Effects of Too Much Capacitance (Leading Power Factor) on a Motor 1

[vobprecis]

If the power factor correction capacitors are oversized for a 50 hp motor causing the power factor at the motor to be .85 or .9 leading, what would be the effect on the motor itself if any?

 

[Marke]

Hello vobprecis

There will be no effect on the motor provided the capacitors are never connected to the motor during the run down period.

If the capacitors remain connected to the motor after the supply is removed, (main contactor is opened) there will be major problems.

When a motor is disconnected from the supply (switched OFF), it will act as a generator for a period of time. The frequency of the voltage generated is speed dependent, so it reduces as the motor slows.
If a motor is under corrected (as it should be) there is a resonant frequency between the inductance of the motor and the capacitors which is above line frequency. A slowing motor will not pass through resonance. If the motor is critically corrected, (corrected to 1.0) then the moment that the contactor is opened, the motor is at resonance with the capacitors and it will generate a very high voltage potentially causing capacitor failure and insulation breakdown. There can be major mechanical damage as well due to the torque transients associated with the insulation breakdown.
If the motor is over corrected, the motor will pass through resonance as it slows and can cause major damage.
So the problem is not while the motor is running, rather it is when the motor is shut down.

In order to prevent such problems, you should only correct to 80% of the magnetising current of the motor, or use a separate contactor to control the capacitors such that it must disconnect the capacitors when the motor is disconnected from the supply.

When the motor is running, over correction may increase the voltage on the motor terminals but should not cause issues other than supply resonance issues, but these are not related to over correction.
see

http://www.lmphotonics.com/pwrfact.htm

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[vobprecis]

Mark, Appreciate the response. Makes sense and was helpful.

Vince