1. INDUCTION MOTOR Scalar Control (squirrel cage)
MEP 1523
ELECTRIC DRIVES
INDUCTION MOTOR Scalar Control (squirrel cage)

2. Scalar control of induction machine: Control of induction machine based on steady-state model (per phase SS equivalent circuit): Is
Lls
Ir’
Llr’ Rs + Vs – +
Eag – Lm
Rr’/s Im

3. Scalar control of induction machineTe
Pull out Torque
(Tmax)
rotor TL Te
Intersection point (Te=TL) determines the steady –state speed sm
rated
Trated r s s

4. Scalar control of induction machine
Given a load T– characteristic, the steady-state speed can be changed by altering the T– of the motor:
Pole changing
Synchronous speed change with no. of poles
Discrete step change in speed
Variable voltage (amplitude), frequency fixed
E.g. using transformer or triac
Slip becomes high as voltage reduced – low efficiency
Variable voltage (amplitude), variable frequency
Using power electronics converter
Operated at low slip frequency

5. Variable voltage, fixed frequency
e.g. 3–phase squirrel cage IM
V = 460 V Rs= 0.25 
Rr=0.2  Lr = Ls = 0.5/(2*pi*50)
Lm=30/(2*pi*50)
f = 50Hz p = 4
Lower speed  slip higher
Low efficiency at low speed

6. Variable voltage, variable frequency
Constant V/f operation
At low slip

7. If Φag is constant  Te α slip frequency
Variable voltage, variable frequency – Constant V/f
If Φag is constant  Te α slip frequency

8. Approximates constant air-gap flux when Eag is large
Variable voltage, variable frequency – Constant V/f
How do we make constant ?
Approximates constant air-gap flux when Eag is large
Eag = k f ag
= constant
Speed is adjusted by varying f - maintaining V/f to approximate constant air-gap flux

9. Characteristic with constant
Variable voltage, variable frequency – Constant V/f
Characteristic with constant

10. Constant  constant V/f
Variable voltage, variable frequency
Constant  constant V/f Vs
Vrated
Constant slope
frated f

11. Constant V/f – open-loop
Variable voltage, variable frequency
Constant V/f – open-loop
Rectifier
3-phase supply
VSI IM C f
Ramp
Pulse Width Modulator V
s* +
rate limiter is needed to ensure the slip change within allowable range (e.g. rated value)

12. Constant V/f – open-loop
Variable voltage, variable frequency
Constant V/f – open-loop
Simulation example: 415V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= H, Lm = , J = kgm2 , Load: k2

13. Constant V/f – open-loop
Variable voltage, variable frequency
Constant V/f – open-loop
Simulation example: 415V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= H, Lm = , J = kgm2 , Load: k2
constant_vhz_withoutBoost/Signal Builder : Group 1
Signal 1 50 40 30 20 10
0.5 1
1.5 2
2.5 3
3.5
Time (sec) 13

14. Constant V/f – open-loop
Variable voltage, variable frequency
Constant V/f – open-loop
Simulation example: 415V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= H, Lm = , J = kgm2 , Load: k2 14

15. Constant V/f – open-loop
Variable voltage, variable frequency
Constant V/f – open-loop
Simulation example: 415V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= H, Lm = , J = kgm2 , Load: k2
With almost no rate limiter 15

16. Constant V/f – open-loop
Variable voltage, variable frequency
Constant V/f – open-loop
Simulation example: 415V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= H, Lm = , J = kgm2 , Load: k2
With 628 rad/s2 16

17. Constant V/f – open-loop low speed problems
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems
Problems with open-loop constant V/f
At low speed, voltage drop across stator impedance is significant compared to airgap voltage - poor torque capability at low speed
Solution:
(i) Voltage boost at low frequency
(ii) Maintain Im constant  stator current control

18. Constant V/f – open-loop low speed problems (i) voltage boost
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) voltage boost
Torque deteriorate at low frequency – hence compensation commonly performed at low frequency
In order to truly compensate need to measure stator current – seldom performed

19. Constant V/f – open-loop low speed problems (i) voltage boost
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) voltage boost
With voltage boost of Irated*Rs
Torque deteriorate at low frequency – hence compensation commonly performed at low frequency
In order to truly compensate need to measure stator current – seldom performed

20. Constant V/f – open-loop low speed problems (i) voltage boost
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) voltage boost
Voltage boost at low frequency
Vrated
Linear offset
Boost
Non-linear offset – varies with Is
frated

21. Constant V/f – open-loop low speed problems (i) voltage boost
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) voltage boost
Idc
Rectifier +
Vdc -
3-phase supply
VSI IM C f
Ramp
Pulse Width Modulator
s* + V +
Vboost

22. Constant V/f – open-loop low speed problems (i) Constant Im
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) Constant Im
ag, constant → Eag/f , constant → Im, constant (rated)
Controlled to maintain Im at rated Is
Lls
Llr’
Ir’ Rs + Vs – Lm +
Eag –
Rr’/s
maintain at rated Im

23. Constant V/f – open-loop low speed problems (i) Constant Im
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) Constant Im
From per-phase equivalent circuit,
Current is controlled using current-controlled VSI
The problem of stator impedance drop is solved
Dependent on rotor parameters – sensitive to parameter variation

24. Current reference generator
Variable voltage, variable frequency
Constant V/f – open-loop low speed problems (i) Constant Im
3-phase supply
VSI
Rectifier IM C
Current controller
Tacho
slip * +
|Is| PI - + s r
Current reference generator +

25. Problems with open-loop constant V/f
Variable voltage, variable frequency
Constant V/f
Problems with open-loop constant V/f
Poor speed regulation
Solution:
(i) Slip compensation
(ii) Closed-loop control

26. Constant V/f – poor speed regulation: (i) slip compensation
Variable voltage, variable frequency
Constant V/f – poor speed regulation: (i) slip compensation
Motor characteristic AFTER slip compensation T
ωr (rad/s)
ωs1*
Tload
ωs2*=ωs1*+ωslip1
ωslip1
Motor characteristic BEFORE slip compensation
ωr2≈ωs1* T2
ωr1 T1
ωslip1

27. Constant V/f – poor speed regulation: (i) slip compensation
Variable voltage, variable frequency
Constant V/f – poor speed regulation: (i) slip compensation
Idc
Rectifier +
Vdc -
3-phase supply
VSI IM C f
Ramp
Pulse Width Modulator
s* + + V + +
Vboost
Slip speed calculator
Vdc
Idc

28. Constant V/f – poor speed regulation: (i) slip compensation
Variable voltage, variable frequency
Constant V/f – poor speed regulation: (i) slip compensation
How is the slip frequency calculated ? +
Vdc 
Idc
INV
Pdc= VdcIdc
Pmotor,in= Pdc – Pinv,losses
Pmotor,in
Stator Copper
lossess
Stator Core
losses
ROTOR
STATOR
Pair-gap 28

29. Constant V/f – poor speed regulation: (i) slip compensation
Variable voltage, variable frequency
Constant V/f – poor speed regulation: (i) slip compensation
How is the slip frequency calculated ?
Pair-gapc = Tesyn
Te =  Pair-gap/syn
For constant V/f control, 29

30. Constant V/f – poor speed regulation: (i) closed-loop speed
Variable voltage, variable frequency
Constant V/f – poor speed regulation: (i) closed-loop speed
Require speed encoder
Increase complexity 30