measuring rotor current

[AYahya]

how can I measure rotor current in running slip ring AC motor ? I know that usual clampmeters can not measure low frequency current ?

 

[ScottyUK]

A shunt, an isolating amplifier and a 'scope would do nicely. As the current is a sinusoid you cane easily convert from peak voltage to RMS.


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If we learn from our mistakes I'm getting a great education!

 

[AYahya]

thanks

if the scope is not available, what can be done ?
and if i used the clamp anyway, what would be the error in measuring?

 

[ScottyUK]

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!

 

[AYahya]

fr=0.4 to 0.5 Hz depends on load
rotor current = 600 A
rotor voltage = 1820 V

 

[roydm]

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.

 

[ScottyUK]

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!

 

[LionelHutz]

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.

 

[ScottyUK]

Which takes us back to the problem of finding a meter which will be accurate at 0.5Hz...


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If we learn from our mistakes I'm getting a great education!

 

[roydm]

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

 

[LionelHutz]

Yes, a good scope would be a good idea.

 

[ScottyUK]

Oh well, at least we seem to be agreeing on the methods all through this thread.


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If we learn from our mistakes I'm getting a great education!