Power Factor Correction Calculations

[maguire.david.c]

I'm an instructor myself and we teach to correct power factor with capacitive reactance by hooking the appropriate size capacitor in parallel with the inductive load. One of my students questioned as to why you would do this instead of in series stumped me and unfortunately the book I was teaching from lacked the reasoning behind the action as well. Can any one give me reason. I understand capacitors are additive in parallel but when youre dealing with a series parallel circuit that would be formed (say a capacitor that is in parallel connected to a (resistor + inductor in series)) how would you go about explaining that? Thanks.

 

[beanland]

Think Energy

Think Energy

I try to address this by thinking of energy. Iron cores consume reactive power. Capacitors generate it. Where you consume reactive power you provide a shunt capacitor to generate reactive power to feed it.

Placing capacitors in series is done on utility transmission lines to compensate for series reactance BUT they create complex voltage issues and problems during faults.

If the motor faults, the capacitor can limit fault current to a point that circuit breakers will not trip. Also, extreme voltage can be present.

Note that capacitor run motors use series capacitors to create magnetic field phase shift in single-phase motors.

 

[magtiger]

But Energy Storage

But Energy Storage

The inductor stores energy (reactive power) in the iron core using a magnetic field. The Capacitor stores energy in a dielectric medium using an electric field. In a capacitor instantaneous voltage change is disallowed. Current must first flow, thus current is leading voltage. In an inductor instantaneous current change is disallowed. so current must lag voltage. The cap releases the stored energy as current so it must be in parallel. Were it in series, the cap couldn't change the current, it's a passive device and the current in is identical to the current out. It can provide current into a 3 point node causing a change in the ratio of the other 2 current lines so long as the 3 points sum to 0. The cap draws leading current from the source at the same time the load is drawing lagging current. The 2 meet in the node a sum to the source current. which if the cap is sized properly will have a unit PF.

 

[Electric-Light]

Capacitors create leading reactive power.
Inductors create lagging reactive power.

In other words, they shift in opposite directions. Capacitors act as counterweights. If you apply too much kVAr, you overcompensate it.

Sandbags in the truck bed helps traction, but you don't want to shift the weight to the back to the point your front wheels are up in the air.