smkettner wrote:
Salvo wrote:
The microwave and converter have a power factor around 0.7. Those currents are 40% greater than you think. Putting a scope on the ac line would be interesting. Have you measured voltage with a true rms meter?
I thought PF increased the apparent power needed.... not the actual power used.
true, it increase the current draw and that's what trips breakers, not the power. A breaker will generally trip near the rated rms current regarless of the load power factor. If you have a load that is primarily drawing current at the peak of the waveform, it gets complicated since it will then depend somewhat on the thermal time constant of the breaker to see if it trips on peak current loads with a short thermal time constant or RMS current with a longer time constant. I suspect most thermal breakers will act somewhere in between on non sinusoidal loads. Breaker must trip within a given time for a specific overload, so the time constant is relatively short. fractions of a second on big overloads.
The issue is partly the load. is it purely a reactive component where current leads or lags the voltage but is sinusoidal or is it reactive and non sinusoidal like the load from a switching power supplies which draws current for part of the cycle near the peak voltage and looks reactive as well.
Power companies hate this since it causes the voltage to become slightly non sinusoidal with harmonics. hence the ratio of peak to rms (crest factor) goes wacky and they must maintain a crest factor tolerance as well.
However, the label on a device should list the max current draw and that takes into account the power factor. So if an AC lists 12A max current, that is the max continous current draw, same for a microwave.
And yes, microwaves and converters have terrible power factors, 0.7 is pretty typical. that raises havoc with many portable generators that the "power" rating is really a VA rating not power. AC units that I have measured have a pf of around 0.9 not bad.
and there are a small handfull of converters that are power factor corrected.
