1. EEEB443 Control & 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
Dr. Ungku Anisa, July 2008

2. DC Drives Outline Introduction to DC Drives
Separately Excited DC Motor
Speed Control Methods
Speed Control Strategy
Operating Modes
References
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

3. 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
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

4. Introduction Some limitations: Commonly used DC motors
High maintenance (commutators & brushes)
Expensive
Speed limitations
Sparking
Commonly used DC motors
Separately excited
Series (mostly for traction applications)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

5. Separately Excited DC MotorLf Rf if + ea _ La Ra ia vt vf
Armature circuit
Field circuit
Electromagnetic torque
Armature back e.m.f.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

6. Separately Excited DC Motor
Motor is connected to a load.
Therefore,
where
TL= load torque
J = load inertia (kgm2)
B = viscous friction coefficient (Nm/rad/s)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

7. Separately Excited DC Motor – Steady State Condition
Time derivatives = 0. Therefore,
(1)
(2)
(3)
(4)
The developed power
(5)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

8. Speed Control Methods for Separately Excited DC Motor Te
From equation (3),
Three possible methods for speed control:
Armature voltage Va
Armature resistance Ra
Field flux  (by changing field resistance Rf)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

9. Speed Control Methods – Va control TL
Va↓
Requires variable DC supply Te
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

10. Speed Control Methods – Ra control TL
Simple control
Losses in external resistor
 Rarely used.
Ra ↑ Te
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

11. Speed Control Methods –  control
 ↓ TL
Not possible for PM motor
Normally employed for speed above base speed Te
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

12. 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
base 
Va control
 control
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

13. 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
base Va Ia
1.0
If, Te,  
Va control
 control
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

14. Speed Control Strategy
Per unit
quantities Va
1.0 Ia
If, Te, 
Va control
 control
base
Torque and power relations below and beyond base
P, T P Te
P =K 
Te = KIa
constant torque
constant power
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

15. 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.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

16. Operating Modes Motoring Back EMF Ea < Va Ia and If are positive
Motor develops torque to meet load demand (i.e. Te =TL )
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

17. 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
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

18. 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)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

19. Operating Modes Plugging Supply voltage Va is reversed
Assists Ea in forcing Ia in reverse direction
Rb connected in series to limit current
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

20. 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
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

21. 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.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives