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Thread: Motor troubleshooting help

  1. #21
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    Quote Originally Posted by GoldDigger View Post
    None of those tests would detect a shorted turn, which would greatly increase the current in one winding (therefore in two phase wires.) That seems to match the symptoms.
    An ohm measurement using AC as the applied voltage or using an impedance bridge would conclusively diagnose this problem.
    If you cannot measure one winding at a time (with delta corners being permanently bonded) you just have to realize that each measurement gives you a different series-parallel combination of all three windings.

    The amperages seen could correspond to the combination of effects of voltage imbalance and a shorted turn.

    Sent from my XT1585 using Tapatalk
    What I meant to say was that the simplistic L-G megger test would not catch that, not that a L-L would... sorry, I wrote that wrong.
    __________________________________________________ ____________________________
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  2. #22
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    run it at a lower pressure
    run it at no pressue
    run it with the belts off
    measure i for all 3

    reciprocating type?
    how many pistons?

  3. #23
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    Quote Originally Posted by Jraef View Post
    I'm with MTW on this.

    You have a significant current imbalance, about 22%. That's going to cause most SSOL relays to trip, because a severe current imbalance will cause extra motor heating even though the current is not exceeding the setting. That's what makes SSOLs better than heater type. So DO NOT "bypass" it! It's likely doing EXACTLY what it needs to do to keep that motor from burning up!

    So the issue is, what's causing the current imbalance? Your voltage readings are showing that the current imbalance is not the result of a line voltage imbalance, so you ruled that out.
    He only checked at the panel supply end, according to the OP. I wouldn't consider it ruled out just yet. I would want to see voltage readings at the load side of the starter, while loaded, before going further.

    MTW

  4. #24
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    Quote Originally Posted by DrSparks View Post
    They use CTs don't they?

    Sent from my A574BL using Tapatalk
    Cts are not heat sensitive.
    Bob

  5. #25
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    Quote Originally Posted by Jraef View Post
    I'm with MTW on this.

    You have a significant current imbalance, about 22%. That's going to cause most SSOL relays to trip, because a severe current imbalance will cause extra motor heating even though the current is not exceeding the setting. That's what makes SSOLs better than heater type. So DO NOT "bypass" it! It's likely doing EXACTLY what it needs to do to keep that motor from burning up!

    So the issue is, what's causing the current imbalance? Your voltage readings are showing that the current imbalance is not the result of a line voltage imbalance, so you ruled that out.

    When the IR guy said he "checked the motor", what does that mean? Megger test? If not, whatever he did was meaningless. If so, was he just checking L-G, not L-L? it could have an insulation breakdown turn-to-turn or phase to phase, but nothing going to ground. If all he did was the basic test of hooking all of the motor leads together to one megger lead and the other to ground, that would not find those other problems.
    20% is not that bad. especially when the motor is as underloaded as it is. in fact, those readings suggest to me the motor might not have been loaded at all when the readings were taken and maybe when it is loaded the readings are different.

    ETA: It appears the default setting in AB E300 electronic OL relays for current imbalance is 35% which is more than 50% higher than this situation.
    Last edited by petersonra; 03-07-18 at 04:58 PM.
    Bob

  6. #26
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    Quote Originally Posted by petersonra View Post
    20% is not that bad. especially when the motor is as underloaded as it is. in fact, those readings suggest to me the motor might not have been loaded at all when the readings were taken and maybe when it is loaded the readings are different....
    A good point. I think that to a first approximation the counter-EMF will be the same for all windings, and will have its greatest effect at low slip and low power. So under those conditions a small % difference in applied voltage can make a larger % difference is effective voltage (applied voltage minus counter-EMF.)

    We might be looking for a condition that causes the OLs to trip only under certain loading conditions or only intermittently.

  7. #27
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    do you have a pic of the name plate?
    rated hp?
    rated v ?
    rated pf and eff?

  8. #28
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    Just replace the motor and get done with it, IMO.

  9. #29
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    Quote Originally Posted by Ingenieur View Post
    do you have a pic of the name plate?
    rated hp?
    rated v ?
    rated pf and eff?
    Quote Originally Posted by topgone View Post
    Just replace the motor and get done with it, IMO.
    The OP said he is already done with it, he was just looking for ideas on why it might have been tripping.
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  10. #30
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    Quote Originally Posted by ishium 80439 View Post
    While it was running I got readings of A- 29.7A, B- 23.5A, C- 18.9A.
    This is a severe imbalance and is NOT normal despite what anyone may tell you.

    I would have tested voltage right at the motor splices a-b, b-c, a-c. If voltage was within spec at the motor with a current imbalance like that, I would tell them to replace the motor.

    Nothing more on your part needs to be done at this point, no OL's need to be bypassed, etc. They just need to replace the motor. That's the first step.

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