Motor capacator value

Discussion in 'General Electronics Chat' started by franky5, Jul 29, 2011.

  1. franky5

    Thread Starter New Member

    Jul 29, 2011
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    I have an old Janett gear motor that is a 1/40 HP, 125 volt, 0.9 amp. It is missing the capacitor. I tried a cap that I had from another motor and it ran but not at the correct speed. It also became very hot quickly. I have two of these motors and one has the proper capacitor, however, it has no stamp of what it is. I swapped the capacitor to the other motor and it runs at the proper speed and stays within the temperature rise stated.

    I need both motors. Can anyone advise me on the proper capacitor value to use? There is no internal switch so the cap is in the circuit at all times.

    Thank you for any help, franky5
     
  2. electronis whiz

    Well-Known Member

    Jul 29, 2010
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    27
    is it a starting or running cap?
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Check this out. page 6. It's not your brand and it's not your horsepower, but it's somewhere to start. they list 1/50hp at .4A using a 3.0MFD 250VAC cap. They list a 1/15hp at .9A using a 10.0MFD 250V cap. So maybe shoot for the middle of those two and see what happens?
     
  4. franky5

    Thread Starter New Member

    Jul 29, 2011
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    I have to assume that the capacitor is in the circuit at all times since there is no internal centrifugal switch. So it is both a start and run cap.

    Thanks, franky5
     
  5. franky5

    Thread Starter New Member

    Jul 29, 2011
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    Thanks Strantor

    I checked out the page you suggested and I think I will try your suggestion of the middle-of-the-road.

    Thanks again, franky5
     
  6. #12

    Expert

    Nov 30, 2010
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    Just run 120 volts through the good capacitor, measure the curent, and calculate the value.
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    DOH! Or you could just do that.

    +1
     
  8. #12

    Expert

    Nov 30, 2010
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    Oil filled run caps are so very bad about changing capacitance with the voltage applied that normal capacitor checkers are useless, so this is the every-day method to check capacitors on industrial motors. Use the voltage they are designed for and see if they pass the current called for by the capacitive reactance formula.
     
  9. franky5

    Thread Starter New Member

    Jul 29, 2011
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    Thanks everyone. I think doing the calculation is a good idea just to make sure I am in the ball park. The SWAG method sometimes bites. It's been about 40 years since I did any calculations like that so I'll have to dig out my reference book.

    By-the-way, I didn't look to see if you can post pictures on this forum. If so, I'll post a picture of what the motor is for. I have a sever neck problem that requires traction therapy to keep the nerves to my left arm from being cut off. I built a traction unit. Should anyone have a similar problem I will be glad to share the plans.
    Thanks again, franky5
     
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