Quote:
Originally posted by bigben2k
I thought those were startup caps, to prevent a large power draw, when you turn it on?
Wouldn't a meter compensate for the inductive load? I mean, if it measures current, and the current is cyclic (i.e. AC), then shouldn't it still give off a good reading?
From what I remember in electronics courses, P=VI, but in AC circuits, you have to use the RMS values: 127 Vac, is actually rms, aka averaged, as the peak is somewhere around 170 volts. RMS is the actual average value over the sine wave in the AC signal.
I don't see the numbers being off by too much: if the pump was almost completely obstructed, you should get the max power rating of 45 Watts, no?
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For pure sinusoidal AC signals:
P = V * I * cos(phase angle)
In a resistive (or power factor corrected) load, the current is in phase with the voltage. (phase angle = 0)
In a purely inductive load, the current "lags" the voltage by 90 degrees. (phase angle = -90 degrees)
In a real world load like a pump, there is both a resistive and an inductive component to the load, so the current will "lag" the voltage by some angle between 0 and 90 degrees. The yellow line in the picture below shows current lagging the voltage by 60 degrees.
The power consumed in the out of phase case shown, is half as much as the in phase case, even though the amplitude of the current is the same in both cases.