Typically I would check the motor leads (or motor branch circuit conductors) with a clamp-on ammeter with the motor running, and check my readings against the nameplate full-load current of the motor with the motor fully loaded. Among other things, the driven equipment (including pulleys, belts, bearings, wheels, and such) should be given a once over to ensure it's nothing mechanical in nature causing the overload.
That's what I usually do when they run, but this thing let the smoke out as soon as the 480 hit it.
However, when you see smoke pouring out of the motor, its a pretty safe bet those windings are shot...Maybe the motor overload was set too high (if there was one. If not you may consider installing either a motor overload unit or finding a motor with a built-in thermal overload. It would prevent burning up another motor.) BTW, what's the pump's job here?
Motor had overloads and I believe they were set correctly. Pump is one of three feeding a manifold that feeds irrigation circles.
You may find a shop that can rebuild that motor, or you may have to consider replacing it, but you will need motor overload protection here...[/QUOTE]
Got it, the farmer will rewind/replace the motor and hopefully be back in business. Thanks.
If there are no obvious mechanical causes the first test I do when a motor trips out or blows a fuse is to check the ohmic readings between the motor leads as far back as possible, usually at the overload or fuse block.
Keep in mind the windings are basically a short circuit like a transformer winding so the value is very low but they MUST be exactly equal.
Even a difference of value of lets say 1 Ohm between 2 legs and 1.2 Ohms between the other legs is enough to trip an overload and the motor needs a rewind.
If the windings are balanced (equal ohmic readings between windings) then I check insulation resistance between each leg and then each leg to ground.
If a motor runs but trips out regularly I use the current clamp to check for overloading and/or equal current flow in each leg.
Sounds like I need to start testing the windings. Another one just 2 weeks ago I had a 50 kva transformer that was also tripping the breaker at startup with no load on the secondary. Megged fine and had only been in service 6 mos. Replaced transformer and all is well. I figured shorted windings on this one too. I'll try ohming one next time and see if it tells me anything. I don't like having to throw a 480v breaker into a dead short for testing...there's gotta be a better way to do this.
Thanks.
I may be a bit weak in this area - I assume you are referencing using a megger? I have limited experience here, so I could use any input on how and when to check insulation resistance. It's one thing I've never used to troubleshoot motors...
My Fluke 1507 has been a very handy tool. If you do service, especially checking wiring and motors, you should definitely have one. I probably use mine about 3 times a month or so. Up until these last two weeks with the transformer and this motor it's made it obvious where the problem was.
Just a couple examples:
We do work on a large dairy utilizing four small motors to drive a carousel for milking cows. With the wet environment a dairy is, motors and wiring seem to be getting checked and replaced on a regular basis. Being able to isolate the VFD from the wire from the motor, testing each one is invaluable. Right then and there you know which one is testing poorly and needs replacing. No guessing why the drive is showing ground fault.
One about 3 weeks ago, pump panel is blowing one of the two control fuses. Meg the control wire on that side of the fuseblock, it's reading zero. Disconnect starter coil, still reading zero. Pull wires off of HOA/start button, meg again, reads great. Put megger lead on individual wiring terminals of the HOA, reads great. Throw it on the start button, one terminal shorted to ground, couldn't physically see the problem but the Megger caught it. Put the T+ Pro tester on it just for curiousities sake and it didn't show continuity. I would of needed a handful of control fuses to get this one sorted out without the megger if I was just using a regular multimeter.
Sorry for the long post.:smile:
