1. AC Vector Controlled Drives Induction Motor Drives
School of Electrical and Electronic Engineering
AC Vector Controlled Drives
Induction Motor Drives
Greg Asher
Professor of Electrical Drives and Control

2. Revision of Induction motors
Part I
Revision of Induction motors
Equivalent circuit
Power Flows
Torque-speed characteristic

3. THE 3 PHASE INDUCTION MACHINE
60% of world's generated energy  rotating machines
>90% of this  induction machines
The induction machine consumes more of world’s generated electricity than any other piece of electrical equipment
Power Range
W small fans
1-50kW fans, pumps, conveyors, escalators
500kW water pumping, coal cutting,
1MW high speed train motor (eg. x4)
10MW warship/cruise ship motor (X2)

4. Introduction – construction of cage IMB B’ A A’
Iron
Al bars VA
End rings
Rotor
(side view)
Stator has 3 windings AA’, BB’, CC’ wound 120 apart in space
Stator windings connected to 3-phase mains at e = (2) 50Hz mains
Fed by 3-phase currents 120 apart in time to create rotating magnetic field
Rotor has NO windings
It has a cage of Aluminium bars; currents will be induced in it

5. Speed of rotating fields
Rotating field set up by stator currents rotates at synch speed s fe
P (poles)
rads-1
rpm 50
314 2
3000 4
157
1500 6
105
1000 8 78
750 10 63
600
If each phase spans 60° in space, then get 4-pole distribution
1 rpm = 2 radians/minute = 2/60 radian/second (rads-1)
Therefore 1 rads-1 = 60/2  10 rpm
Stator windings of an IM can only be wound in one way. P is fixed for an individual machine. An IM can either be a 2-pole machine, or a 4-pole machine or ….etc.

6. Concept of torque increasing with rotor slip
Rotor bars see magnetic field rotating past them (conductors in moving field)
Currents induced in rotor bars to establish torque; rotor travels at in attempt to catch up with rotating field
Have ;
Bigger slip, bigger torque T
Low Rr
Slope =
High Rr
Rated Operation
Irat
(Stator current increases with slip) r
s = 1
s = 0.5
s = 0

7. Per phase equivalent circuitIm RS VS IR lS lR LM IS
Power losses
Mechanical power
Vs , IS rms stator volts, current per PHASE (not line-line)
IR rms rotor current referred to the primary (also component of Is flowing to cancel magnetic field of rotor currents)
Im rms component of stator current which magnetises machine (sets up rotating field
L0 magnetising inductance
lr ls rotor leakage inductance, stator leakage inductance
Lr rotor self inductance,
Ls stator self inductance,
Rs stator resistance, Rr rotor resistance
Stator an rotor leakage coefficients

8. Per phase equivalent circuit – full speed range
Leakage effects reduce torque for a given slip, also causing maximum torque and shape of torque curve at large slips
Torque-slip curve now given by:
Real T-speed curve
Final speed determined by load
Tacc P1
Typical fan-pump load shown
When motor switched to mains:
- motor goes to P1
- motor too large or too small? r
Tstart
4Irat
5Irat
3Irat T
s=0
s=0.5
s=1
2Irat
Irat P2
Smaller fan-pump load shown
When motor switched to mains:
- motor goes to P2
Lift, hoist load shown in green
- constant due to gravitational force
- slight increase due to friction etc

9. Rotating field and rotating flux

10. Rotating field, flux and applied voltage

11. Rotating field and induced rotor currents

12. Torque on induced currents

13. Field due to rotor currents - cancelled!!

14. Stator & rotor current fields – increasing load

15. Stator current components

16. Effect of rotor leakage -1

17. Effect of rotor leakage - 2

18. T Tacc Variable frequency (and voltage) operation
Motor torque for given motor voltage Vs and frequency e:
put Vs = ke since
: this keeps Im (and field)  constant when
applied frequency changes
0.25
0.5
0.75 1 T
Tacc
Torque expression becomes:
Only dependent on sl

19. Field weakening esp. at higher speed
In Vs = ke, k is such that Vrated (eg 415V) occurs at e-rated (eg 50Hz)
If Vrated is the maximum voltage of the converter, then Im and the field must reduce if we wish e > e-rated
Seen that as field of flux  1/e ; hence T  1/e for a given current (Ir)
Eventually, leakage effects impose
Te= constant
Te2 = constant
T = constant
Field weakening region often called “Constant Power”
Frequencies to 2e normal
Employed if load also has constant power characteristic (so that good motor-load matching can be got)

20. The PWM converter IDC IDC
Variable Vs and e synthesized by “modulating” the transistor switching pattern
Motor speed r may be +ve or –ve depending on phase sequence of VS
Regeneration occurs when r > e
Under region, current reverses into DC link,, charging C
Voltage increases!
Is = Is rated
Is = -Is rated e
Is = 0
Generating region
IDC

21. The PWM converter - regeneration
IDC E
Called “dynamic braking”
If E rises to Enom+E, then transistor turned on. If E falls to Enom-E, then turned off
Cheap but energy wasteful, especially if load has many braking instances
IDC
Called PWM rectifier or “active font-end”
Can draw near sinusoidal currents form supply
Can inject reactive power into supply
Line inductors required to “decouple” supply voltage from PWM output

22. (where accurate speed-holding not required)
Open- loop V-f control
(where accurate speed-holding not required)
Ramp generator ramps fe to fe* at rate k (fe = kt )
K reduced (or set to zero) if IDC > Imax or E > E+E + - 6
reduce k
set fe*
set Imax fe V f
PWM
E+E Vm
Ramp generator with slope k
Voltage-frequency characteristic
Irat A B
e1
2Irat
3Irat
e2
Irat
2Irat
e2
e1

23. Low speed voltage boost
Open- loop V-f control
Low speed voltage boost Im Rs Lm
Rr/s Vm Vs Is
Aim is to adjust Vs to keep Im constant -
When e is not small 
When e is small 
The voltage boost Vb (normally 20-40V) is required to overcome the voltage drop due to Rs when e is small fe Vm k Vb
Field weakening
1pu

24. Summary for PWM V-F drives
About 25-30% of IM drives are driven by PWM converters
Open-Loop V-f drive most common – 60% of total
- many drives esp. pumps and fans are just switched on and left running for long
periods under constant speed
V-f drive operation based on steady state sinusoidal operation – only controlling rms values
V-f drive has poor torque control and poor low speed performance
- but OK for just starting loads requiring low torque at low speed
Need to control instantaneous values of current
to get fast control of torque and flux (and hence speed)
This is done by “vector control” of IMs

25. Revision of Induction motors
Part II
Revision of Induction motors
Equivalent circuit
Power Flows
Torque-speed characteristic