voltage vs amps. 1

[delhi81]

have you seen a point where voltage and amps are NOT inversely proportional ? i have a .75 hp 120/240 volt single phase split capacitor motor running at 16.7 no load amps @ 128 volts. When we run it at 120 volt, it runs back within its' rated amperage.

 

[ScottyUK]

Your motor has an iron core. The iron magnetically saturates if you apply a voltage higher than it is designed to accept. The magnetising current increases in a highly non-linear manner when the iron reaches saturation.


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[electricpete]

Maybe I am missing something.

Your currents sound too high for no-load.

Yes, overvoltage can increase the current. But
128 volts does not seem like much of an overvoltage to cause that high current if 120 volts is the nameplate voltage.

Are you sure the motor is at no-load? Do you have the right cap on there.

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[delhi81]

I have never seen that only worked with it in theroy. typicaly, most any electrical components can take up to 10%
variation. this is close.

 

[waross]

I concur with electricpete. You should be able to go 10% overvoltage on a NEMA motor with no problems whatsoever. That would be 132 volts. I'd try changing the capacitor. I'd also check the calibration of the voltmeter. Are you on a generator? Low frequency would explain your problem. If you're down to 55Hz on a 60Hz motor, Then you have used up your 10% overvoltage margin with de-rating and the saturation that ScottyUK describes will start on almost any overvoltage.
I usually see similar single phase voltages on shrimp and lobster boats with nominal 120/208 volt systems. The panel meters are line to line, and the crews can't accept that 208 volts is normal and correct and jack the line to line voltage up to 220V or 230V.
As well as checking the capacitor, I'd consider reconnecting the motor and running it on 230V. (Or 208V)
yours