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Power meter behaviour with rectifying load

Diver300

Well-Known Member
Most Helpful Member
I was checking the characteristics of a hairdryer with one of these:-
https://uk.rs-online.com/web/p/energy-meters/1785370 although mine isn't branded the same and is a few years old.

The hairdryer has a big diode in series with the motor and the heater when on the slow fan setting. There is a big DC component of the current.

The power factor shown by the meter was around 88%. I've calculated that the power factor of a resistor with a diode in series should be 71%, or 1/sqrt(2)

Any idea why there is such a discrepancy?
 

Diver300

Well-Known Member
Most Helpful Member
Well the motor only took about 100 W, and as soon as any heater was on the power was at least 500 W, so the effect of the motor would be small.

On the fast fan setting, the diode is shorted out. Then the power factor was very close to 1. On the highest power setting nearly all the current is going into the heater. When that current is only turned on half the time, I would expect the power factor to be worse.
 

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