1. EEEB443 Control & Drives Induction Motor – Scalar Control 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. Outline Introduction Speed Control of Induction Motors
Pole Changing
Variable-Voltage, Constant Frequency
Variable Frequency
Constant Volts/Hz (V/f) Control
Open-loop Implementation
Closed-loop Implementation
Constant Airgap Flux Control
References
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

3. Introduction
Scalar Control - control of induction machine based on steady-state model (per phase SS equivalent circuit)
Rr’/s + Vs – Rs
Lls
Llr’ E1 Is
Ir’ Im Lm
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

4. Requires speed control of motor
Introduction Te
Pull out Torque
(Tmax)
rotor TL 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
Requires speed control of motor
Dr. Ungku Anisa, July 2008
EEEB443 - Control & 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
EEEB443 - Control & Drives

6. Speed Control of IM Pole Changing
Machines must be specially manufactured (i.e. called pole changing motors or multi-speed motors)
Need special arrangement of stator windings
Only used with squirrel-cage motors
Because number of poles induced in squirrel cage rotor will follow number of stator poles
Two methods:
Multiple stator windings
stator has more than one set of 3-phase windings
only energize one set at a time
simple, expensive
Consequent poles
Discrete step change in speed
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

7. Speed Control of IM Pole Changing Consequent poles
single winding divided into few coil groups
No. of poles changed by changing connections of coil groups
Change in pole number by factor of 2:1 only
A two-pole stator winding for pole changing. Notice the very short pitch (60 to 90) of these windings.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

8. Speed Control of IM Pole Changing Consequent poles
Close up view of one phase of a pole changing winding.
In Figure (a): the 2-pole configuration, one coil is a north pole and the other is a south pole.
In Figure (b): when the connection on one of the two coils is reversed, they are both north poles, and the magnetic flux returns to the stator halfway between the two coils. The south poles are called consequent poles. Hence the winding is now 4-pole.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

9. Speed Control of IM Variable-Voltage (amplitude), Constant Frequency
Controlled using:
Transformer (rarely used)
Thyristor voltage controller
thyristors connected in anti-parallel
motor can be star or delta connected
voltage control by firing angle control
(gating signals are synchronized to phase voltages and are spaced at 60 intervals)
Only for operations in Quadrant 1 and Quadrant 3 (requires reversal of phase sequence)
also used for soft start of motors
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

10. Speed Control of IM Variable-Voltage (amplitude), Constant Frequency
Voltage can only be reduced from rated Vs (i.e. 0 < Vs ≤ Vs,rated)
From torque equation, Te  Vs2
When Vs , Te and speed reduces.
If terminal voltage is reduced to bVs, (i.e. Vs = bVs,rated) :
Note: b  1
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

11. Speed Control of IM Variable-Voltage (amplitude), Constant Frequency
Suitable for applications where torque demand reduces with speed (eg: fan and pump drives where TL  m2)
Suitable for NEMA Class D (high-slip, high Rr’) type motors
High rotor copper loss, low efficiency motors
get appreciable speed range
Practical
speed range
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

12. 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 (harmonics introduced by thyristor switching)
Poor line power factor (power factor proportional to firing angle)
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
EEEB443 - Control & Drives

13. Speed Control of IM Variable Frequency
Speed control above rated (base) speed
Requires the use of PWM inverters to control frequency of motor
Frequency increased (i.e. s increased)
Stator voltage held constant at rated value
Airgap flux and rotor current decreases
Developed torque decreases Te  (1/s)
For control below base speed – use Constant Volts/Hz method
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

14. 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  and enters saturation region oh B-H curve):
- excessive stator currents flow
- distortion of flux wave
- increase in core losses and stator copper loss
Hence, keep ag = rated flux
stator voltage Vs must be reduced proportional to reduction in f (i.e. maintaining Vs / f ratio)
Assuming small voltage drop across Rs and Lls
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

