Induction Motor Working Principle

Induction motors operate on the principle of current induction in the rotor which must rotate at a speed less than synchronous speed for induction to occur. This is referred to as slip speed and should not be confused with pole slipping.

Why induction motors have no pole slipping and why other motors have pole slipping? I will attempt to answer this for you but first let's define poles and synchronous speed in regards to electric motor operation.

Poles in an electric motor refer to the magnetic circuit poles and come in sets of two just like a common magnet. One is north (N) and one is south (S). If a motor has two poles, it will have one N-pole and one S-pole. If the motor has four poles, it will have 2 N-poles and 2 S-poles and so on.
Induction motor
Synchronous speed in an electric motor is the speed that is produced by the traveling magnetic filed wave as it rotates around the stator magnetic circuit. The synchronous speed of a motor is equal to 120*f/p where f = the system frequency in Hz and p is equal to the number of poles in the induction motor.

All AC motors have two basic parts: (1) a stator winding and iron core and (2) a rotor winding and iron core that is free to rotate and is connected to the motor shaft. For a 3-phase induction motor, when the stator is energized by a three phase voltage source, a magnetic field will be produced that rotates at synchronous speed. As this magnetic flux cuts across the rotor winding it induces a current (via Faraday's law of induction) in the rotor winding which intern produces a second magnetic flux. These two magnetic fluxes both rotate at synchronous speed and couple together like two magnets and thus transfer torque directly to the rotor shaft.

Now here is the key thought you must have to understand the induction motor working principle and why there is no pole slipping. In order for the stator filed to induce a current in the rotor circuit, the rotor must turn slower than synchronous speed such that Faraday's law of induction holds and the flux cuts across the rotor windings. If the rotor turned at synchronous speed, the stator flux would not cut across the rotor windings and no current would be induced in the rotor and the second magnetic field would be zero. Therefore, the rotor must turn at a speed slower than synchronous speed for there to be induction in the rotor circuit. This is referred to as slip speed and should not be confused with pole slipping.

The other type of (AC) motor is a synchronous motor where there is a separately excited field winding on the rotor. For a synchronous motor to work, the rotor shaft turns at synchronous speed and the magnetic field produced by the field winding couples with stator field and torque is transmitted to the shaft. In the case when the shaft torque exceeds the magnetic field force acting on the rotor, then a pole slip can occur which results in very high magnitudes of stator current and shaft torque. This condition can cause severe motor damage and trip circuit breakers.

Study it. Understand it. Ask and answer your own questions by getting a good book on motors and studying the energy conversion principles.

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