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Thread: Amps and motor overheating

  1. #1
    Join Date
    Sep 2006
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    Amps and motor overheating

    When electric motors in good mechanical condition "overheat", does this imply that that they must be drawing more than their rated amps?

    The question occurs to me while I'm working on my evaporative cooler, but it seems applicable to motors found in power tools. It would be comforting to know that any tool motor that was drawing its proper amps should be working against the proper load and running at its proper temperature. (I just got a new AC ammeter.) Sounds too simple to be true.

  2. #2
    Mostly, yes.

    If the voltage is constant (which it is), then the power into the motor is the voltage times the current. If the current into the motor does not exceed the motor's nameplate rating, then there is no way to put more power into the motor than it was designed for.

    One small exception: Many induction motors have a fan mounted on the shaft that cools the motor. If you load the motor too heavily, the speed will decrease (but the torque will increase). Since power = Torque * speed, the electrical power you read into the motor may stay unchanged (or even decrease). HOWEVER - the fan is no longer spinning fast enough to cool the motor. Thus, it may overheat that way.

  3. #3
    The motor overheating does normally mean the motor is drawing more than rated current in order to cope with the overload it is subjected to. Any electrical appliance and machine produces heat under normal operating condition, but then it is designed to handle that heat with proper heat transfer and ventilation mechanism. The heat generated is proportional to the square of the current flowing through the equipment. So in theory 10% increase in current means 21% increase in heat, 50% increase in current means 225% increase in heat ! But since, the equipment is not designed to handle that additional 21% or 225% heat, the temperature starts rising.

    However, overload is not necessarily alway the reason for overheating. Failure of heat transfer mechanism or blockage in ventilation can also cause heat to accumulate and temperature to rise. This is true for any device or component subjected to generate heat. For example, in your computer motherboard, the CPU is equipped with heatsink that transfers the heat out of the CPU. But if the heatsink fins get covered with layers of dust, it wont to be doing its job causing the CPU to overheat. Same is true for motor, too.

  4. #4
    Join Date
    Apr 2006
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    If the motor has an adjustable pulley, you set the pulley using an ammeter on the motor. I never had to do it, but that was always in the back of my mind when working on coolers. Also make sure the motor hp is up to the rated cfm of the squirrel cage. Putting a lower hp motor in will cause problems as there's too much air resistance for the motor. Then there's always the problems inherent in many cooler instalations, using little extension cords instead of proper wiring for example, though you don't usually see those problems showing up as an overheating motor.

    Usually though, cooler motors overheat due to bad bushings. They need to be oiled at the beginning of the season (at an absolute minimum). Once they're causing problems the motor has had it. Lube often and a cooler motor should last far more than the 3-5 years most seem to.

  5. #5
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    Induction motors will try to maintain design RPMs regardless of load or voltage. An increase of load or a decrease in voltage will increase current draw and of course heat. If the wire feel very warm to the touch, then one or both factors exist.

  6. #6
    Join Date
    Nov 2007
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    Mid Missouri (Brazito/Henley)
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    Better motors have a 1.0 or 1.15 service factor. This means the motor can be run at 100% or 115% of it's rated hp. for a period of time with no damage. Of course, proper ventilation and proper size wiring is a must.

    One of my hardest working motors has been the 2hp, 12A, 230V TEFC motor on my old Rockwell RC33 13" planer. After a lengthy session of planing 12"wide 8/4 oak, the motor is too hot to touch. Yet it has never tripped the overload or circuit breaker. It's one tough motor!
    Necessisity is the Mother of Invention, But If it Ain't Broke don't Fix It !!

  7. #7
    Join Date
    May 2009
    Location
    fargo ND
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    whenever i encounter electric motors that overheat i always check for low voltage or voltage sag. improper gauge wiring before the motor is often a major cause for this - lower voltage means more amps drawn and more heat.

    a prime example of this is a blower motor running off 100' of 16/3 stranded - way to small of conductors; the motor will work (apparently) fine for a while and then overheat or trip breakers. i find this at my workplace very often - i just cant seem to explain this and get through to them about why they need 10/3 if they insist of going 100' with it.

  8. #8

    As an HVAC contractor, I can tell you...

    Quote Originally Posted by Stephen Tashiro View Post
    When electric motors in good mechanical condition "overheat", does this imply that that they must be drawing more than their rated amps?

    The question occurs to me while I'm working on my evaporative cooler, but it seems applicable to motors found in power tools. It would be comforting to know that any tool motor that was drawing its proper amps should be working against the proper load and running at its proper temperature. (I just got a new AC ammeter.) Sounds too simple to be true.
    The proper amps is not the rated amps on the motor.

    The problem with your cooler motor is due to the drive pulley being adjusted too large and the result is too much load on the motor.

    We always adjust the drive pulley to keep the motor draw at 50 to 60% of the rated amps. Any more than that and you will cause the motor to overheat.

    Whenever we see a motor that is drawing it's rated amps, we know that there is a problem. A motor that is drawing 100% of it's rated amps has a very dim and short future.

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