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Pf Improvement Capacitor


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Hi to All !


I have a 932kW, 960 RPM, 415V, 50Hz Slip Ring (Wound Rotor) Motor. The application is Steel Rolling Mill duty. I propose to connect a 400KVAr, 415V fixed capacitor bank to the stator terminals to improve the power factor. The capacitor bank will have it's own set of HRC fuses.

Does anyone forsee any problem with this arrangement ?




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Hello sunilji


Welcome to the forum.


This is a common method of power factor correction and is called static correction.


The major issue here is that because the capacitor is directly connected to the stator terminals, it is extremely important that you do not over correct. You must not correct to mor that 80% of the magnetising current of the motor.

If you over correct (or correct greater than 80% of the magnetising current of the motor), you will create a resonant circuit between the motor and the capacitor that is below the line frequency.

When you disconnect the motor form the supply, is behaves as a generator and the frequency of the voltage generated is dependant on the speed of the rotor. If this passes through resonance, very high voltages will result and this will cause damage to the motor windings and the capacitors. There can also be mechanical damage due to high currents when the insulation breaks down.


Typically for a motor of this size, the magnetising current will be in the order of 20 - 25% of it's rated full load current. I would expect that you probably need correction in the order of 150 - 200KVAR. The figure that you quote is much higher and this may be due to a high magnetising current in which case it is OK. Please check the calculations for the value!!


If you are using the same contactor to control the stator and the capacitors, it will be subjected to a higher transient current on closing. This could shorten the contactor life.

It is best to use a separate contactor to switch the capacitors if possible.


Best regards,

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