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Thread: electric motors with safety overheat shut off sensor

  1. #1

    electric motors with safety overheat shut off sensor

    Anyone know if an electric motor that has a sensor on it that shuts the motor down when it overheats, would be less likely to shut the motor off ; if it is used outside in COLD weather?

  2. #2
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    Well, yes, for a little while anyway. If the motor is being operated within its ratings, it should never trip the thermal overload. Not just the load on the motor, but supply voltage, ambient temperature, and cooling air flow all have to be withing ratings. If the OT is tripping, it could be the motor is too heavily loaded, the supply voltage is too low, the ambient temperature is too high, or there isn't sufficient cooling air flow. All of those things will cause the motor temperature to rise higher and faster than it would under normal conditions, perhaps to the point where the OT trips. If you move the motor to an area with low ambient temperature, it means the motor has to heat up more before the OT trips. If the motor is only mildly overloaded, say, the added cooling provided by the low ambient temperature may be enough so the OT never trips. But I wouldn't count on it. Better to correct the root cause. Repeatedly overheating the motor will shorten its life.

    [edit:] Another common cause of overheating on big motors is failing bearings.....

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    I have had a DeWalt 733 for several years, I got it secondhand at an auction for $120 or something like that. I use it occasionally to plane a few boards but maybe 5 years ago it got so that the overload protector would trip before I even finished one board. I decided to remove the overload protector and just splice the wires together since I had gotten some years out of the machine and didn't have much to lose, and it has run perfectly ever since. I believe that the protector was defective, but I don't know for sure.
    Zach

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    Zach, Of course a faulty sensor is always possible. On small motors they are pretty crude devices and often have fairly wide tolerance. It's possible yours was just a bit too sensitive. Or there could have been a real problem that just wasn't bad enough to cause immediate serious damage.

  5. #5
    I agree with Paul's post but I'd ask one additional question: Is the motor really hot when the OT trips? There's always a possibility that the OT device is tripping in error.

    If it's uncomfortable to put your hand on the motor, it's probably too hot.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

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    Quote Originally Posted by Mike Henderson View Post
    I agree with Paul's post but I'd ask one additional question: Is the motor really hot when the OT trips? There's always a possibility that the OT device is tripping in error.

    If it's uncomfortable to put your hand on the motor, it's probably too hot.

    Mike
    Many motors are too hot to hold a hand on when operating near full load and within design parameters. Most TEFC motors are that way.

  7. #7
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    Quote Originally Posted by Clarence Martinn View Post
    Anyone know if an electric motor that has a sensor on it that shuts the motor down when it overheats, would be less likely to shut the motor off ; if it is used outside in COLD weather?
    That's an issue regardless of which thermal overload protection the motor has.

    A magnetic starter with thermal overloads will also be affected by ambient temperature.....Regards, Rod

  8. #8
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    Quote Originally Posted by Clarence Martinn View Post
    Anyone know if an electric motor that has a sensor on it that shuts the motor down when it overheats, would be less likely to shut the motor off ; if it is used outside in COLD weather?
    Ambient temperature will have little effect on the likelihood of the sensor activating once it's trip temperature is reached. It will affect the amount of current and or time it takes to reach that trip temperature. So, lower ambient temperature means more current or a longer time is needed to shut the motor off.

    Does "less likely" imply some randomness in the operation? As has been noted by other posters if the trip point seems unpredictable you probably have a defect in the sensor or elsewhere. If that's the case the effect of ambient temperature could also be random.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  9. #9
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    We borrowed a TS and it heated up and tripped under moderate loading. Long story short, it was full of sawdust. Once cleaned out it was fine.

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