Leading PF and Ind Motor

[ccdubs]

Hi,

What would happen if you have a leading power factor feeding an induction motor?

 

[infomaniac]

ccdubs, it depends.

First of all, if the motor is running and you have a parallel capacitor connected in the circuit to compensate just for the motors power factor, there really is no direct affect on the motor if the power factor is over sized. The motor impedance has not changed and therefore draws the same current before or after the capacitor is added. The power factor IS improved from the capacitor connecttion on up through the feeding transformer, but the power factor between the motor and the capacitor is the same as it was before adding the capacitor. The main concern with an oversized capacitor then during motor operation is the amount of voltage rise the capacitor will create on the bus due to the leading power factor. Believe me, I have a lot of experience with capacitors, you would have to have one very large oversized capacitor in most cases before an appreciable voltage rise would occur while the motor is running. This is because most of the capacitive affect is cancelled out by the motors inductive charactersitics. The 'extra' vars is what causes additional voltage rise. The voltage rise on a totally unloaded transformer with a pure capacitive load is calculated as follows:

Vrise = (KVAR*%Ztransformer)/KVAtransformer

Second, you can have a serious problem however, if the capacitor is isolated with the motor when the motor is turned off (i.e., the capacitor is connected to the load side of the motor starter contactor). In this case, if you have a oversized capacitor, the extra vars can kae the motor begin to behave similar to a generator and cause excessive voltage to occur in the closed system. This can cause damage to both the motor and the capacitor.

This is a bit confusing but read it carefully and I think you will understand what I am saying.