AC Motor run capacitor question...

Discussion in 'General Electronics Chat' started by jeffbrog, Mar 9, 2014.

  1. jeffbrog

    Thread Starter New Member

    Mar 9, 2014
    16
    1
    Hello!
    I have a question about a 5 hp 220 volt compressor motor.
    It uses two parallel 25uf caps for the run circuit, the housing that holds them is cast aluminum. The caps are 2" in diameter and about 4" long. Why does the motor use/need two 25uf caps instead of one 50uf? I was told by a motor repair shop that a single cap does not work (he's tried it), he said I'd have to talk to an engineer as to why. Since two 25's in parallel = 50uf, I don't see why. I'm sure the motor mfg. would use a single cap if it worked to cut the cost of the 2nd cap and the larger housing. Can someone supply an answer? Thanks.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Is there a motor schematic?

    Are you sure that they are parallel and both in the run circuit?

    Metal can?
    "Run" are more commonly oval.

    Maybe a single cap would not fit.

    Compressor motors are usually capacitor start.

    I see no reason one would not work.
     
    Last edited: Mar 9, 2014
  3. AfdhalAtiffTan

    Active Member

    Nov 20, 2010
    117
    11
    I never seen that before, thanks for sharing.

    Maybe for the ESR reduction?
     
  4. jeffbrog

    Thread Starter New Member

    Mar 9, 2014
    16
    1
    A few more comments to my post:
    Yes these are "run" capacitors, metal shell, wired in parallel, there is a phenolic shelled start cap that is a 216-259uf rating. It is in it's own bump out case like you normally see on a motor. There are 50uf run caps that are the same size as the 25 cap. The run caps are rated 370 volts. The cast alum. cover was made for 2 caps.
    Any more ideas why they use/need 2- 25's instead of one 50, and why one won't work (according to the motor shop)???
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Are the 50's that are the same size 370vac also?
     
  6. jeffbrog

    Thread Starter New Member

    Mar 9, 2014
    16
    1
    Yes, you can get a 50uf that is 370 volt in the same size shell as the 25's.
    The motor guy apparently thought it would work too (some time ago, he's been rebuilding motors for many years), but he said one doesn't work. I can't figure out why, neither could he! My thought was that 50uf is 50uf regardless of two or one! Since the mfg. of the motor spent the extra money for a special "dual" cap case and the expense of the second cap, it would make sense there is a reason, but it's beyond me!
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    They must have found from experience that using two lasts longer. (cooler)

    I would have to see, to believe that it wouldn't work at all.
     
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