Testing an AC 3ph motor
1 leg of a motor pulls 10% less current than the other two legs? U20A, V20A, W16AI measured the winding resistance with a milliohmeter and did not find any appreciable dif. The Voltage on each leg is within .5 volts. If the Voltage is even and the amperage is different the prob should be with the motor? By ohms law current draw=Volt/Res. True resistance of an AC motor is reactance? How do I measure this?? Please jump in on this think tank! Thanks for any help. "Al the Man" had mentioned the wave form and I am going to scope it Friday.
This is a new thread because I am curious on how other techs perform motor checks
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Is this the motor that is on a VFD? As I believe it may be that particular phase output that may be the problem, if so swap two of the phases temporarily to see if the lower current stays on the same phase.
Or if on a VFD supply, hook it up to 3 phase direct, if possible, and recheck
If it is on a fixed 3 phase supply, then the ammeter should be a simple indication of the health of the motor.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
What is your opinion on the I inrush if i connect The 75HP directly to the Line? Any precautions? The machine is a Screw compressor. I don't want the motor to go backwards but I could swap U & V then V & W And I would still Be going the right direction ?
It may be a problem without reduced voltage start, what about swapping the vfd motor phases and see if it stay on the same phase?
If the opposite direction is a problem, you would have to change all three phases instead of the usual two.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
Gotcha, This will isolate either the drive or the motor. Also I am going to use your suggestion of scoping the phases. I have to re-program the unit to run at a constant Hz. Thanks for your help Al and I will post my results 3/14/8.
These VFD's are intriging! They can amplify EMI and create ringing and I have found that with out proper shielding they can effect(false trigger) old TTL CPU,PLC systems, What are your thoughts on this comment? EX: I was working on a Heald 2EF Grinder with the Acramatic 700G and when I upped the carrier frequency on 150HP VFD the The Acramatic would malfunction with a multitude of errors. I disconnected the motor shield and the problems were amplified. I did not have time or the equipment to experiment further but it was interesting. We moved the machine away from the VFD and no more wierd shut-downs
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