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  1. #1
    Join Date
    Mar 2002
    Posts
    516
    I been replacing quite a few bad run caps lately. Instead of just putting back the same value as what was removed, I have started looking at the label on the comp. All the Copeland scrolls have the run cap value on the label. Sometimes I'll take out a bad 35+5 370 and then look at the label and see 40,45 or even 50 mfd 370VAC. What's going on here? The comp. obviously ran for quite a while with the small run cap. What are the effects on the comp. (current draw, rpm, voltage etc.) due installing a run capacitor that's too small? Could the fact that the cap was undersized have lead to it's early failure?
    Do some techs just replace any bad run cap. with a 35+5 370 because that's what they have on the truck and in most cases will work?

  2. #2
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    Generally a smaller cap will reduce the starting torque, and may prevent it from starting properly under moderate load.

    paul

  3. #3
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453
    As tecman said, it will reduce torque, both starting & running. This in turn will cause the motor to pull more current trying to either get the load started or keep the load going. Increased current results in increased heat inside the windings, and Wa La !! the windings don't last as long.
    Have seen undersized cap on fan motor, motor lasted two weeks.

  4. #4
    Join Date
    Mar 2002
    Posts
    516
    The current draw goes up as the run cap. value goes down right? So you never want to undersize the run cap. What about oversizing the cap? Say the label specs a 40 mfd and all you have on the truck is a 35 and a 45. We know the 35 is a no no but what about the 45?

  5. #5
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    Oversizing has it's downside as well. The current in the start winding will increase above design, increasing heat and shortening life. All that being said, most manufacturers rate the capacitor value with a 10% tolerance, so a small change in value, say 5 mfd is within the nominal tolerance range and should not have a significant effect.

    paul

  6. #6
    Join Date
    Apr 2004
    Location
    Palmdale, CA
    Posts
    199
    Correct me if I am wrong. I was taught that + - 5 MFD was ok. The right one is best but let's say for some reason you don't have the right one. Is it ok? I was taught it was.

  7. #7
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    This subject actually came up at a Lennox training class I was in a few years ago.
    The guy teaching the class was an engeneer and a tech rep for Lennox.
    I don't remember how the subject came up, but someone asked a question about it.
    His answer was that the capacitor rating on the nameplat of the compressor, or in the compressor manufactuers literature, is the capacitor used for the benchmark ratings for that compressors capacity and whatnot.
    Changing the size of the run cap actually changes the capacity of the compressor, and capacitor matchups is one of the ways equipment manufacturers fine tune the capacity matchups of compressors to the equipment they build. Acording to him, you should use the capacitor called for by the equipment manufacturer if the unit has an OEM compressor in it.

    Thats the only time I have seen the subject come up, the answer made sense to me, and I trusted the guys word, so I havn't thought about it or researched it since then.

    One similar example of this would be the "rescue" type motors that some motor manufacturers sell. In addition to using different leads off the motor for different HP ratings, you also have a range of run cap sizes to pick from, depending on the HP needed and the operation of the motor after it is installed.
    If more government is the answer, then it's a really stupid question.

  8. #8
    Join Date
    Mar 2002
    Posts
    516
    Another bad run cap. today Copeland ZR34K1-PFV. This is an old comp. without the cap. value data on the label. I looked it up on the Copeland website and it doesn't show a run cap spec. either. Anybody know the run cap value spec. for this old comp? Thanks, Lee

  9. #9
    Join Date
    Dec 2003
    Posts
    165
    Mark, I agree with you. I always replace with a direct match. I just happened to replace a codenser fan motor today with a rescue. On the motor it said to use a 7.5 plus 10 or minus 5%. Plus 10% would be an 8.25 and minus 5% wouold be a 7.125. I only have 5, 7.5, and 10's on my truck. I know you can get it going with the closest one but......but......I dont want to have to replace burned up motors and have to explain how I did wrong. Just my 2 cents worth!!

  10. #10
    when replacing a cap,voltage should be =to or >than the original.
    run cap. uf+or-10% of original.
    start cap. uf=to but not >than 20% of original
    start cap=75-600uf
    run cap.=3-75uf
    when running caps in parallel, add uf. for total microfarads
    do reciprocal method on caps. in series of more than three

  11. #11
    Join Date
    Dec 2003
    Posts
    165
    Or just put back the correct one.

  12. #12
    Join Date
    Mar 2002
    Posts
    516
    Copeland ZR34K1-PFV spec calls for 50 uf 370vac. Unit had a bad 35+5 370 that looked like it had been in there a few years. Replaced with correct cap and unit is working again. Most of the caps. that I have been replacing are just like this ie. prior replacement that was too small. Does too small lead to early failure? Anybody else seeing this?

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