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Restart Of Induction Motor


bonny

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If an induction motor is restarted before its rotor stop rotating, then what will be the amount of current drawn?

Is this current will be the same as motor starting current?

 

bonny, it is a favourable situation.

If the motor is restarted when the speed is high, slip will be low and restarting current will be a little higher than nominal current.

 

If the motor is restarted when the speed is medium, slip will be medium and restarting current will be 2 to 4 times the nominal current.

 

If the motor is restarted when the speed is low, slip will be high and restarting current will be a little bit less than normal locked-rotor current.

 

Regards

Mario

 

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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In addition to Mario's reply, there is one other consideration.

When the motor is running, there is a rotating magnetic field in the stator which is created by the applied voltage. This induces a rotor current that is dependent on the slip.

When the stator voltage is removed, there is still rotor current which will decay at a rate determined by the rotor time constant.

If the stator voltage is reapplied while there is residual rotor current, there will be a transient current and torque which is dependent on the magnitude of the rotor current and the phase difference between the rotor current and the stator voltage at the instant of reconnection.

 

Typically, this occurs within 0.5 seconds of switch off, so if you have a minimum OFF time of 1 second or more, the "auto reclose" effect will not occur.

 

Best regards,

Mark.

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