Amps per leg on three phase??

antique lathe · Feb 24, 2017

Decided to check the amps on a three hp motor. I got 9-.6-9 It sounds good. What is causing the
.6 reading on one leg? I should of checked the voltage. I will do that tomorrow.

Jraef · Feb 24, 2017

First rule out measurement error. A 3 phase motor will not start and run with virtually no current on one leg. It may CONTINUE to run if it was ALREADY running, but it would not re-start.

It's far more likely here that you didn't let your clamp-on ammeter close all the way, that's a common error most people have made once or twice.

JST · Feb 24, 2017

Whenever you see a result from some test that makes no sense, it is wise to recheck it by repeating the measurement, but varying the procedure. In this case, put the meter in a different place on the same wire, for instance.

It's pretty easy to have the clamp close but be slightly jammed open because another wire got trapped where the clamp closes, holding it open a bit. that's one way you can get a way-off reading and not see why.

Putting it on in a different place reduces the chance of repeating the same error.

jim rozen · Feb 24, 2017

"Whenever you see a result from some test that makes no sense, ..."

One way to make sense of the 9,9,1 amp readings is that your motor is being run off a converter, and not under
any mechanical load.

The 9 and 9 readings are the utility provided leads, and that is mostly reactive current. For some reason motors
running off a converter apparently do not show the reactive current on the manufactured phase.

If my surmise is correct, and you put a mechanical load on your motor, the 9 numbers will stay the same, and
the one amp leg will rise in current. If the motor is fully loaded (actually tough to do) then all the numbers will match.

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Amps per leg on three phase??

antique lathe

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JST

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jim rozen

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