In article , Ted Edwards says...

If your scope is dual trace, you can do what you want with a current
transformer. You need a 120V to 12V transformer rated for a dozen or so
amps. Connect a 10 Ohm, 10 Watt resistor across the 120V winding.
Connect the 12V winding in series with a motor lead. The "backwards"
transformer will have 11A flowing in the "12V" winding which will become
1.1A in the "120V" winding. 1.1A in a 10 ohm resistor is 11 volts so
the other winding (the "12V" winding) will drop only 1.1V. This will
have negligable effect on the motor operation. If you now pick up the
hot motor lead in one channel of the scope and the secondary of the
current transformer in the other channel, you can measure the time
difference between the waveforms and calculate the phase shift.

I don't understand how he is going to display voltage
with this setup, unless he ties one side of the
incoming line (240, yes?) to the scope common rail,
which is typically by definition power ground. That
will cause smoke to leak out someplace.

Jim

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