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

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