In article , jim rozen wrote:
In article , Richard J Kinch
says...

But the compressor runs off of 240 volts. Don't you
need to put the scope across both hot leads?

No, you really just want to know the phase angle between the current versus
voltage. So any picture of the utility AC voltage will do (such as the
internal scope test signal!). You don't need to look directly at the
voltage supplied to the motor.

Not *just* any picture. The signal has to have exactly
the same phase as the incoming 240 volt line - because
when one is doing this, the power is mostly reactive,
so one is measuring a pretty small difference between
two large values. It would be worth checking to see
that the line phase reference really is good. Jim

okay, just how small? a few degrees? that would seem to obviate
the need for this experiment. Richard seems to be saying the internal
test signal is sync'd on the line frequency, more than a few microseconds
variation would be unexpected.

assuming this is a split phase supply (120/240), theoretically 180deg
phase shift would = 0 watts, right? i am expecting min of 30-60 degrees
on the scope which s/b plenty. and much more than that if indeed
the "compessed output" votes prove true.

of course i would check "calibration" to the internal signal before
measuring, just to be sure, but one less scope lead to my xfmr/resistor
kludge would be helpful. --Loren