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Thread: Motor troubleshooting help

  1. #1
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    Motor troubleshooting help

    I've got an issue with a motor that I'm hoping someone more knowledgeable than I can shed some light on.

    It's a 3 phase 208V motor for a compressor. The nameplate rating is 38.4A. It is being fed with #8 THHN and the panel is approximately 100' away. I was called because they were having issues with it and the tech from Ingersoll-Rand requested I come out and and look at the feed side of things as he could not find an internal issue.

    While it was running I got readings of A- 29.7A, B- 23.5A, C- 18.9A. Voltage was consistent at the panel lugs and the breaker at A-B 209.4, A-C 209.9, B-C 205.3. These voltages held steady both under load and unloaded. All connections were secure and there were no signs of overheating or corrosion. The panel this is being fed from is practically a dedicated panel with only a lighting circuit and a circuit for a tube heater in it.

    To me it seems that it is obvious that the problem is in the motor but for some reason the engineers keep looking to the premise wiring as the culprit. What am I missing?
    "If you don't do it this year , you'll be one year older when you do" Warren Miller

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    180307-1120 EST

    Measure the voltages at the motor. This may mean several places around the compressor. Such as, where the wires first terminate near the compressor, and directly at the motor.

    .

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    I would say about 80% of the time trouble calls on motors are are caused by a problem that has nothing to do with the motor.

    You gave us voltage and amperage, those sound ok, but you didn't tell us what the problem is.

    Check all connections, check the pressure switch, check the unloaded on the compressor head, check the check valve, check and make sure the leads aren't wired up for 480V.
    If you don't think too good, don't think too much.

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    You didn't tell us what the symptoms were

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    According to the operations manager the call originated because the the internal thermal overload kept kicking out and stopping the motor. That compressor has been there for about 3 years or so and had been running fine until a few months ago. It apparently started as an intermittent problem and got progressively worse.

    Thanks for the replies so far.
    "If you don't do it this year , you'll be one year older when you do" Warren Miller

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    Quote Originally Posted by ishium 80439 View Post
    According to the operations manager the call originated because the the internal thermal overload kept kicking out and stopping the motor. That compressor has been there for about 3 years or so and had been running fine until a few months ago. It apparently started as an intermittent problem and got progressively worse.

    Thanks for the replies so far.
    Faulty heater?

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    The overloads are currently internal and electronic. The plan they are going with is to bypass them and use a conventional thermal overload in their place. But how would that explain the disparity in the current draw?
    "If you don't do it this year , you'll be one year older when you do" Warren Miller

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    Quote Originally Posted by ishium 80439 View Post
    The overloads are currently internal and electronic. The plan they are going with is to bypass them and use a conventional thermal overload in their place. But how would that explain the disparity in the current draw?
    Didn't see that. The only thing I can think of is a bad winding

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    Quote Originally Posted by DrSparks View Post
    Didn't see that. The only thing I can think of is a bad winding

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    If one of the windings are dropping out it would essentially single-phase the motor and cause the overload to trip.

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    Thanks, that is exactly what I was thinking also so can't figure out why they are resistant to the idea of changing the motor.....

    And in interest of full disclosure at this point this really is just curiosity for me. Once the wires hit the lugs on the unit I walk away and hand it over to the techs from the manufacturer.
    "If you don't do it this year , you'll be one year older when you do" Warren Miller

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