3 phase motor problem

Discussion in 'General Electronics Chat' started by mik3, Jan 1, 2009.

  1. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    Hi guys,

    I have a 3 phase motor working on a single phase supply. The problem is that it blows one of the two capacitors connected to it. Do you know what might cause this?
    I believe it is a partially shorted internal winding.
     
  2. PointGiven

    New Member

    Sep 10, 2005
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    Why do you have a 3 phase motor working on a single phase supply? I would use the correct type of motor (single phase).
     
  3. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    It is a car elevator and that is how it came. Do you know what might be the problem?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Just a guess here, but the caps might be wired in to "fake" the other two phases by phase shifting. The caps might be too high in their ESR, which would cause excessive internal heating, or too low in their voltage rating. I suppose you could also have some partially shorted windings.

    You might take the motor down to a repair place that has a high-pot tester and have them check it out.
     
  5. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    Thanks for your answer Sgt. What is a high-pot tester?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    They use a high voltage low current source to see if the insulation on the windings is failing. If there's current from the windings to the frame, your winding insulation is shot.
     
  7. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    Thanks for the information Sgt but I think there is no serious leakage from the windings to the frame because the GFI does not trip. I haven't made my tests yet but the owner of the motor changed the capacitor and it blew again. Thus I believe there is a short on the winding itself (one point of the winding to another) which causes its resistance and inductance change and cause a larger current to flow. The other good point is the one for the high ESR of the capacitor you stated before.
     
  8. floomdoggle

    Senior Member

    Sep 1, 2008
    217
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    Hey Mik,
    What is a car elevator? Unless you have three phase power coming into the building, the motor must be internally wired to operate in three phase.
    Where, and what is the motor used? Can it be spun by hand without feeling resistance, or crunchy things? And is the cap blowing while unattached to the load of the motor? Your motor may not meet the manufacturer's specs.
    Over sized motors are just as inefficient as undersized motors, and just as likely to burn up.
    Dan
     
  9. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    I replaced both the running and starting capacitors and it works fine. Because at starting both capacitors are involved and the running capacitor was not good the current was flowing all through the starting capacitor it blew up.
     
  10. ron_o

    New Member

    Mar 1, 2009
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  11. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    If you connect two of the windings to the single phase supply and use a capacitor between the single phase supply and third winding you can create a fake 3 phase system. It works but with reduced performance.
     
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