# Physics Question--How to test a pm dc motor for its voltage and current

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james joule
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Physics Question--How to test a pm dc motor for its voltage and current

Hi,
I've just joined and this is a great site, I wondered if anyone could help me, if I had a permanent magnet dc motor and didn't know the voltage or current it should be run on, how would I test for this so that I don't damage the motor, and also how do I test for the back emf, many thanks James.

amesmich
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Re: Physics Question--How to test a pm dc motor for its ...

Run as a generator. Hook it up to another motor to drive it and measure the voltage. This is a rough idea if you know nothing about the motor (permanent magnet dc motor). Maybe put a small small load across it when you measure the voltage. This will give you the voltage but not the capacity or current max.

marylandbob
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Re: Physics Question--How to test a pm dc motor for its ...

If you have a suitable adjustable power supply and an oscilloscope, you could slowly increase voltage to the motor, which should be wired so that the oscilloscope continually monitors the voltage at the motor terminals, while you periodically interrupt(pulse it off for perhaps 1/10th second) the supply voltage, and note the back emf voltage, as the motor continues to spin. as you progress, a back emf will be reached that fails to increase in proportion to the supply increase, and at this point, absolute maximum voltage will have been reached. Further increase will result in excessive heating, due to saturation of the magnetic structure. For good RELIABILITY, the cmotor should be operated at a voltage lower than bthis "absolute" voltage, perhaps only 50 to 75% of the measured value. Series wound D.C. motors should NOT be tested without a mechanical load, as the RPMs may reach destructive levels!-Bob

Robert M. Curry

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