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  1. 1. Induction Motor Review By Mr.M.Kaliamoorthy Department of Electrical & Electronics Engineering PSNA College of Engineering and Technology 1
  2. 2. Outline  Introduction  Construction  Concept  Per-Phase Equivalent Circuit  Power Flow  Torque Equation  T- Characteristics  Starting and Braking  References 2
  3. 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 3
  4. 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 4
  5. 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 5 a b b’ c’ c a’ 120o 120o 120o
  6. 6. Induction Motor – Construction 6
  7. 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) 7 f P P s    4 2   f P ns 120  m s sl      s m s s     
  8. 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: 8   1 2 E I L πf j I R V s ls s s s    sf fr      r lr r r r r lr r r r I L πf j I s R E I L πf js I R sE 2 2          
  9. 9. Induction Motor – Concept • E1 and Er related by turns ratio aeff • Rotor parameters can be referred to the stator side : 9 r eff r r eff r eff r r r eff L a L R a R a I I E a E 2 ' 2 ' ' 1     Rr/s + Vs – Rs Lls Llr + E1 – Is Ir Im Lm + Er –
  10. 10. 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 10 Rr’/s + Vs – Rs Lls Llr’ + E1 – Is Ir’ Im Lm
  11. 11. Induction Motor – Power Flow 11  cos 3 L T in I V P  m L out T P   Stator Copper Loss (SCL) s s SCL R I P 2 3  Rotor Copper Loss (RCL) ' 2 ' 3 r r RCL R I P  Airgap Power Pag Converted Power Pconv Rotational losses Prot (Friction and windage, core and stray losses) s R I P r r ag ' 2 ' 3          s s R I P r r conv 1 3 ' 2 ' Electrical Power Mechanical Power ag RCL sP P  Note:   ag conv P s P   1
  12. 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) 12   ag conv P s P   1   s r s     1                       2 2 ' 2 ' 3 lr ls r s s s r e X X s R R V s R T  m conv e P T   s r r s ag e s R I P T   ' 2 ' 3  
  13. 13. Induction Motor – T- Characteristic • T- characteristic of IM during generating, motoring and braking 13
  14. 14. Induction Motor – T- Characteristic  Maximum torque or pullout torque occurs when slip is: (8)  The pullout torque can be calculated using: (9) 14             2 2 2 max 2 3 lr ls s s s s X X R R V T   2 2 ' max lr ls s r X X R R s     r s Trated Pull out Torque (Tmax) Te 0 rated smax s 1 0
  15. 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 15 r s Trated Pull out Torque (Tmax) Te 0 rated smax s 1 0
  16. 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 16 s
  17. 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 17
  18. 18. Induction Motor – Starting • Methods for starting: – Stat-delta starter – Autotransformer starter – Reactor starter – Soft Start 18
  19. 19. Induction Motor – Starting • Star-delta starter – Special switch used – Starting: connect as ‘star’ (Y) • Stator voltages and currents reduced by 1/√3 • Te  VT 2  Te reduced by 1/3 – When reach steady state speed • Operate with ‘delta’ ( ) connection – Switch controlled manually or automatically 19
  20. 20. Induction Motor – Starting • Autotransformer starter – Controlled using time relays – Autotransformer turns ratio aT • Stator voltages and currents reduced by aT • Te  VT 2  Te reduced by aT 2 – Starting: contacts 1 & 2 closed – After preset time (full speed reached): • Contact 2 opened • Contact 3 closed • Then open contact 1 20
  21. 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 21
  22. 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 22
  23. 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) 23
  24. 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 24
  25. 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. 25

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