What is working principle of three phase induction motor ? | How 3 phase induction motor start ?
When we talk about induction motor first question arise that, What is working principle of induction motor ? How induction motor start ?
Lest find the answer in most easy way,
However, in a.c. motors, the rotor does not receive electric power by conduction but by induction in exactly the same way as the secondary of a 2-winding transformer receives its power from the primary.That is why such motors are known as induction motors.
Working principle of three phase induction motor
The reason of induction motor rotor to set into rotation is ,
When the 3-phase stator windings of induction motor are fed by a 3-phase supply then, RMF(rotating magnetic field) is produced, RMF is a magnetic flux of constant magnitude, but rotating at synchronous speed, is set up,
This flux passes through the air-gap, sweeps past the rotor surface and so cuts by the rotor conductors which, as yet, are stationary. Due to the relative speed between the rotating flux and the stationary conductors, an e.m.f. is induced in conductor of rotor winding according Faraday's laws of electromagnetic induction.
The frequency of the induced emf. is the same as the supply frequency.
Magnitude of e.m.f. is induced in conductor of rotor winding is proportional to the relative velocity between the flux and the conductors and its direction is given by Fleming's Right-hand rule.
Now the rotor bars or conductors form a closed circuit. rotor current is produced whose direction as given by Lenz's law, is such as to oppose the very cause producing it in this case, the cause which produces the rotor current is the relative velocity between the rotating flux of the state and the stationary rotor conductors.
Hence, to reduce the relative speed, the rotor starts running in the direction as that of the flux and tries to catch up with the rotating flux.
The setting up of the torque for rotating the rotor is explained below:
In Fig (a) is shown the stator field which is assumed to be rotating clockwise. The relative motion of the rotor with respect to the stator is anticlockwise.
Applying Right-hand rule, the direction of the induced emf in the rotor is found to be outwards. Hence, the direction of the flux due to rotor current alone, is as shown in Fig.(b).
Now, by applying the Left-hand rule, or by the effect of combined field (c) it is clear that the rotor conductors experience a force tending to rotate them in clockwise direction. Hence, the rotor is set into rotation in the same direction as that of the stator flux (or field).
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