15. Constant Volts/Hz (V/f) Control
Max. torque remains almost constant
For low speed operation:
can’t ignore voltage drop across Rs and Lls (i.e. E1  Vs)
poor torque capability (i.e. torque decreased at low speeds shown by dotted lines)
stator voltage must be boosted – to compensate for voltage drop at Rs and Lls and maintain constant ag
For above base speed operation (f > frated):
stator voltage maintained at rated value
Same as Variable Frequency control (refer to slide 13)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

16. Constant Volts/Hz (V/f) ControlVs
Vs vs. f relation in Constant Volts/Hz drives
Boost - to compensate for voltage drop at Rs and Lls
Vrated
frated
Linear offset
Non-linear offset – varies with Is
Boost
Linear offset curve –
for high-starting torque loads
employed for most applications
Non-linear offset curve –
for low-starting torque loads f
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

17. Constant Volts/Hz (V/f) Control
For operation at frequency K times rated frequency:
fs = Kfs,rated  s = Ks,rated (1)
(Note: in (1) , speed is given as mechanical speed)
Stator voltage: (2)
Voltage-to-frequency ratio = d = constant:
(3)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

18. Constant Volts/Hz (V/f) Control
For operation at frequency K times rated frequency:
Hence, the torque produced by the motor:
(4)
where s and Vs are calculated from (1) and (2) respectively.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

19. Constant Volts/Hz (V/f) Control
For operation at frequency K 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
EEEB443 - Control & Drives

20. Constant Volts/Hz (V/f) Control
Constant Torque Area
(below base speed)
Field Weakening Mode (f > frated)
Reduced flux (since Vs is constant)
Torque reduces
Constant Power Area
(above base speed)
Rated (Base) frequency
Note:
Operation restricted between synchronous speed and Tmax for motoring and braking regions, i.e. in the linear region of the torque-speed curve.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

21. Constant Volts/Hz (V/f) Control
Constant Torque Area
Constant Power Area
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

22. Example
A 4-pole, 3 phase, 400 V, 50 Hz, 1470 rpm induction motor has a rated torque of 30 Nm. The motor is used to drive a linear load with characteristic given by TL = K, such that the speed equals rated value at rated torque. If a constant Volts/Hz control method is employed, calculate:
The constant K in the TL - characteristic of the load.
Synchronous and motor speeds at 0.6 rated torque.
If a starting torque of 1.2 times rated torque is required, what should be the voltage and frequency applied at start-up? State any assumptions made for this calculation.
Answers:
K = 0.195, synchronous speed = rpm & motor speed = rpm, At start up: frequency = 1.2 Hz, Voltage = 9.6 V
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

23. Constant Volts/Hz (V/f) Control – Open-loop Implementation
PWM
Voltage-Source Inverter
(VSI)
Note: e= s = synchronous speed
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

24. Constant Volts/Hz (V/f) Control – Open-loop Implementation
Most popular speed control method because it is easy to implement
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 (shown as the ‘G’ in slide 23)
Boost voltage Vo is added at low speeds
Constant voltage applied above base speed
Sinusoidal phase voltages (vabc*) is then generated from Vs* & s* where s* is obtained from the integral of s*
vabc* employed in PWM inverter connected to motor
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

25. Constant Volts/Hz (V/f) Control – Open-loop Implementation
Problems in open-loop drive operation:
Motor speed not controlled precisely
primary control variable is synchronous speed s
actual motor speed r is less than s due to sl
sl depends on load connected to motor
sl cannot be maintained since r not measured
can lead to operation in unstable region of T- characteristic
stator currents can exceed rated value – endangering inverter-converter combination
Problems (to an extent) can be overcome by:
Open-loop Constant Volts/Hz Drive with Slip Compensation
Closed-loop implementation - having outer speed loop with slip regulation
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

26. Constant Volts/Hz (V/f) Control – Open-loop Implementation
Open-loop Constant Volts/Hz Drive with Slip Compensation
- Slip speed is estimated and added to the reference speed r*
Slip Compensator
Idc
Vdc = Vd
sl
r*
Note: e= s = synchronous speed
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

