1. Induction Motor ReviewBy
Mr.M.Kaliamoorthy
Department of Electrical & Electronics Engineering
PSNA College of Engineering and Technology
Dr. Ungku Anisa, July 2008

2. Outline Introduction Construction Concept Per-Phase Equivalent Circuit
Power Flow
Torque Equation
T- Characteristics
Starting and Braking
References

3. Introduction Induction motors (IM) most widely used
IM (particularly squirrel-cage type) compared to
DC motors
Rugged
Lower maintenance
More reliable
Lower cost, weight, volume
Higher efficiency
Able to operate in dirty and explosive environments

4. Introduction IM mainly used in applications requiring constant speed
Conventional speed control of IM expensive or highly inefficient
IM drives replacing DC drives in a number of variable speed applications due to
Improvement in power devices capabilities
Reduction in cost of power devices

5. Induction Motor – Construction
Stator
balanced 3-phase winding
distributed winding – coils distributed in several slots
produces a rotating magnetic field
Rotor
usually squirrel cage
conductors shorted by end rings
Rotating magnetic field induces voltages in the rotor
Induced rotor voltages have same number of phases and poles as in stator winding
120o a c’ b’ c b a’

6. Induction Motor – Construction

7. Induction Motor – Concept
Stator supplied by balanced 3-phase AC source (frequency f Hz or  rads/sec )
field produced rotates at synchronous speed s rad/sec
(1)
where P = number of poles
Rotor rotates at speed m rad/sec (electrical speed r = (P/2) m)
Slip speed, sl – relative speed (2)
between rotating field and rotor
Slip, s – ratio between slip speed
and synchronous speed (3)

8. Induction Motor – Concept
Relative speed between stator rotating field and rotor induces:
emf in stator winding (known as back emf), E1
emf in rotor winding, Er
Frequency of rotor voltages and currents: (4)
Torque produced due to interaction between induced rotor currents and stator field
Stator voltage equation:
Rotor voltage equation:

9. Induction Motor – Concept
E1 and Er related by turns ratio aeff
Rotor parameters can be referred to the stator side :
Lls Is
Llr Ir Rs + Vs – + E1 – + Er – Lm
Rr/s Im

10. Induction Motor – Per Phase Equivalent Circuit
Rr’/s + Vs – Rs
Lls
Llr’ E1 Is
Ir’ Im Lm
Rs – stator winding resistance
Rr’ – referred rotor winding resistance
Lls – stator leakage inductance
Llr’ – referred rotor leakage inductance
Lm – mutual inductance
Ir’ – referred rotor current

11. Induction Motor – Power Flow
Airgap Power Pag
ConvertedPower Pconv
Mechanical Power
Electrical Power
Rotational losses Prot
(Friction and windage, core and stray losses)
Rotor Copper Loss (RCL)
Note:
Stator Copper Loss (SCL)

12. Induction Motor – Torque Equation
Motor induced torque is related to converted power by:
(5)
Since and , hence
(6)
Substituting for Ir’ from the equivalent circuit:
(7)

13. Induction Motor – T- Characteristic
T- characteristic of IM during generating, motoring and braking

14. Induction Motor – T- Characteristic
Maximum torque or pullout torque occurs when slip is:
(8)
The pullout torque can be calculated using:
(9) r s
Trated
Pull out Torque
(Tmax) Te
rated
smax s

15. Induction Motor – T- Characteristic
Linear region of operation (small s)
Te  s
High efficiency
Pout = Pconv – Prot
Pconv = (1- s )Pag
Stable motor operation r s
Trated
Pull out Torque
(Tmax) Te
rated
smax s

16. Induction Motor – NEMA Classification of IM
NEMA = National Electrical Manufacturers Association
Classification based on T- characteristics
Class A & B – general purpose
Class C – higher Tstart (eg: driving compressor pumps)
Class D – provide high Tstart and wide stable speed range but low efficiency s

17. Induction Motor – Starting
Small motors can be started ‘direct-on-line’
Large motors require assisted starting
Starting arrangement chosen based on:
Load requirements
Nature of supply (weak or stiff)
Some features of starting mechanism:
Motor Tstart must overcome friction, load torque and inertia of motor-load system within a prescribed time limit
Istart magnitude ( 5-7 times I rated) must not cause
machine overheating
Dip in source voltage beyond permissible value

18. Induction Motor – Starting
Methods for starting:
Stat-delta starter
Autotransformer starter
Reactor starter
Soft Start

19. Induction Motor – Starting
Star-delta starter
Special switch used
Starting: connect as ‘star’ (Y)
Stator voltages and currents reduced by 1/√3
Te  VT2  Te reduced by 1/3
When reach steady state speed
Operate with ‘delta’ ( ) connection
Switch controlled manually or automatically

20. Induction Motor – Starting
Autotransformer starter
Controlled using time relays
Autotransformer turns ratio aT
Stator voltages and currents reduced by aT
Te  VT2  Te reduced by aT2
Starting: contacts 1 & 2 closed
After preset time (full speed reached):
Contact 2 opened
Contact 3 closed
Then open contact 1

21. Induction Motor – Starting
Reactor starter
Series impedance (reactor) added between power line and motor
Limits starting current
When full speed reached, reactors shorted out in stages

22. Induction Motor – Starting
Soft Start
For applications which require stepless control of Tstart
Semiconductor power switches (e.g. thyristor voltage controller scheme) employed
Part of voltage waveform applied
Distorted voltage and current waveforms (creates harmonics)
When full speed reached, motor connected directly to line

23. Induction Motor – Braking
Regenerative Braking:
Motor supplies power back to line
Provided enough loads connected to line to absorb power
Normal IM equations can be used, except s is negative
Only possible for  > s when fed from fixed frequency source
Plugging:
Occurs when phase sequence of supply voltage reversed
by interchanging any two supply leads
Magnetic field rotation reverses  s > 1
Developed torque tries to rotate motor in opposite direction
If only stopping is required, disconnect motor from line when  = 0
Can cause thermal damage to motor (large power dissipation in rotor)

24. Induction Motor – Braking
Dynamic Braking:
Step-down transformer and rectifier provides dc supply
Normal: contacts 1 closed, 2 & 3 opened
During braking: Contacts 1 opened, contacts 2 & 3 closed
Two motor phases connected to dc supply - produces stationary field
Rotor voltages induced
Energy dissipated in rotor resistance – dynamic braking

25. References
Chapman, S. J., Electric Machinery Fundamentals, McGraw Hill, New York, 2005.
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd ed., Pearson, New-Jersey, 2004.
Trzynadlowski, Andrzej M. , Control of Induction Motors, Academic Press, 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.