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Reason and remedy for AC motor overheating?

  1. Apr 12, 2014 #1
    Hi All

    I am asking this question as a mechanical engineer, so please bare this in mind if you have the time to post a reply.....

    I have a basic DIY type cement mixer that has given up today. It is a AC 230V model with the most basic of drive setups (pulley and drive belt attached to the motor shaft).

    I set everything up today to mix some concrete and turned the mixer on. Without the barrel fitted (no load on the output shaft) the motor turned no problem, then after around 20 seconds the motor slowed down and stopped completely over a period of around 10 seconds. I turned it off and noticed a smell, the familiar smell of electrical equipment going 'poof'.

    Having then removed the protective cover from the motor to investigate it became apparent that the motor was too hot to touch with my bare hand. The motor will start when it has cooled down, then stops running as described above.

    I have removed the motor from the machine in order to fit a replacement but have some questions about the existing motor before I splash out on a new one......

    1. Why would a motor overheat so quickly?
    2. Is there potential for a cheap/simple repair to the existing motor?

    I am genuinely interested to understand the technicalities of such devices.

    I've attached a picture of the rating sticker.

    Much respect

    Attached Files:

  2. jcsd
  3. Apr 12, 2014 #2
    Possible faults, check the bearings maybe they get stuck or are worn heavily
    if rotor moves freely , then you could check the resistance of the windings , possibly a winding short circuit, that would explain the motor heating heavily with no load.

    I would put my bet on faulty winding isolation personally , fropm what i have read.
  4. Apr 12, 2014 #3


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    Almost certainly an insulation breakdown resulting in a shorted turn.
    Maybe the motor has a thermal cutout or thermistor in the single phase supply, that recovers when it cools.
    There is probably no quick or cheap fix.
  5. Apr 12, 2014 #4
    When I was in the motor manufacturing business we called it a thermal overload. See picture here. It can usually be found attached to, or buried inside, the main winding.

    I agree with the others. Most likely cause is shorted windings. 20 seconds is a little quick for a locked rotor.
  6. Apr 12, 2014 #5


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    If the motor has a starting capacitor then it may not be getting switched out when the unit gets up to speed.
  7. Apr 12, 2014 #6

    jim hardy

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    do you have an inexpensive multimeter?

    Check motor for continuity between both prongs of the power plug and the metal frame. If you get continuity between windings and frame the motor has failed insulation.

    My dishwasher motor burnt up like that, fortunately it was just the start winding. I was able to see the short - two adjacent wires had got together and melted one of them in two... i was able to separate them, unwind one turn and solder the ends of the broken one. Then i painted the whole winding heavily, about four coats of polyurethane wood varnish.
    It ran for four more years and by that time i had found a replacement motor.

    You might be able to adapt a salvaged washing machine motor - better ones are 2/3 horsepower

    here's a typical hookup for US 120 volt machines


  8. Apr 13, 2014 #7
    Thank you. The motor and drive shaft turns freely with little effort, all bearings are good. I'd ask how to test the isolation but think this is covered in one of the other replies so I will comment on that one.

    Thank you. It does have a capacitor: 10µF ±5% SH, 450V AC 50/60Hz EN60252. Is there an easy way of testing this cap?

    Thank you. I have a cheap multimeter, I'll carry out this basic test tomorrow and report back. Looking at the windings, there's no obvious flaws.

    It's probably quicker to buy a new motor if necessary rather than making a new mounting bracket for a washing machine motor.

    Thanks again for the interest.

  9. Apr 13, 2014 #8

    jim hardy

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    that's a GOOD sign. My start winding had turned black.

    yes, a cheap multimeter will do the job if it's got a high ohms scale, at least RX1k.

    Place meter on highest ohms scale across capacitor leads.
    The meter will charge cap to value of meter's internal battery and while it's doing that you will see a brief "blip" on needle of an analog meter so watch close.
    Do same again after say a half minute and if capacitor held charge , like it should, you'll see NO blip because cap is already charged.
    Then reverse leads and connect again, you should see a blip twice as large because cap first discharges through meter then continues charging in other direction..

    If cap is shorted your meter tells you immediately.

    If your meter is digital you'll see ohms start low and build to max reading as cap charges.
    This build will be slower on higher ohm scales because meter applies less current. Probably RX10K will give reasonable rate of climb on 10 uf.
    Current that a digital meter applies makes perhaps 1 volt at meter's highest reading, so on RX1k it'd make 1 volt at 100K which is 10 microamp, which charges 10uf at one volt per second. RX10K would be a better range for testing a 10 uf cap, it'll take ~ 10 seconds to reach max reading.

