troubleshooting drive and motor

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jcole

Senior Member
Hell guys and gals.

I had a fault on a powerflex 400 drive called "hardware overcurrent" today. After disconnecting and reconnecting power a fault labeled " UV shorted" came up. "U" and "V" are output terminals for the motor from the drive. The motor does not move at all and the fault happens so fast I cant get a current draw.

I disconnected conductors from drive to motor. I ohmed with VMM phases to phases and phases to ground. Nothing. Then I megged conductors at 1000v the same way and nothing.

I then megged motor to ground at 1000v. Nothing.

I then checked for mechanical overload. Nothing.

I then ohmed phase to phase the motor which has 3 leads. They all were shorted together. But isnt this common for ac induction motors because all you are doing is measuring wire resistance of each coil. Correct?

I connected a motor that is a lot smaller than the one in question and it ran fine.

So I assume the problem is at motor. How would you guys have went about trying to find the problem?

What could possibly be wrong that my measurements are not telling me?

Thanks for replies.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Hell guys and gals.

I had a fault on a powerflex 400 drive called "hardware overcurrent" today. After disconnecting and reconnecting power a fault labeled " UV shorted" came up. "U" and "V" are output terminals for the motor from the drive. The motor does not move at all and the fault happens so fast I cant get a current draw.

I disconnected conductors from drive to motor. I ohmed with VMM phases to phases and phases to ground. Nothing. Then I megged conductors at 1000v the same way and nothing.

I then megged motor to ground at 1000v. Nothing.

I then checked for mechanical overload. Nothing.

I then ohmed phase to phase the motor which has 3 leads. They all were shorted together. But isnt this common for ac induction motors because all you are doing is measuring wire resistance of each coil. Correct?

I connected a motor that is a lot smaller than the one in question and it ran fine.

So I assume the problem is at motor. How would you guys have went about trying to find the problem?

What could possibly be wrong that my measurements are not telling me?

Thanks for replies.

I had a customer call me on the weekend with the EXACT same problem, same PF400 as you and same fault code. His turned out to be a melted down cord end he had installed right before the motor.

SOP for me:

Disconnect T-leads from drive. Meg wiring/motor together. If low reading, isolate motor from T-leads and meg each seperately.

If everything megged great,:confused: I'd then hook just the T-leads up, capped off in the motors peckerhead and run the drive. If it didn't fault out, I'd hook the motor up. If it then faulted out, I'd look hard at the motor leads to see if any insulation was rubbed off just to be sure, etc etc. If it all looked good, I'd assume the windings were shorted together and tell the customer to replace the motor.

One other thing that really throws a wrench in it when you're megging is if there's moisture in the conduit hosing up your readings or if there's an intermittent short where the wires have to be wiggled just right.....
 

jcole

Senior Member
Thanks for the reply.

That exactly what I did. I even connected the tempory motor to the conductors from the drive at the peckerhead of the motor in question and it ran fine.

How would I know if the windings were shorted together? What type of test would the local motor repair shop do to find out what is wrong with it?
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
How big is the motor? Usually you can get a good idea by just connecting an ohm meter. Zero it first. 1-2, 1-3 & 3-2 resistance should all be the same. Separate the windings even further and test again if you get a funky reading. Worked on the last 250 HP anyway. After that you may need help with different testing equipment. The 250 HP is stretching the limits for typical VOM I do believe.
 

jcole

Senior Member
How big is the motor? Usually you can get a good idea by just connecting an ohm meter. Zero it first. 1-2, 1-3 & 3-2 resistance should all be the same. Separate the windings even further and test again if you get a funky reading. Worked on the last 250 HP anyway. After that you may need help with different testing equipment. The 250 HP is stretching the limits for typical VOM I do believe.

It is a 60hp 3 lead motor. I was getting .8 ohms across each winding.
 
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