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Thread: 3 phase motor problem

  1. #11
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    Quote Originally Posted by Jraef View Post
    That's either a really inefficient 5HP motor or a really efficient 7-1/2HP motor.
    Good point. I did the calcs and they don't equate to 5HP (3.7kW) motor.

  2. #12
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    Thank you for all of the input. A little more info on this. It's a 7.5 hp motor that has a fan wheel on it for a filtering system so it's not under any real load at start up. I can spin the fan wheel and it spins easily and for quite a long time before it comes to a stop.

    I checked the voltage on the line side of the motor contactor and it's good. I'll try to stop there again today and see if I can get some voltage readings while it's starting up.

    Thanks again.

  3. #13
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    Quote Originally Posted by ActionDave View Post
    Bad connection somewhere in the circuit, maybe a bad bearing.

    Did you check voltage and current on all three phases?

    Has anything else in the building changed?
    Nothing else has changed in the building. I'll try to stop there today and do some more checking.

  4. #14
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    My first thought was single phasing too. I checked the voltages on each phase and they were good but haven't done so under a start up condition to see if there's a bad connection. I'll try to stop there again today to do some more testing. Thank you.


    Quote Originally Posted by Jraef View Post
    Electrically, on a 3 phase motor the only thing that could cause this is a bad connection, i.e. a partial single phasing condition. If one phase was completely lost the motor would not start at all, but if one phase is connected but not well, i.e. high resistance, it might try to spin, but is not creating enough torque. The fact that it SOMETIMES works fine would make me look at the contacts in the motor starter.


    That's either a really inefficient 5HP motor or a really efficient 7-1/2HP motor.


    that's basically Locked Rotor Amps, normal, other than the fact that it doesn't get to speed.


    There would not be any caps inside of a 3 phase motor...


    If it's not a bad contact in the motor starter, it's going to be a mechanical problem; either a bad bearing somewhere as previously mentioned or something in the load that is causing the motor to stall, in which case it is not an electrical issue.

  5. #15
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    I went back this morning. The load side of the motor contactor had one phase that wasn’t making a good connection. I took it out from under the screw, stripped it, and reinstalled it and it works again. Thanks for all of the help and input. Greatly appreciated.

  6. #16
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    Quote Originally Posted by ammklq143 View Post
    I went back this morning. The load side of the motor contactor had one phase that wasn’t making a good connection. I took it out from under the screw, stripped it, and reinstalled it and it works again. Thanks for all of the help and input. Greatly appreciated.
    Glad it worked out.
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