What frequency you are trying to measure? It's obviously slip - if you are looking at about 0.5Hz or so then you might get away with a fast bipolar DC ammeter. If the meter has a peak hold then that would be useful.
How big is this machine? Post some details!
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If we learn from our mistakes I'm getting a great education!
You can use a field effect type ammeter, they will work with AC/DC or one of the old style clipon ammeters, the ones that worked on magnetic field strength alone (no transformer windings), one of the old shops in your area might have one.
Yeah, the really old style tong tester would probably work.
Considering the amount of power involved the less interfacing you do with it the better. I still think the isolation amp and 'scope is your best bet in terms of accuracy and safety.
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If we learn from our mistakes I'm getting a great education!
If you have some resistance left in the circuit you can measure the voltage across that resistor. You can connect the meter once running (using the appropriate safety gear) because the rotor voltage should be less than 20V. Don't leave the meter connecting when starting though unless it's a 2000V capable probe.
Scotty,
"Which takes us back to the problem of finding a meter which will be accurate at 0.5Hz"
Obviously not a digital, but an old analog meter either center zero moving coil swinging back and forth or a moving iron clampon should be able to indicate that. Perhaps you just take the peak and back calculate the RMS.
I wonder if it's really a true sinosoidal frequency though, there might be lots of harmonics and other effects caused by the winding slots.
Interesting thread.
Roy
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