Induction Motor Review By Mr.M.Kaliamoorthy Department of Electrical & Electronics Engineering PSNA College of Engin

1. Induction Motor Review By Mr.M.Kaliamoorthy Department of Electrical & Electronics Engineering PSNA College of Engineering and Technology

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. 120o 120o 120o 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 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 : Ir Lls Llr Is Rs + Vs – + E1 – + Er – Rr/s Lm Im

10. Lls Is Llr’ Ir’ Rs + E1 – + Vs – Rr’/s Lm Im Induction Motor – Per Phase Equivalent Circuit • 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 ConvertedPowerPconv Airgap Power Pag 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 Te • Maximum torque or pullout torque occurs when slip is: (8) • The pullout torque can be calculated using: (9) Pull out Torque (Tmax) Trated r 0 rated smax s s 1 0

15. Induction Motor – T- Characteristic Te Linear region of operation (small s) • Te s • High efficiency • Pout = Pconv – Prot • Pconv = (1- s )Pag • Stable motor operation Pull out Torque (Tmax) Trated 0 rated smax r s s 1 0

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 Tstartand 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 Tstartmust 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.