Electric motor fail??

Discussion in 'The Garage' started by D.T., Apr 19, 2013.

  1. D.T.

    D.T. Difficult but useful

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    Have a weird problem at work with a 3 phase 13.2kW motor. It blows fuses but the overload doesn't trip.

    We checked the windings for a ground and it's not grounded. Brought it to a motor place and they had a "good" and "bad" tester for it and they say it's fine. :huh I think they used a megohmmeter.

    Some dude says it's the bearings. So I just replaced them. The front one looked like it overheated and had a bit of drag.

    Any ideers? We haven't tried it yet. 480VAC @ 22 amps at nameplate.

    Oh, and we tried it without the leads hooked up to the motor so it isn't the leadwires.
    #1
  2. the_gr8t_waldo

    the_gr8t_waldo Long timer

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    you seem to be totally focused on the motor...sure the controler is working?
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  3. tommu56

    tommu56 Long timer

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    What size are the fuses
    Are they time delay
    What type of starter
    What voltage
    What load are you starting?

    Motors can take 3-5 times their running current to start.

    the overloads have a time delay inherent to them.

    fuses protect wire ol is suppose to protect motor if sized correctly.


    tom.
    #3
  4. Flashmo

    Flashmo Whatever...

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    This.

    Don't know what the installation is, but vibration can cause the feeders to chafe and only short periodically...they then blow themselves clear and wait for the next time.
    #4
  5. River

    River Gee

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    Did you try the motor un-coupled?
    What is the service it is in, pump, fan?
    What brand controller/starter is it?
    Motor control and automation is my forte:D
    #5
  6. D.T.

    D.T. Difficult but useful

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    Fuses were 40A, I think there is 25A in there now. I think they ran out of 40A's.

    OL is set for 15 amps. Standard contactor and OL.

    Motor for a hot oil pump. 365 degrees hot oil.

    Schematic says 35A fuses with 10-16A OL setting. 12.7 A is standard current draw.
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  7. River

    River Gee

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    I am assuming the conductors are #10 or larger(depending on the distance from the contactor), so 40A would be proper protection. 15A O/L on a 12.7 FLRA is around 125%, which should be correct as well. Blowing fuses on startup would indicate that there is a short somewhere(fuses can take a limited amount i of inrush). You could try engaging the contactor with the leads disconnected at the motor. This will let you know if the cable is good up to that point. It is a good idea to meggar the cable before it is re-energized. Also check the contactor for any carbon tracking and that it will move in and out freely. Check the resistance of the motor windings, making sure they are balanced. There is also the posibility that the control transformer is shorted (480-120VAC most likely). Try to eliminate each component until you find the culprit.
    I hope this helps and good luck...if it is really important, I'm not afraid of flying:evil
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  8. PeterW

    PeterW Long timer

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    Start capacitor is gone ?.

    Pete
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  9. Benesesso

    Benesesso Long timer

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    3 phase motors don't use caps.
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  10. D.T.

    D.T. Difficult but useful

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    Well, of course it's do or die. We have been trying to get them to buy us a spare pump for a while. But they have to make our lives a PITA. Boss wants to get a good megger now though.

    Tried it with the motor disconnected and no problems with the fuses. It isn't VFD controlled. Learned recently that they can generate voltage and destroy bearings, so our 125 hp motor has a grounding brush on the shaft. I tested the windings at .8 ohms across all 3 phases. Boss got around 1.2 ohms. Could be the contactor. Lead wires are 10 gauge I'm pretty sure of.

    Just had to replace a 100A circuit breaker because it was tripping. Internally it got fried. This was for another piece of equipment.

    Electrical gremlins have been kicking my ass lately.
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  11. Hastelloy-X

    Hastelloy-X Been here awhile

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    Have you tried replacing the fuses with buss bars?

    Try it and let us know what happens...

















    :hide
    #11
  12. tommu56

    tommu56 Long timer

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    Are the time delay fuses? If the are fast blow fuses to protect electronics they won't hold for starting current.

    Post the exact fuse # so we can look it up.

    A good meger will cost as much as that motor if it is a standard one.

    I have literary hundreds of AC drives from 1/4 to 800hp. in our plant and haven't had any bearing failures we do use good VFD Faraday type shielded cable on them.

    fat fingered on a droid
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  13. trc.rhubarb

    trc.rhubarb ZoomSplat!

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    Have you measured voltage on both sides of the contactor at each leg?
    Have you measured amperage on the load side of the contactor at each leg?
    Are you blowing all the fuses?

    It really sounds like a low voltage/high amperage on a single leg caused by a bad contact or corroded terminal on the contactor.
    #13
  14. the_gr8t_waldo

    the_gr8t_waldo Long timer

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    cut to the chase- pay an electrician to look at it. an industrial setting is a poor choice for a home hobbist to learn industrial electrics
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  15. machinebuilder

    machinebuilder Long timer

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    There are some good suggestions here but some by people who don't know a damn thing about 3 phase and motors

    1. are all the fuses blowing, if not is it the same ones

    2. are they time delay fuses (the 25a ones probably won't work)

    3. have you checked the contactor? if it cycles alot there is a good chance the contacts are burnt.

    4. do the fuses blow instantly? if not try checking the current on each leg.


    In my years working on this type of thing, shorting windings are hard to tell, current readings tell more than resistance (unless it's shorting to Ground)

    doing a quick look the 40 amp fuses may be a little small for the motor.

    try just the motor without the load hooked up, the pump may be bad.
    #15
  16. victor441

    victor441 Long timer

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    Is the phase rotation correct?? It is possible that someone switched two wires anywhere downstream, even on the electrical utility side...it happens once in awhile and causes all sorts of problems...not real likely but did not see phase rotation mentioned yet

    http://ecmweb.com/content/determining-motor-rotation-direction
    #16
  17. joexr

    joexr Banned

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    The motor would run either way. The pump just wouln't pump.
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  18. victor441

    victor441 Long timer

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  19. gmiguy

    gmiguy You rode a what to where?

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    If I read this right it sounds like somebody replaced a 40A fuse with a 25A fuse; for a circuit that the schematic says is supposed to have a 35A fuse.

    If so 40>35>25; so of course you're blowing fuses.

    The RLA and overload setpoint don't matter; motor protectors aren't instantaneouse and RLA doesn't address inrush at startup.

    Confirm that you're running with the exact type and rating fuses specified by the equipment manufacturer or designer before you suspect the motor.

    For three phase motors if the shaft spins by hand, the leads don't show continuity to ground, and ohm out lead-to-lead close to the rated resistance then there's about a 98% chance that the motor itself is fine.
    #19
  20. D.T.

    D.T. Difficult but useful

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    My boss is going to put the motor in tomorrow and try it. Probably strap it to a pallet and try the motor by itself.

    I'd think a bad contact in the contactor could make it single phase on startup and blow a fuse. That's all I got for now.

    No overtime for me, since I had to work last Sunday. Calling an electrician to come out and work on it is an ABSOLUTE last resort where I work. Getting a PO could take days.
    #20