1. EEEB283 Electrical Machines & Drives
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
Dr. Ungku Anisa, July 2008

2. 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

3. Introduction
Scalar Control - control of induction machine based on steady-state model (per phase steady-state equivalent circuit)
Rr’/s + Vs – Rs
Lls
Llr’ E1 Is
Ir’ Im Lm
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

4. Introduction Te Pull out Torque (Tmax) rotor rated Trated r sTL Te
Intersection point (Te=TL) determines the steady –state speed sm
rated
Trated
What if the load must be operated here? r s
rotor’ s
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

5. Speed Control of IM
Given a load T– characteristic, the steady-state speed can be changed by altering the T– curve of the motor
Varying voltage (amplitude) 2
Varying line frequency 3
Pole Changing 1
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

6. 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

7. 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

8. 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,:
Note: b  1
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

9. 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

10. 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

11. Constant Volts/Hz (V/f) Control
Airgap flux in the motor is related to the induced stator voltage E1 :
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
Assuming small voltage drop across Rs and Lls
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

12. Constant Volts/Hz (V/f) Control
Max. torque remains almost constant
For low speed operation:
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

13. Constant Volts/Hz (V/f) ControlVs
Vrated
frated
Linear offset
Non-linear offset – varies with Is
Boost
Vs vs. f relation in Constant Volts/Hz drives f
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

14. Constant Volts/Hz (V/f) Control
For operation at frequency  times rated frequency:
fs = fs,rated  s = s,rated (1)
Stator voltage: (2)
Voltage-to-frequency ratio = d = constant:
(3)
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

15. Constant Volts/Hz (V/f) Control
For operation at frequency  times rated frequency:
Hence, the torque produced:
(4)
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
For operation at frequency  times rated frequency:
The slip for maximum torque is:
(5)
The maximum torque is then given by:
(6)
where s and Vs are calculated from (1) and (2) respectively.
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

17. Constant Volts/Hz (V/f) Control
Constant Torque Area
Field Weakening Mode (f > frated)
Reduced flux
Torque reduces
 Constant Power Area
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

18. Constant Volts/Hz (V/f) Control
Constant Torque Area
Constant Power Area
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

19. Constant Volts/Hz (V/f) Control – Open-loop Implementation
PWM
Voltage-Source Inverter
(VSI)
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

20. 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

21. 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