    With an analog meter your eye gets to where it can estimate capacitance from size of "blip".
    With a digital you can guesstimate from time to reach full scale.

    Lastly, that might be a run capacitor and this a split phase motor. Clues to that are its a 5% which is incredibly precise for a start cap, and 2800 RPM not 3450 or so.. You can probably find another run cap in a junk airconditioner. If you have a good electrical supply house they should run six to ten dollars US.

    Try this if you can -

    mechanically disconnect motor from its load, and remove capacitor. Check for continuity between both capacitor leads and power plug. You should find it.
    Then tape leads for there'll be a lot of voltage on them.

    Energize motor, it'll hum loudly so you want to be quick with next step.

    Give shaft a spin, motor should accelerate and run fine.
    Be careful to not let it grab your finger or shirtsleeve, acceleration may be brisk.
    Repeat, this time spinning motor the opposite direction. It should run fine that way also.

    If motor passes that test i'd venture you have capacitor trouble. If not, you probably need a motor.

    Motor start caps are cheap plastic enclosed things that often fail shorted.
    Motor run caps are usually metal encased and filled with oil. They tend to fail open when the little wire going in from the terminal fractures inside.
    I suspect you have a split phase motor and a run cap.
    CAVEAT i have a cheap Chinese drill press with a split phase motor and plastic run cap . I replaced the plastic cap with a metal one from a furnace blower motor.

    We handymen gotta stick together. Good Luck !!!

    And learn your meter's secrets - a cheap meter can do more than one thinks.
    one of my dmm's only makes 200 millivolts full scale, another makes two volts but on diode setting it applies one milliamp at up to four volts..

    old jim
    Last edited: Apr 13, 2014
  10. Apr 13, 2014 #9
    To add to what Jim said about the cap. When you test the cap it needs to be disconnected from the motor. At least one side of it anyway. The reason is that the motor windings are connected in parallel with the cap and they will read like a DC short to the meter after a short settling time. Disconnecting a cap is an obvious step to an EE but may not be to others. I've seen plenty of electricians and techs replace perfectly good caps.

    Also, many meters, even the cheapos, can read capacitance. Mine will tell you if it's open circuited, short circuited, or it will read the Farads. Look for the little capacitor symbol on the meter.

    Many motor caps have three terminals. The reason for that is that the motor requires a large start capacitance and a smaller run capacitance. One terminal is a common and the other two terminals provide different amounts of Farads. One or both sides of the cap can fail. It's not likely you have this kind of cap though.
  11. Apr 13, 2014 #10


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    If it is a starting capacitor and the unit gets up to speed it is unlikely that it is a capacitor that is at fault. More likely the centrifugal switch.
  12. Apr 13, 2014 #11

    jim hardy

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    thanks Okee' and 'nova...

    re-reading i guess i didnt make it clear that a consumer type split-phase motor (which i think you have) uses a run capacitor and does not switch it out after start. So if you do replace the cap make sure it's a run type not start. You may not find a centrifugal switch on that motor, but if you do i was wrong - that's all.
    Only 10 uf won't cost much.
  13. Apr 14, 2014 #12


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    A split phase can also have a high resistance start winding.
  14. Feb 3, 2017 #13
    Could it have been caused by a a bad or underpowered generator, I had a nice sds drill which I used on a suspect generator, the next time I used it on mains it got up to full speed then slowed down to almost a stop and started to smoke, seems very similar to yours but maybe yours has a thermal cut out switch and mine does not, I'm only guessing and by no means no what I'm talking about just trying to join the dots!
  15. Feb 4, 2017 #14


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    If this is a DIY design, your gearing may be too high - which could be causing the motor never to run without the start winding (mentioned above)? Mixing concrete is a heavy job.
  16. Feb 4, 2017 #15

    jim hardy

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    This thread being a couple years old, is 'necropost' the right word ?

    Indeed a too long extension cord will burn up an induction motor,
    and what OP describes is exactly what they do in early stages of winding failure. The first winding to short heats up, expanding the faulted area to adjacent windings....
    If one catches it early and is lucky he can gently push the shorted turns apart with something akin to the orange cuticle stick from a woman's cosmetic kit, then apply varnish or paint to lock the wires down and re-insulate. I fixed a room "air purifier" with the dregs of a can of engine enamel, in its third year since repair now and diong fine.

    Meanwhile back at the ranch,
    we've had a couple threads on how to wire a washing machine motor.
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