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

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

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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

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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

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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’ 120 o

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Induction Motor – Construction 6

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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

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Induction Motor – Concept Relative speed between stator rotating field and rotor induces: – emf in stator winding (known as back emf), E 1 – emf in rotor winding, E r 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

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Induction Motor – Concept E 1 and E r related by turns ratio a eff Rotor parameters can be referred to the stator side : 9 R r /s +Vs–+Vs– RsRs L ls L lr +E1–+E1– IsIs IrIr ImIm LmLm +Er–+Er–

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Induction Motor – Per Phase Equivalent Circuit R s –stator winding resistance R r ’ –referred rotor winding resistance L ls –stator leakage inductance L lr ’ –referred rotor leakage inductance L m –mutual inductance I r ’ –referred rotor current 10 R r ’/s +Vs–+Vs– RsRs L ls L lr ’ +E1–+E1– IsIs Ir’Ir’ ImIm LmLm

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Induction Motor – Power Flow 11 Stator Copper Loss (SCL) Rotor Copper Loss (RCL) Airgap Power P ag Converted Power P conv Rotational losses P rot (Friction and windage, core and stray losses) Electrical Power Mechanical Power Note:

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

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

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Induction Motor – T- Characteristic Maximum torque or pullout torque occurs when slip is: (8) The pullout torque can be calculated using: (9) 14 rr s T rated Pull out Torque (T max ) TeTe 0 rated s max ss 1 0

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Induction Motor – T- Characteristic Linear region of operation (small s) T e s High efficiency P out = P conv – P rot P conv = (1- s )P ag Stable motor operation 15 rr s T rated Pull out Torque (T max ) TeTe 0 rated s max ss 1 0

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

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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 T start must overcome friction, load torque and inertia of motor- load system within a prescribed time limit – I start magnitude ( 5-7 times I rated ) must not cause machine overheating Dip in source voltage beyond permissible value 17

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

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Induction Motor – Starting Star-delta starter – Special switch used – Starting: connect as ‘star’ (Y) Stator voltages and currents reduced by 1/ √ 3 T e V T 2 T e reduced by 1/3 – When reach steady state speed Operate with ‘delta’ ( ) connection – Switch controlled manually or automatically 19

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Induction Motor – Starting Autotransformer starter – Controlled using time relays – Autotransformer turns ratio a T Stator voltages and currents reduced by a T T e V T 2 T e reduced by a T 2 – Starting: contacts 1 & 2 closed – After preset time (full speed reached): Contact 2 opened Contact 3 closed Then open contact 1 20

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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

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Induction Motor – Starting Soft Start – For applications which require stepless control of T start – 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

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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

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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

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References Chapman, S. J., Electric Machinery Fundamentals, McGraw Hill, New York, Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3 rd ed., Pearson, New-Jersey, Trzynadlowski, Andrzej M., Control of Induction Motors, Academic Press, Nik Idris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM,

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