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Thread: Single phase motor tripping breaker - bad capacitor or centrifugal switch?

  1. #1
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    Single phase motor tripping breaker - bad capacitor or centrifugal switch?

    We were called out to a farm that has a single phase, 7.5hp, 230v Baldor motor that trips the breaker as soon as the motor gets up to speed. It has 3 start capacitors and 3 run-capacitors (I couldn't tell you at this time if they are in series or parallel). The start capacitors tested okay, one of the run capacitors tested okay, the other two showed 0 mfd's (failed).

    Could it just be that the two run-caps happen to be bad, or is something else causing them to go bad? And how might this cause the brkr to trip? The OL's aren't tripping. Current goes up to around 110 or so during start-up, then drops down to 40 (during the times that it didn't trip the brkr), which is nameplate for this motor. Voltage is around 240 before start-up, then dips to around 217 during start-up (the transformer is close, so this doesn't seem right?)

    I don't know a ton about motor troubleshooting. Talking to a co-worker, he mentioned that if the start-caps were bad it would trip the OL's, but if the brkr is tripping, it's most likely the run-caps. If this is true can someone explain how this works and why?

    Thanks for any input,
    Sky

  2. #2
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    Is the centrifugal switch disengaging the start caps? check the terminal ends to see that they are closed

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    Would there be a relatively easy way to check this without tearing the motor apparatus apart? could this be checked by the accessible wires where the caps are located?

    Thanks

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    Quote Originally Posted by sw_ross View Post
    Would there be a relatively easy way to check this without tearing the motor apparatus apart? could this be checked by the accessible wires where the caps are located?

    Thanks
    Yes unplug the cap and check the terminals.

  5. #5
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    What is driven load? If high torque load the run caps are absolutely needed to develop enough phase shift to create necessary torque. If they are in series and you lose one you have really lost them all as you have opened the series (unless it is shorted). If they are in parallel (most of the time they are) you are just making a higher microfarad value capacitor bank with every one added in parallel to the bank.

    If high enough current is being drawn sometimes branch circuit protection still opens before overload protection. They are not the same time/current ratios, and the trip patterns are not necessarily a straight line if graphed out.

  6. #6
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    Quote Originally Posted by sw_ross View Post
    Would there be a relatively easy way to check this without tearing the motor apparatus apart? could this be checked by the accessible wires where the caps are located?

    Thanks
    Sounds like the starting switch is working based on what you said in the OP. If the switch were bad it would be likely that the start caps would blow. If you want to test put your amp clamp around the wire between the switch and the start cap.

    Did you test the motor unloaded?

    If it coasts a long time after cutting the power in may need new bearings.
    Baldor will also put grease zerks on the motor and shieled berings inside. People will start packing them with grease which can lead to troubles, which leads them to add more grease.
    If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

  7. #7
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    Quote Originally Posted by ActionDave View Post
    Sounds like the starting switch is working based on what you said in the OP. If the switch were bad it would be likely that the start caps would blow. If you want to test put your amp clamp around the wire between the switch and the start cap.

    Did you test the motor unloaded?

    If it coasts a long time after cutting the power in may need new bearings.
    Baldor will also put grease zerks on the motor and shieled berings inside. People will start packing them with grease which can lead to troubles, which leads them to add more grease.
    I have never run into a Baldor replacement motor with zerks and with shielded or sealed bearings. I have run into many motors that the equipment manufacturer likely put zerks on the motor yet there was shielded/sealed bearings inside so the zerks accomplished nothing. I have also taken apart motors just about filled with grease from operators that think they have to grease thes things often, not only often but pump grease until the handle (manual pump grease gun) starts to offer resistance or until he decides it will never offer resistance and maybe he pumped it enough.

    Dry type start caps will puke all over the place if left in the circuit too long. Oil filled run caps have an internal fuse that opens the circuit if the pressure inside case starts expanding the case - this prevents pressure from further building and rupturing the case.

  8. #8
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    Like I mentioned, I don't do a lot of motor troubleshooting, but I want to be more knowledgeable in all aspects of electrical troubleshooting so bear with me and my ignorant questions. And thanks for your patience, comments and input.

    I'm not even for sure that there is a centrifugal switch in this motor. That was just mentioned to me as a possibility by a co-worker. If there are start-caps and run-caps is it safe to assume that there would be a centrifugal switch that would disconnect the start-caps, thereby allowing the run caps to continue operating? That does make sense. Are there other methods of disconnecting start-caps? I was reading about potential relays used to disconnect start-caps, but they seemed to be used in motors that have start windings in addition to run winding. This motor is used to operate a drying aeration fan on a grain bin.

    You're saying to check the teminals, after disconnecting the start cap (after the motor gets up to speed) to see if the switch is closed or not?

  9. #9
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    Quote Originally Posted by sw_ross View Post
    I'm not even for sure that there is a centrifugal switch in this motor.
    There is one.

    If there are start-caps and run-caps is it safe to assume that there would be a centrifugal switch that would disconnect the start-caps, thereby allowing the run caps to continue operating?
    Correct.

    Are there other methods of disconnecting start-caps? I was reading about potential relays used to disconnect start-caps, but they seemed to be used in motors that have start windings in addition to run winding.
    All single phase motors have a start winding. There are other ways to drop power to the start winding or start caps. Most have a centrifugal switch.
    You're saying to check the teminals, after disconnecting the start cap (after the motor gets up to speed) to see if the switch is closed or not?
    No, I am saying if want to check the operation of a starting switch without taking apart the motor put your clamp on ampmeter around the wire that comes from the start switch to the start cap and start the motor. If the amps drop to zero or close to it the switch is opening.
    If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

  10. #10
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    Quote Originally Posted by kwired View Post
    I have never run into a Baldor replacement motor with zerks and with shielded or sealed bearings..
    I see this all the time and only Baldor.

    I have run into many motors that the equipment manufacturer likely put zerks on the motor yet there was shielded/sealed bearings inside so the zerks accomplished nothing. I have also taken apart motors just about filled with grease from operators that think they have to grease thes things often, not only often but pump grease until the handle (manual pump grease gun) starts to offer resistance or until he decides it will never offer resistance and maybe he pumped it enough.
    .
    I think grease is the farmer's and maintanence man's version of duct tape and WD-40 for motors. They always have plenty of it around, they use it all the time, and if there is a problem they use more of it.
    If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

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