Speed Control of DC Motors By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr.
Download ReportTranscript Speed Control of DC Motors By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr.
Speed Control of DC Motors By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering
Dr. Ungku Anisa, July 2008
EEEB283 – Electrical Machines & Drives 1
DC Drives Outline
Introduction to DC Drives Separately Excited DC Motor Speed Control Methods Speed Control Strategy Operating Modes References
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EEEB283 – Electrical Machines & Drives 2
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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EEEB283 – Electrical Machines & Drives 3
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
R a L a L f R f i a + + i f + v t _ e a _ v f _
v a T
R a i a e
L a di a dt
K t i f i a
e a
Electromagnetic torque
e a
K v i f
Armature back e.m.f.
v f
R f i f
L f di f dt K t K v K v
= torque constant = voltage constant (V/A-rad/s) = K
t Dr. Ungku Anisa, July 2008
EEEB283 – Electrical Machines & Drives 5
Separately Excited DC Motor
Motor is connected to a load.
Therefore,
T e
J d
dt
B
T L
where
T L
= load torque J = load inertia (kg/m 2 ) B = viscous friction coefficient (Nm/rad/s)
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Separately Excited DC Motor – Steady State Condition
Time derivatives = 0. Therefore,
V f
R f I f
E a
K v I f
(1)
V a T e
(2)
R a I a
R a I a K t I f
I a E a K
I v f B
T L
(3) (4) The developed power
P d
T e
(5)
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EEEB283 – Electrical Machines & Drives 7
Speed Control Methods for Separately Excited DC Motor
From equation (3), intercept
K V a v I f
V a
R a I a
K v I f
V a K v I f V a K v I f
K R a v I f
R a K v I f
2
T e
T e K t I f
slope
K R a v I f
2 T e Three possible methods for speed control: Armature voltage V
a
Armature resistance R
a
Field current I
f
(by changing field resistance R
f
) flux
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EEEB283 – Electrical Machines & Drives 8
Speed Control Methods – V
a K V a v I f
control
V a K v I f
K v R a I f
2
T e
T L
V a
↓ Requires variable DC supply T e 9
Dr. Ungku Anisa, July 2008
EEEB283 – Electrical Machines & Drives
Speed Control Methods – R
a
control
V a K v I f K V a v I f
slope
K R a v I f
2
K v R a I f
2
T e
T L Simple control Losses in external resistor Rarely used.
R a
↑ T e
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EEEB283 – Electrical Machines & Drives 10
Speed Control Methods –
control
V a K v I f
K v R a I f
2
T e
T L
V a K v I f
slope
K R a v I f
2
I f
↓ Not possible for PM motor Normally employed for speed above base speed T e 11
Dr. Ungku Anisa, July 2008
EEEB283 – Electrical Machines & Drives
Speed Control Strategy for Separately Excited DC Motor
Base speed base = Speed at rated V
a
, I
f
= 0 to base speed control by V
a
> base and I
a
speed control by flux weakening ( , i.e.
I f
↓ ) T
Dr. Ungku Anisa, July 2008 V a
control base EEEB283 – Electrical Machines & Drives control 12
Speed Control Strategy for Separately Excited DC Motor
T P d = 0 to base > base
I a
= E
a I a
For maximum torque capability, I
a
= I a max = (K speed control by flux weakening (
v I f
)I speed control by V
a a
= constant when > base in order to go beyond base ,
I
)
f
(1/ ) Per unit quantities I a V a 1.0
I f , T e ,
V a
control base EEEB283 – Electrical Machines & Drives control
Dr. Ungku Anisa, July 2008
13
Speed Control Strategy
Per unit quantities I a 1.0
V a I f , T e ,
V a
control base control Torque and power relations below and beyond base P, T P P =T
e
T e T e = K
v I f I a Dr. Ungku Anisa, July 2008
constant torque EEEB283 – Electrical Machines & Drives constant power 14
Operating Modes
Motoring Back EMF E
a
< V
a
I a
and I
f
are positive Motor develops torque to meet load demand (i.e.
T e =T L
)
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Operating Modes
Regenerative Breaking Motor acts as generator Develops E
a
> V
a I a
negative (flows back to source)
I f
positive Machine slows down until E
a
= V
a
Used only when there are enough loads to absorb regenerated power
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EEEB283 – Electrical Machines & Drives 16
Operating Modes
Dynamic Breaking Similar to regenerative breaking But V
a
removed, replaced by R
b
Kinetic energy of motor is dissipated in R
b
(i.e. machine works as generator)
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EEEB283 – Electrical Machines & Drives 17
Operating Modes
Plugging Supply voltage V
a
reversed is
V a
assists E
a
in forcing I
a
in reverse direction
R b
connected in series to limit current
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Operating Modes Four Quadrant Operation
+V
a
Q2
, +E
a -I a
-T + Power = -ve +V
a
Q1
, +E
a
+ +I
a
+T Power = +ve
-V a
Q3
, -E
a -I a
-T Power = +ve
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EEEB283 – Electrical Machines & Drives
-V a
Q4
, -E
a
+I
a
+T Power = -ve 19
References
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3 rd ed., Pearson, New-Jersey, 2004.
Dubey, G.K., Fundamentals of Electric Drives, 2 nd Science Int. Ltd., UK, 2001.
ed., Alpha 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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