Compressor Motor Capacitor Problem

Discussion in 'The Projects Forum' started by jledwell, Jan 30, 2009.

  1. jledwell

    Thread Starter New Member

    Jan 30, 2009
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    I have a small air compressor that has 2 capacitors a 70uF and a 80uF. The 80uF capacitor is blown and has been removed. The compressor seems to work fine. I originally assumed the 80uF capacitor was the start and the 70uF was the run. Now I believe they are both run(maybe in parallel). I have tried the 70uF with each set of wires and it seems to function the same.

    Does some motors have 2 run caps and no start caps? If so, why the different values? What are the consequences of running with just one? I assume I am reducing the life of the cap, but am I reducing the life of the motor as well?
    Thanks in advance for the response.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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    One possibility is that you have a two-speed motor, and that it will now not run at the low speed. I've never heard of a two speed motor in a compressor, but the universe is full of things I've never heard of.
     
  3. jledwell

    Thread Starter New Member

    Jan 30, 2009
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    Thanks for the reply. I very seriously doubt that is the case. It's a small cheap direct drive compressor (central pneumatic) from harbor freight.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Single phase motors need a starting capacitor to start rotating and some need a run capacitor too for smoother operation. The 80uF should be the starting capacitor and the 70uF the running one or be both starting capacitors connected in parallel.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    It's a two speed motor all right. They are often marketed as "twin capacitor" motors. Central Pneumatics engages in this exact hyperbole.

    You've got less starting torque now than you used to.

    To address part of the original question, there are some types of motors with start caps and no run caps. There are no motor types with run caps & no start caps.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    It is possible that one cap is supposed to be switched out by a centrifugal switch that has failed. Harbor Freight doesn't sell cheap stuff because it's high quality.
     
  7. jledwell

    Thread Starter New Member

    Jan 30, 2009
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    Thanks for the great responses.
    Does this mean they are both starts? Or is one start and the other run? I'm assuming one start and one run, but why does the motor perform the same no matter which pair of wires I connect the one good cap to?

    Can you explain two speed motors? Do they run at different speeds depending on the load?

    Thanks


     
  8. jledwell

    Thread Starter New Member

    Jan 30, 2009
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    Thanks for the response.
    I'm seeing voltage on the wires that are supposed to be connected to the start cap(unless there both start caps), does that mean the centrifugal switch has failed? If so, this could explain why the start cap was blown.

     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    They've got two sets of start and run windings, and two caps. These can be wired either in parallel or in series to get the two speed/torque combinations. With one of those caps missing, you won't be able to start in series. So I'm guessing you are starting in "parallel," but with only one start winding.
     
  10. jledwell

    Thread Starter New Member

    Jan 30, 2009
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    This cap is kind of weird, it has a stud sticking out of the back for mounting. The only place I can find one is from Harbor Freight parts dept and they only have the 70uF cap. With the caps in parallel, do you think it will make much of a difference to have a 70 and 70 instead of a 70 and 80?
     
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