Hello everyone, today I had a phase UW (T1&T3) short fault on an ab powerflex 4m drive that powers a 1hp 3ph 460v induction motor. When I reset the drive the fault code comes right back up after receiving a run signal. I took the leads off the drive and tested resistance between the 3 leads that were still connected to the motor each lead read 13.8 ohms amongst each other (T1 to T2, T1 to T3, T2 to T3). Then I megged each lead to ground at 1000v each reading was infinite, 1Mohm on my tester. Assuming the motor was ok, I went ahead and replaced the drive, which we have numerous spares. Same problem. Swapped T1 and T2 and tried to run and the drive faulted out with a VW (T2&T3) short. Disconnected motor from gearbox and spun both shafts by hand and everything spun freely including the load. Then tried to run motor while disconnected, same problem. I unwired the motor and temporarily wired in a spare and it ran fine without fault. After assisting the machinist with the install of the new motor I looked up specs on the motor and resistance from line to line should be 3.5 ohms, but I was getting 13.8 ohms from line to line, however T1 to T4&T7, T2 to T5&T8, etc all read around 3.5ohms. As I have only been an industrial electrician for a little over 2 years this is leaving me puzzled. Even our senior electrician was lost. Any tips, suggestions would be greatly appreciated. The motor is still on my bench and I’d like to try to troubleshoot it more. Thanks in advance
Phase UW short on powerflex drive
- Thread starter Rob442
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ptonsparky
Tom
- Occupation
- EC - retired
Isolate each winding and Meg to the others. It may pass to ground but fail winding to winding. Check each winding resistance again.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
if I understand what you are telling us correctly, you may have a short in the wires going from the drive to the motor. did you isolate them and test them phase to phase?
I tested for resistance with a standard rms multimeter from T1 to T2, T2 to T3, T1 to T3 with windings 4&7, 5&8, 6&9 spliced together and got 13.5 ohms on each. I was told not to meg from winding to winding as it could damage them. Thoughts?
ptonsparky
Tom
- Occupation
- EC - retired
Meg winding to winding. You won’t hurt them. Low readings will not be your fault, they were existing.
ptonsparky
Tom
- Occupation
- EC - retired
13.8 ohms seems high to me, but I haven’t checked one lately.
Yes I did forgot to put that in, wires from the drive to the motor are not shorted
I agree but they were all consistent
ptonsparky
Tom
- Occupation
- EC - retired
Compare with another new motor.
Will do ?
ptonsparky
Tom
- Occupation
- EC - retired
Comparing a 1.5 hp 3600 rpm, 3 ph we have wired for 240 at the shop, your 13.5 ohms is close enough for government work. Comparing tangerines to oranges, but close.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
You megged L-G, the short is likely L-L, but not to G, hence the megger reading OK.
Will meg line to line tomorrow and post results
Thanks
Confused??
Confused??
Megged line to line 1 to 2 1 to 3 2 to 3 with all windings disconnected and everything is reading infinite. Y connected motor ohms between 1 to 4 2 to 5 3 to 6 reading 3.5 ohms which is within specs. 7 to 8 7 to 9 8 to 9 all reading about 6.7ohms. Infinite resistance between 1 to 2,3,5,7,8,9 between 2 to 3,4,6,7,8,9 etc. I’m lost
Confused??
Megged line to line 1 to 2 1 to 3 2 to 3 with all windings disconnected and everything is reading infinite. Y connected motor ohms between 1 to 4 2 to 5 3 to 6 reading 3.5 ohms which is within specs. 7 to 8 7 to 9 8 to 9 all reading about 6.7ohms. Infinite resistance between 1 to 2,3,5,7,8,9 between 2 to 3,4,6,7,8,9 etc. I’m lost
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