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How hot should the motor get?

Discussion in 'Electronic Basics' started by David Harmon, Oct 1, 2003.

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  1. David Harmon

    David Harmon Guest

    I've got a motor, 110V shaded pole type with attached gearbox, made by
    Brevel. I haven't found any useful Brevel web site. The motor gets quite
    hot while running with no load, not too hot to touch, but too hot to pick
    up and hold on to comfortably. It has no fan. How hot is something like
    this supposed to run? Is it suitable for continuous operation?

  2. The motor is probably perfectly comfortable. Humans have pretty low
    threshold for temperature based discomfort. Metal objects of something in
    the vicinity of say 50 deg. C isn't normally comfortably holdable. So that
    is probably around where the case temperature of the motor is at.
    Internally it might be somewhat more.

    Magnet wire comes in a number of temperature ratings, but all the stuff I've
    ever seen is intended to last quite a long time at temperatures above 100
    deg. C. Since your motor doesn't sound like it is running anywhere near
    that hot (it would nearly instantly burn you) the life expectancy is
    probably extremely long, at least in the no load condition you mention.

    Core loss is roughly constant independent of motor loading (or transformer
    loading for transformers for that matter). Core loss is often designed to
    be roughtly equal to the copper loss when the device is fully loaded. When
    the motor is unloaded the copper loss should be quite small. So in the case
    of your motor being fully loaded, expect it to increase in temperature
    (above ambient) about twice what it currently doing.

    IE: Assume ambient temperature of 25 deg. C. Assume under no load motor is
    running at 50 deg. C. There is a 25 deg. C delta between ambient and no
    load, so expect a 50 deg. C delta between ambient and full load. So the
    motor should operate around say 75 deg. C under full load. These are
    probably very rough estimates. The internal operating temperature will
    likely be somewhat higher.

    Nevertheless it sounds like the motor is still going to be a fair amount
    under the insulation temperature rating so it should last an extremely long
    time. Wether or not the motor is suitable for continuous operation is
    somewhat uncertain, but it doesn't sound like it will be substantially
    thermally limited. The motor specifications are the best place to determine
    for sure if it can handle continuous operation or not. My first guess is it
    could handle it alright.
  3. David Harmon

    David Harmon Guest

    I have made some measurements and, maybe, answered my question.

    I powered up the motor and let it run. It ran for about 20 minutes,
    reaching about 190 F (88 C) on the laminations before apparently a
    thermal cutout went "thunk" and it wouldn't run any more until it
    cooled down again. It restarts itself at about 170 F (77 C).
    Ambient here is 80 F (27 C).

    Incidently, it draws about 1.45 A AC with no noticeable change
    whether loaded or unloaded.

    That's considerably hotter than the temps you suggest would be
    normal for continuous operation. Do you agree that letting it cycle
    between 190 F and 170 F continuously would constitute abuse?

    The odor of warm oil from the bearings is not very strong, and no
    smoke yet.
    That would be nice, but I don't have any.
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