modes
 References
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Introduction
 DC Drives – Electric drives employing DC
motors as prime movers
 Dominated variable speed applications before
introduction of Power Electronic converters
 Still popular even after Power Electronics
 Advantage: Precise torque and speed control
without sophisticated electronics
 Applications: Rolling mills, hoists, traction, cranes
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Introduction
 Some limitations:
 High maintenance (commutators & brushes)
 Expensive
 Speed limitations
 Sparking
 Commonly used DC motors
 Separately excited
 Series (mostly for traction applications)
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Separately Excited DC Motor
Ra
ia
+
Lf
La
Rf
+
if
+
vt
ea
vf
_
_
_
Armature
circuit
Field
circuit
dia
va  Raia  La
 ea
dt
Te  Kia  Kbia
ea  K  Kb
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EEEB443 - Control & Drives
v f  Rf i f  Lf
dif
dt
Electromagnetic torque
Armature back e.m.f.
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Separately Excited DC Motor
 Motor is connected to a
load.
 Therefore,
d
Te  J
 B  TL
dt
where
TL= load torque
J = load inertia (kgm2)
B = viscous friction
coefficient (Nm/rad/s)
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Separately Excited DC Motor –
Steady State Condition
 Time derivatives = 0. Therefore,
 Vf  Rf I f
(1)


Ea  Kb  K
Va  Ra I a  Ea
 Ra I a  K
(2)
(3)
 Kb I a  KI a  B  TL (4)
 The developed power
(5)
Pd  Te
 Te
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Speed Control Methods for
Separately Excited DC Motor V
 From equation (3),
Va  Ra I a Va
Ra  Te 





K
K K  K 

intercept 
Va
Ra
 

T
2 e
K   K 
a
K
Ra
slope 
K 2
Te
 Three possible methods for speed control:
 Armature voltage Va
 Armature resistance Ra
 Field flux  (by changing field resistance Rf)
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Speed Control Methods
– Va control

Va
Ra


T
2 e
K  K 
Va
K
TL
Va↓
Requires variable
DC supply
Te
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Speed Control Methods
– Ra control

Va
K
Va
Ra


T
2 e
K  K 
Ra
slope 
K 2
TL
Simple control
Losses in external resistor
 Rarely used.
Ra ↑
Te
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Speed Control Methods
–  control

Va
Ra


T
2 e
K  K 
TL
↓
Va
K
Ra
slope 
K 2
Not possible for PM motor
Normally employed for
speed above base speed
Te
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Speed Control Strategy for
Separately Excited DC Motor
 Base speed base = Speed at rated Va, If and Ia
  = 0 to base  speed control by Va
  > base  speed control by flux weakening ()
T

Va control
Dr. Ungku Anisa, July 2008
base
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 control
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Speed Control Strategy for
Separately Excited DC Motor




 = 0 to base  speed control by Va
 > base  speed control by flux weakening ( )
T  Ia  For maximum torque capability, Ia = Ia max
Pd = EaIa = (K)Ia = constant when  > base
in order to go beyond base,   (1/)
Per unit
quantities
Ia
1.0
Va
If, Te, 

Va control
Dr. Ungku Anisa, July 2008
base
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 control
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Speed Control Strategy
Per unit
quantities
1.0
Ia
Va
If, Te, 

Va control
base
 control
 Torque and power relations below and beyond base
P, T
P
P =K 
Te
constant torque
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Te = KIa
constant power
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Speed Control of DC Motor –
Example
A 220 V, 500 A, 600 rpm separately excited motor has armature
and field resistance of 0.02 and 10  respectively. The load
torque is given the expression
TL = 200 – 2N,
where N is the speed in rpm. Speeds below the rated are
obtained by armature voltage control and speeds above the
rated are obtained by field control.
i) Calculate motor terminal voltage and armature current
when the speed is 450 rpm.
ii) Calculate field winding voltage and armature current when
the speed is 750 rpm. Assume the rated field voltage is the
same as the rated armature voltage.
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Operating Modes
Motoring
 Back EMF Ea < Va
 Ia and If are positive
 Motor develops
torque to meet load
demand (i.e. Te =TL )
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Operating Modes
Regenerative Braking
 Motor acts as generator
 Develops Ea > Va
 Ia negative (flows back
to source)
 If positive
 Machine slows down
until Ea = Va
 Used only when there
are enough loads to
absorb regenerated
power
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Operating Modes
Dynamic Braking
 Similar to
regenerative
breaking
 But Va removed,
replaced by Rb
 Kinetic energy of
motor is dissipated
in Rb (i.e. machine
works as generator)
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Operating Modes
Plugging
 Supply voltage Va is
reversed
 Assists Ea in forcing
Ia in reverse
direction
 Rb connected in
series to limit
current
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Operating Modes Four Quadrant Operation
Note: In the figure, Eg = Ea
Q2
+Va , +Ea  + 
-Ia  -T
Power = -ve
Q1
+Va , +Ea  + 
+Ia  +T
Power = +ve
Q3
-Va , -Ea  - 
-Ia  -T
Power = +ve
Q4
-Va , -Ea  - 
+Ia  +T
Power = -ve
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References
 Chapman, S. J., Electric Machinery Fundamentals, McGraw Hill,




New York, 2005.
Rashid, M.H, Power Electronics: Circuit, Devices and
Applications, 3rd ed., Pearson, New-Jersey, 2004.
Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha
Science Int. Ltd., UK, 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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