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Thread: Motor Over-Amping

  1. #1
    Join Date
    May 2007
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    Question Motor Over-Amping

    What could cause a motor to over amp? We have a 75 horsepower motor (460V-3ph) that has a design full load amps of 86 amperes. A motor test was run and the loads are phase A - 85.9 amps, phase "B" - 92.8 amps; phase "C" - 91.7 amps. Should e be concrned?

  2. #2
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    Question

    It sounds like you have very unbalanced line voltage.

    Did you take voltage readings?

    Was the motor mechanically connected to anything or just running by itself?

  3. #3
    Join Date
    May 2007
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    The motor is for a chilled water pump. The voltages were 487-490-483. How could we deal with the unbalanced voltages?

  4. #4
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    Quote Originally Posted by timosborne
    How could we deal with the unbalanced voltages?
    I really do not know, but someone here will.

    Here is a link that may be helpful.

    Resolving Voltage Problems

  5. #5
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    Your pump may be pumping too much water. If the motor is doing more work it will draw more

  6. #6
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    Quote Originally Posted by timosborne
    What could cause a motor to over amp? We have a 75 horsepower motor (460V-3ph) that has a design full load amps of 86 amperes. A motor test was run and the loads are phase A - 85.9 amps, phase "B" - 92.8 amps; phase "C" - 91.7 amps. Should e be concrned?
    You are not much out of balance. You can try to balance the load a little better or you can ask the Utility to change the taps and reduce the voltage,
    that is if the transformer has taps.

  7. #7
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    Oshkosh, Wisconsin
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    Is the motor fed by it's own transformer, or are the measured voltages directly from the POCO?

    The voltage is a bit high, but the phase differance is not out of the ordinary, acording to my experiances. Our voltage from our substations is about 480.
    We use isolation transformers between them and our machines, had one wired 1:1, and had 505VAC out, retappped to the 1:0.9 tap and got 493VAC out, wound incorrectly I guess.

    Your motor probably has a service factor of 1.15, so the current is within that range.

    Most pumps come with throttling valves, if the overcurrent causes OL tripping, you can throttle the flow back slightly.

  8. #8
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    Current imbalance is 4.7%. To much. Hollow shaft motor, submersible or centrifigal pump?

    What is the transformer set up? Open Delta?
    Try rotating the leads. Move A to B, B to C, and C to A. Then again, until you have the least amount of imbalance. Amazing how well that works sometimes.
    Tom
    TBLO

  9. #9
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    Cool

    Quote Originally Posted by ptonsparky
    Try rotating the leads. Move A to B, B to C, and C to A. Then again, until you have the least amount of imbalance. Amazing how well that works sometimes.
    What a great suggestion, I will remember that.

    If it works great, if it does not you have not spent anything.

  10. #10
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    Fernley, NV. (near Reno)
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    A few questions here, then some answers.

    How accurate are your current measurements? Were they done with recently calibrated equipment, or a clampmeter that's been kicking around the toolbox for 10 years?

    What is the service factor of the motor? Also, does the nameplate list S.F. amps.

    The current balance doesn't concern me in the slightest. In my experience, it seems very reasonable.

    If the service factor is higher than 1.0, I wouldn't be concerned about the slight overcurrent. Lets say it's 1.15, (very common). You actually have an 86.25 HP motor which has been de-rated to 75 HP to account for overloads, voltage imbalance, high ambient, etc. If the S.F. amps are listed, that is the theoretical maximum current the motor can handle continuously in its maximum ambient temperature (usually 40C or 104F). If S.F. amps are not listed, it's not full load amps X 1.15, you'll have to guess. 93 amps on an 86 amp motor is not very far into the service factor. I normally don't advocate operating motors in their service factors, mainly because you're now what we would call an 'non-compensated test engineer', but you might not be overloading it at all.

    Alot of centrifugal pumps will load the motor into the service factor when operated a low pressure. Some, particularly high pressure designs will grossly overload. The HP required by a centrifugal pump is related to the flow through it, not the pressure difference across it.

    If it were me, I'd basically leave it alone. Make sure the overloads are properly sized, and it's not operated in too high of ambient temp., and you'll be fine. If you want to see lower current, throttle the discharge a bit (partially close a valve). This will reduce flow, and thus reduce HP.

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