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**Induction Motor Review**

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

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**Outline Introduction Construction Concept Per-Phase Equivalent Circuit**

Power Flow Torque Equation T- Characteristics Starting and Braking References

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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

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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

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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 120o a c’ b’ c b a’

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**Induction Motor – Construction**

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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)

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**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:

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**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

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**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

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**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)

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**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)

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**Induction Motor – T- Characteristic**

T- characteristic of IM during generating, motoring and braking

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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) r s Trated Pull out Torque (Tmax) Te rated smax s

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**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

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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 Tstart (eg: driving compressor pumps) Class D – provide high Tstart and wide stable speed range but low efficiency s

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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 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

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**Induction Motor – Starting**

Methods for starting: Stat-delta starter Autotransformer starter Reactor starter Soft Start

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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 Te VT2 Te reduced by 1/3 When reach steady state speed Operate with ‘delta’ ( ) connection Switch controlled manually or automatically

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**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

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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

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**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

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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)

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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

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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.

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