# Power Factor and Resonance

Discussion in 'Physics' started by amilton542, Oct 22, 2014.

1. ### amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
I'm always told you should strive for unity power factor, but unity p.f. implies the circuit is at resonance. I don't know how this can be a good thing in power engineering. There is a tendency for high circulating currents to be established, that of which will increase the active power demand because the copper losses equate to I_r.m.s.^2 x R.

Another thing, even if the circuit is at resonance I'm told it only consumes real power. Well this can't be entirely true because the inductors and cap's still need to be charged to some degree in order to feed off one another, so there is reactive power in the circuit even at resonance.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The two reactances cancel, leaving only resistance. It is the core definition of resonance.

3. ### amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
Damn! Positive reactive power, negative reactive power. Watch out for your plus and minuses folks, they will ALWAYS catch you out.

4. ### amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
@ Bill Marsden

I've gone away thinking about what you've said, but for the two reactive powers to cancel implies the circuit to be in a sinusoidal steady state condition.

So is it safe to say, resonance and unity power factor only exists in steady state?

5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The initial condition is the circuit is at resonance. So again, it would be assumed the signal is a steady state sin wave where the reactance of both components are equal.

Want to try a little math project I found interesting? Calculate the voltage across the capacitor or inductor in a series LC circuit at resonance. It is counter intuitive.

6. ### crutschow Expert

Mar 14, 2008
13,475
3,362
Yes correcting the P.F. to unity generates a circulating reactive current in the power line, but that reactive current is already there from the reactive load. Correcting it doesn't increase this reactive current. Ideally the correction is done close to the reactive load to minimize the distance that the reactive current must flow, so that it doesn't have to go all the way back to the generator.