Operation of Induction Motor

Operation of Induction Motor

In induction motor, the AC power supplied to the stator creates a magnetic field that is changing according to the rotor and induces an opposing current in the rotor.The rotating magnetic flux induces currents in the windings of the rotor which in turn creates magnetic fields in the rotor that react against the stator field. The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of induced rotor current and torque balances the applied load.OPERATION OF INDUCTION MOTORS MICROSOLUTION.COM.PK

Synchronous speed

When three phase supply is connected across stator windings then a rotating flux isproduced in stator which rotates clockwise at synchronous speed giving the equation

Ns =

Where f is the motor supply’s frequency in Hertz and P is the number of magnetic poles.

When the rotor of three phase motor is placed in air gap of stator, anemf is induced there which causes a current to flow and produces a flux.A torque is produced by the reaction of this flux and the stator flux and rotor rotates in samedirection of rotating flux with a speed less than synchronous speed.

Standard torque

Speed torque curves for induction motor types are as follows:Microsolution.com.PK

Over a motor’s normal load range, the torque’s slope is approximately linear or proportional toslip because the value of rotor resistance divided by slip, , dominates torque in linearmanner. As load increases above rated load, stator and rotor leakage reactance factorsgradually become more significant in relation to  such that torque gradually curvestowards breakdown torque and motor stalls. Althoughpolyphase motors are inherently self-starting, their starting and pull-up torque design limits mustbe high enough to overcome actual load conditions. In two pole singlephase motors, the torque goes to zero at 100% slip.

Starting Torque in Slip Ring Motor Induction Motor

Three resistors are used to start a slip-ring induction motor as shown in following figure:STARTING TORQE IN SLIP RING INDUCTION MOTORS

At start point, three variable resistors present. On starting, all resistances are in the circuit and give highstarting torque. When starting resistance is all cut out andthe switch contacts remain closed, then this is comparable with the resistance of therotor itself.

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Efficiency of the Induction Motor

Efficiency of the induction motor can be determined by following method:



Output power = Input power – losses

Output power = Input power – (stator loss + rotor loss + mechanical loss)

Rotor I2R loss = per unit slip x input power to rotor = s (input power to rotor)


for more tutorial

What is Induction Motor?


for more tutorial

What is Induction Motor?


Operation of Induction Motorclick


 of Induction Motorclick


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