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Motor runs, but not under load

Discussion in 'Troubleshooting and Repair' started by Jagdo Maine, Jul 26, 2014.

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  1. Jagdo Maine

    Jagdo Maine

    4
    0
    Jul 26, 2014
    I'm trying to troubleshoot an electric motor on an old piece of farm equipment.

    The motor turns when removed from the equipment. When it is connected to the equipment (via a drive shaft with several pulleys connected to V-belts), I can turn the engine freely by pulling on the main belt. But it won't start under load. It jerks a bit, then stops and sits there, and if I leave it long enough I can smell it burning a bit.

    The motor is probably not original to the equipment, and it may or may not be the right size/type. What else should I check?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    If the motor stats, will it run under load?

    I guess the question is, will it not start under load, or will it not run under load
     
  3. shrtrnd

    shrtrnd

    3,824
    519
    Jan 15, 2010
    You're interchanging (maybe) the words 'motor' and 'engine', and it makes it a little difficult to try to help.
    Are you trying to identify a problem with and electric motor that is part of or attached to a fuel-driven engine? Or are you just referring to an independent electric motor?
    This is probably going to boil-down to the type of motor, to identify the issue. Electric motors do wear out, but there are capacitor-start/ and capacitor-run motors that have outside parts.
    If possible, tell us if this a stand-alone electric motor problem. And if you can tell us what the name-plate on the motor says it will be helpful.
    The motor nameplate will say who made it, the model number, the voltage/amperage/wattage, and other useful information that will tell us more about what you're dealing with.
     
  4. Jagdo Maine

    Jagdo Maine

    4
    0
    Jul 26, 2014
    Thanks for the suggestions.

    I don't know if it will run under load since it doesn't start under load, and there is no clutch on the equipment. I did gingerly try to nudge the drive belt along with the motor powered up, and it didn't run then. (Folks, I don't recommend doing this. If the drive belt starts and you get caught in it, you could be hurt very badly.) Is there another way of testing whether it will run under load, if you cannot start it under load?

    There is no combustion engine attached to the equipment (an old stationary winnower). The electric motor drives the equipment directly.

    There is no name plate on the motor, at least not any more. But I do wonder whether the motor is appropriate for the equipment. Presumably to work well, it has to give the correct RPMs (for the machine to work the way it is supposed to) with sufficient power (to overcome the internal resistance of the machine). And I don't actually know whether the motor satisfies either condition...

    Any other ideas?
     
  5. Electric Al

    Electric Al

    85
    7
    Jan 16, 2014


    Look up the manual for the equipment to be driven , then it will probably advise you as to what motor is required !
     
  6. neilb

    neilb

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    Jul 27, 2014
    How have you gone jagdo? Is the motor single phase or 3 phase? If single phase does it have external capacitors (look like small cans) probably under a cover?
     
  7. BruceS

    BruceS

    26
    1
    Jun 25, 2014
    I wonder if it's like one of those electric motors you find on automatic washing machines?
    They start on one 'winding' and switch to the other when centrifigul clutch is up to speed.
    Many years ago I had one that wouldn't start up. Cleaned the contacts & away it went.
     
  8. Jagdo Maine

    Jagdo Maine

    4
    0
    Jul 26, 2014
    No manual, and probably never was one. The machine is at least 65 years old. It does exist in multiple copies, fortunately, and I am trying to get in touch with someone who knows a lot more about them than I do.

    Motor is single phase, no capacitors that I can see. It does start when off the machine, just not when it's connected to anything. And it could indeed be a motor pulled off something else.

    Let me try asking the question a different way: If a motor is turning at the correct RPMs, but not with enough horsepower, what is the problem likely to be? Dirty contacts on some of the stator coils? (I'm making up the example, of course, just trying to provoke thought.)
     
  9. BruceS

    BruceS

    26
    1
    Jun 25, 2014
    The obvious .... if someone put a 1/3hp motor on a machine that needs a 5hp motor ???
    A decent electrician could soon test it......... brushes, armature, siezed bearings (part) etc etc
    Does the machine turn over easily by hand? Motor too?
    http://www.wikihow.com/Check-an-Electric-Motor
     
    Last edited: Jul 28, 2014
  10. neilb

    neilb

    2
    0
    Jul 27, 2014
    Good thoughts BruceS, just one point, those type of motors you mention start with 2 windings in circuit to get a rotating magnetic field then the centrifugal switch operates to disconnect the start winding leaving run winding in circuit. So a motor with a faulty or open circuit switch will not start even under no load, just hums but will go if give shaft a spin (in either direction).
    Jadgo - Typically these type of motors are under 1hp i.e. 1/20 - 1/2 hp
    however the pulleys, belts and drive shaft couplings you describe sound like much higher than this.
    Would you be able to post a picture?
    To find out what is on a comparable machine sounds like a very good approach.

    One other thought. Is the supply to the motor good? i.e. terminal connections tight, cable, plugs sockets etc (sorry if this seems to obvious) and if with in your scope or an electrician check voltage at motor terminals under load i.e. still approx 230VAC
     
  11. shrtrnd

    shrtrnd

    3,824
    519
    Jan 15, 2010
    Lots of good ideas here. But don't forget that motors can wear-out.
    The laminations warp, causng drag on the rotor: Depending on the type of motor, brushes wear, commutators wear, mechanical wear in general. The bearings might be dried-out and dragging, etc, ... all things that would let the motor run with no load, but slow or stop with a load.
     
  12. Jagdo Maine

    Jagdo Maine

    4
    0
    Jul 26, 2014
    Thanks to all for the many ideas.

    According to the knowledgeable person I spoke with, this machine takes a 1/6-1/4 hp motor running at ~1700 rpm.

    Don't know how this motor stacks up, of course. But tested with a clamp meter, it was drawing ~3 amps at 120V when running (voltage assumed, not measured). Compare this to the 1/4 hp 120V motors in McMaster-Carr, which are spec'ed to draw 3.4-5.4 amps. So it's on the low side, but in the right ballpark.

    One thing I should add is that this motor has been standing out in the rain for probably 10 years, before I acquired the machine. And it is definitely not a sealed motor. So it wouldn't be surprising if it had "worn out."

    I'm going to do a few more tests and will report back.
     
  13. shrtrnd

    shrtrnd

    3,824
    519
    Jan 15, 2010
    Alright, throw-in corrosion of the mechanical parts as a possible issue. My bet, is your bearings in that motor need to be replaced.
     
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