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#201083 - 05/09/11 11:24 AM meg out motors  
allphase  Offline
Joined: Sep 2007
Posts: 14
Chicago il.
how do you meg out a motor. 75 hp triping out overloads and about every 6th or 7th time takes out 125 a. fuse.
it is all ways b phase fuse

motor has been runing for 4 or 5 years so overloads and fuse are rated fine.
motor turns hydrolic pump for demount mach.
480 v tap 9 lead
went ground to L1 L2 L3 open
any configuration of L1 L2 L3 to each other dead short

any ideas could use some help


Last edited by allphase; 05/09/11 11:25 AM.

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#201084 - 05/09/11 03:04 PM Re: meg out motors [Re: allphase]  
twh  Offline
Joined: Mar 2004
Posts: 916
Regina, Sask.
I think you metered correctly.

What is the running current or will it start? It sounds like an overload. Maybe it's a problem with the pump or the hydraulics. Remove the load and try again.

#201085 - 05/09/11 03:54 PM Re: meg out motors [Re: allphase]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,103
Also check the voltage right at the motor. If you have a low phase it can cause an overload indication. May be a bad contact in the starter.

Greg Fretwell

#201087 - 05/09/11 07:08 PM Re: meg out motors [Re: allphase]  
allphase  Offline
Joined: Sep 2007
Posts: 14
Chicago il.
What is the running current or will it start

current is at 38 a when not under load will go up to 80.9 a when under heavy load it will run fine for 4 to 6 hours then trip over loads
Greg the voltage is good at the motor i have not been there when it has triped so i dont know if the voltage drops when it trips but all the other machines are runing fine in the plant

#201090 - 05/09/11 09:00 PM Re: meg out motors [Re: allphase]  
twh  Offline
Joined: Mar 2004
Posts: 916
Regina, Sask.
Overloads can fail over time, but it doesn't explain the fuse. Motors don't always draw the same current on all phases, so B might be the high phase, or the B phase is failing to ground somewhere.

To test the starter, check the voltage from Line 1 to Terminal 1 and Line 2 to Terminal 2, etc. If you have a problem in the starter, one leg will read higher than the others. I would expect less than 1 volt, but comparing the readings on the three phases will give you an idea if something is out of balance. If you have excessive voltage drop on one leg, you can move your probes and narrow down the source of the problem. Then, try the disconnect.

More likely, though, something is changing on the load. Maybe a solenoid sticking?

#201093 - 05/09/11 10:34 PM Re: meg out motors [Re: allphase]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,103
You could try rolling the phases around one position and see if that changes what fuse blows. I would start with the input to the starter and if it moves, swap that back and roll the motor leads. Just be sure you keep the sequence the same.

Greg Fretwell

#201098 - 05/10/11 04:24 PM Re: meg out motors [Re: allphase]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
Your OP doesn't indicate that you tested for insulation breakdown.

It's a very good bet that you've got a short across one of your windings, perhaps two.

When connected delta ( a pretty good bet ) B fuse feeds two windings.

If those windings are shorted ( just so ) the motor will draw excessive amps across the B fuse -- yet still run. The key thing is such a short must be somewhat close to the taps. Then these two legs produce low back-EMF and draw too much juice.

Meggers test for insulation breakdown AND low resistance.

Go back and re-test both.

None of your comments provide enough information: what are the resistance values across the windings -- what is the breakdown voltage?


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