27. Constant Volts/Hz (V/f) Control – Open-loop Implementation
Open-loop Constant Volts/Hz Drive with Slip Compensation
How is sl estimated in the Slip Compensator?
Using T- curve, sl  Te
sl can be estimated by estimating torque where:
(8)
(9)
(7)
Note: In the figure,
slip= sl = slip speed
syn= s = synchronous speed
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

28. Constant Volts/Hz (V/f) Control – Closed-loop Implementation
Open-loop system
(as in slide 23)
Slip Controller
Note: e= s = synchronous speed
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

29. Constant Volts/Hz (V/f) Control – Closed-loop Implementation
Reference motor speed r* is compared to the actual speed r to obtain the speed loop error
Speed loop error generates slip command sl* from PI controller and limiter
Limiter ensures that the sl* is kept within the allowable slip speed of the motor (i.e. sl*  slip speed for maximum torque)
sl* is then added to the actual motor speed r to generate synchronous speed command s* (or frequency command)
s* generates voltage command Vs* from V/f relation
Boost voltage is added at low speeds
Constant voltage applied above base speed
Scheme can be considered open loop torque control (since T  s) within speed control loop
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

30. Constant Airgap Flux Control
Constant V/f control employs the use of variable frequency voltage source inverters (VSI)
Constant Airgap Flux control employs variable frequency current source inverters or current-controlled VSI
Provides better performance compared to Constant V/f control with Slip Compensation
airgap flux is maintained at rated value through stator current control
Speed response similar to equivalent separately-excited dc motor drive but torque and flux channels still coupled
Fast torque response means:
High-performance drive obtained
Suitable for demanding applications
Able to replace separately-excited dc motor drives
Above only true is airgap flux remains constant at rated value
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

31. Constant Airgap Flux Control
Constant airgap flux in the motor means:
For ag to be kept constant at rated value, the magnetising current Im must remain constant at rated value
Hence, in this control scheme stator current Is is controlled to maintain Im at rated value
Assuming small voltage drop across Rs and Lls
Controlled to maintain Im at rated
Rr’/s + Vs – Rs
Lls
Llr’
E1  Vs Is
Ir’ Im Lm
maintain at rated
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

32. Constant Airgap Flux Control
From torque equation (with ag kept constant at rated value),
since ss = sl and ignoring Rs and Lls,
By rearranging the equation:
Te  sl  sl can be varied instantly  instantaneous (fast)
Te response
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

33. Constant Airgap Flux Control
Constant airgap flux requires control of magnetising current Im which is not accessible
From equivalent circuit (on slide 31):
From equation (10), plot Is against sl when Im is kept at rated value.
Drive is operated to maintain Is against sl relationship when frequency is changed to control speed.
Hence, control is achieved by controlling stator current Is and stator frequency:
Is controlled using current-controlled VSI
Control scheme sensitive to parameter variation (due to Tr and r)
(10)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

34. Constant Airgap Flux Control - Implementation
Current Controlled VSI
Voltage Source Inverter (VSI)
Rectifier
3-phase supply IM
r* +
|Is|
slip C
Current controller s PI r -
Current controller options:
Hysteresis Controller
PI controller + PWM
i*a
i*b
i*c
Equation (10)
(from slide 33) r
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

35. Current-Controlled VSI Implementation
Hysteresis Controller
Motor +
i*a
i*b
i*c
Voltage Source Inverter (VSI)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

36. Current-Controlled VSI Implementation
PI Controller + Sinusoidal PWM
Motor +
i*a
i*b
i*c
PWM PI
Voltage Source Inverter (VSI)
Due to interactions between phases
(assuming balanced conditions)
 actually only require 2 controllers
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

37. Current-Controlled VSI Implementation
PI Controller + Sinusoidal PWM (2 phase)
i*a
abcdq
dq abc
id* PI
PWM
Voltage Source Inverter (VSI)
i*b
iq*
i*c iq
abcdq id
Motor
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

38. 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
EEEB443 - Control & Drives