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Old 09-18-2015, 05:22 PM   #2051 (permalink)
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Originally Posted by thingstodo View Post
We use a megger on motors to see if there are ground faults, resistance line to line to see if the phases are balanced - if the phases are shorted part-way through the resistance won't match ... that's about it. Once or twice we've checked insulation with a hi-pot. Sorry for the jargon - I can look up the *REAL* names of the instruments if you like
No worries on that - I actually have a hi-pot tester - it seemed like a good idea to get something like that when I'm messing w/ 650vdc.

To measure small resistance on large motors we use a ducter. It is an ohm-meter that uses higher current to measure small resistance. 1 amps or 10 amps. The probes are sharp and there are two for + and two for -, so you get a reading on 'corrosion' as well.

The vendor that we use to rewind our motors uses this RLC type of equipment as well as partial-discharge .. a method of measuring insulation breakdown.
Hmm, so it seems the RLC type of equipment ( which basically injects a frequency to measure capacitance, inductance, and resistance) would be very appropriate for this motor control stuff we're doing. After all, we are forcing the stuff to work at various frequencies, not DC.

I did find a cheaper version of the BK precision device - the 875B. It does appear to work in the ranges we're interested in, however the frequency it injects is 1kHz. The more expensive versions allow higher and lower frequency measurements and connection to a PC through a USB port.

As I mentioned - our industrial controllers do their own tuning. It's a 5 minute process when we change a motor to tune to the new motor and away it goes. It just means I have to drive to site some evenings and weekends when motors fail in production ...

Thanks for getting back on that!


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