power factor thru a cct 1

[don01]

hi all,
I've been stretching my poor imagination on this one. If you have an inductive device (say a motor) then at the meter
you have a pf of say .8 for the exercise. Now being progressive engineers we realise this is a waste and put in some capacitors next to the meter and get a perfect pf of 1 (we wish hey).

Thinking about the circuit the motor had(?) a pf of .8 lag. the capacitors match it( .2 lead?) and the world is sweet.

Looking at the motor would the pf be .8 or 1 (likewise the cap) or do we have something like a kirchoffs formula for pf?

Thanks up front
Don

 

[electricpete]

Power factor is computed from angle between voltage and current. Let's say you have a meter that measures power factor by hooking up current and voltage test leads.

The motor and the caps are connected in parallel. Assume that interconnecting wiring has negligible impedance so they both see the same voltage. Your voltage leads will always be connected at that point.

You have a couple of choices of locations for your current leads:

If you measure at a point that includes all the current (motor plus capacitor) in your example, your power factor will be 1.0.

If you measure at a point that includes only the motor current, your power factor will be 0.8.

If you measure at a point that includes only the capacitor current, then your power factor should be 0, leading (not 0.2).

Not sure if this helps. If not please restate the question.

 

[don01]

thanks there for that electricpete,
What you have said was in keeping with my own thoughts but I needed to reassure myself. I had read some where something along the lines that if you correct your pf then you get more "work" out of the motor (say).

I have the feeling the reference was really a per $ thing not product moving etc. But it really did seem to imply pf of 1 at the motor - hence the question.
again thanks
Don