Why the starting current of induction motor is very high compare to running condition.

The starting current of induction motor is objectionably high- this concept we know and utilize this in applications. But why this inrush of current happens in only starting of Induction motor, why not in running condition. Learning this would be a good thing.

At starting- no restrain to stator circuit by rotor circuit

We know the operating principle of induction motor. AC current is supplied to the stator and by induction process, ac current is induced in the rotor, which becomes an electromagnet. Then the rotor rotates by means of magnetic attraction between rotor and stator. In fact the rotating magnetic flux in stator pulls or draws the rotor to rotate continuously, just like a horse pulling its cart, here horse is rotating magnetic flux of stator and cart is the rotor itself… So there is is no direct conduction to the rotor.

Induction motor resembles as a poly phase transformer – the reason behind high starting current

Thus Induction motor is similar to a transformer where the primary is the stator and secondary is the rotor.Now we know the concept of transformer that the primary current and secondary current is inter-related. The secondary current restrains the primary to exceed in value. So in starting condition there is not enough current in secondary, so not enough restraining on the primary current. Thus the primary current severely increases. This is why it is famously said “The secondary of current transformer should not be open while running condition”.

[alvaa]So at starting condition, the resistance of stator is relatively low compare to the reactance . This results a high inrush current but in running condition the secondary circuit that is the rotor circuit provides some restrains that increasing the stator resistance – which keep the current low in running condition in induction motor.

torque current comparison of squirrel cage rotor induction motor
torque current comparison of squirrel cage rotor induction motor

high starting current of induction motor

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