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Electric motor troubleshooting

  1. Mar 22, 2008 #1
    I have a 3HP motor running continuously. After about 5 years it has stopped, flicked breaker and it continued. That happened a few times. More recently, now 10 years, it stopped and wouldn't start. I tried a few things, but turning the motors shaft seemed to get it to work when power was restored. I tried that method again when it failed a number of times, but each time it seemed harder to re-start. It failed again, but this time it doesn't want to start.
    What is likely wrong with the motor that causes these type symptoms? Brushes? Some other item in the motor? The bearings squeak a little so I'd wondered if they would stop it, but the shaft has always rotated, so it mustn't be the reason for it not re-starting (even if it may make the motor fail if overheated/seizing while running).
    I'd like to get an idea of what normally goes wrong/symptoms/troubleshooting in motors, and which parts age/would be best replaced on an overhaul after 10 years, even if they are still working, but are likely to fail because of the motor age.
  2. jcsd
  3. Mar 22, 2008 #2


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    Gold Member

    I don't know enough about motors to help you, other than suggesting a good cleaning if possible. Any motor in my place would have half a pound of cat hair in it after 10 years. :biggrin:
    I would urge caution if you're continuing to use it. Sounds like a possible fire hazard to me. The tripping of breakers is not a good sign.
  4. Mar 22, 2008 #3


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    Staff: Mentor

    They can overheat and melt insulation, gather gunk, etc. Tough to know exactly what it is. But for the price of a 3hp motor (compared with what you are paying to run it), why bother trying to fix it?
    Last edited: Mar 22, 2008
  5. Mar 22, 2008 #4
    The start capacitor could be going bad.
  6. Mar 24, 2008 #5
    Mouse/Rat fur more like.

    Well that's the thing, I'm not sure if it's better to service the beast or buy new. If I had more of an idea of what genrally goes wrong with motors (there's probably not very much to them), and if a simple repair can give it a continued long life, I could make a better decision.

    Are there any sites or books you know of that provide "Electric Motors for dummies" type info?

    Thanks for the info Phrak. The searches I made before posting came up with "start capacitor".

    Actually, I'm not sure it's throwing a breaker. It's on a GFCI, and I'm not sure how they work. Are there just 2 positions the red button can be in "in-on" and "out-off", or can it position itself between the 2? It seems to push in a little from what looks like the "in-on" position.

    When you press the other button "the test button", does that switch the power off, or is it still allowing power to the device?

  7. Mar 24, 2008 #6


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    A GFI is similar to a breaker, except that it's triggered by a ground fault rather than an overload. If the returning (neutral) line shows less current than it should, the device assumes that there's something wrong and trips. The test button is just a simulation system so you can make sure that it's working properly. There is no 'half-way'; it's either on or off.
  8. Mar 25, 2008 #7
    If you're tripping a GFI, you've most likely got some old insulation/mouse hair/grease/general gunk leaking to earth. Is this 230 VAC, single phase?

    First thing I'd do is clean it really well, then examine the brushes (10 years continuous!), and then lubricate the bearings, although if the bearings squeak, that's a bad sign and they probably need replaced.

    Why do you have it on a GFI? And, what kind of GFI?
  9. Mar 25, 2008 #8


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    Science Advisor

    Is this a single phase motor? Is it an AC or DC motor? More information about the motor would help you get a correct answer.

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