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start kit sizing

does anyone know the formula on how to size a start kit for a compressor in the field? i use to have it written down,but have since misplaced it.i know that you take the back emf reading with the compressor running,and then you select a potential relay with a drop out voltage about 80% of that,but i do not remember how to size the start capacitor.when needed i have been using the factory (trane,carrier,etc.) factory start kit when they list one as an option,however the cost of these factory kits are normally4 to 5 times what it costs if you bought the pieces seperatly and wired them up on your own.any assistance would be much appricated.thank you


  • Techman
    Techman Member Posts: 2,144
    edited August 2012
    What formula?

    I'd like that formula also!Are you adding the relay/cap or replacing the relay/cap ? I highly recommend using Copeland caps/relays on Copeland comps and Tecumseh cap/relays on Tecumseh comps and so on .If a Carrier cond unit has a Cope comp in it then it also has a Carrier pot relay which can be replaced w/ a Cope relay. You then contact a Cope-Tecumseh -Bristol wholesaler to get the spec's for the relay/cap for that given comp.I have done what you said w/ the measured BackEMF and put in a pot relay that comes close, but, only TEMPORARILY.There is Hot Pickup Voltage,Cold Pickup Voltage, Dropout Voltage and Continous Coil Voltage to be figured into that formula There is a "one size fits all"pot relay on the market w/ an adjustable Continuous Coil voltage screw but that is not my favorite. Now as far as the Start Cap goes, that is a total crap shoot.What I do is  I put on a 135-156mfd -330vac cap and start the comp w/ my ampmeter on. I rapid start/restart the comp and see how my cap/relay does, then I go up/down one cap size and retest.And then I go up/down another size and retest until I like how the comp starts. Then I'm first in line @ the supply house to get the proper stuff.And I've been dead-on w/my cap sizing only about 50%( ok ok maybe 40%) of the time, which is about 10-12 times in 40 some odd years. 
  • zepfan
    zepfan Member Posts: 358
    adding start kit

    i am talking about installing a start kit to a unit that has never had it,but needs it due to low voltage,long line set, txv installed w/ a recip. compressor.i had the formula from a company i worked with about 20 years ago.i am going to call my boss at the time (who is still there) and see if he still has the formula.i'll check back after i have talked to him.thanks.
  • Techman
    Techman Member Posts: 2,144
    edited August 2012

    Hay zepfan, you got me thinking so I checked and  came up w/ a few things. I knew of a  formula for figuring out an unmarked  run capacitor for mfd's,but not for the voltage of the cap. I did not realize it applied to start caps also until just now ( HUMPH) ! The most important voltage # of the pot relay is the "pick-up voltage" ,( somewhere around 66%-75% of back-emf)it has to less than the next most important voltage #  of the pot relay which is "continous coil voltage" which has to be higher than the measured back-emf. Mars makes a "6 pack" of pot relays ( plus 4 more) and their literature has those #'s in it. It is a lot cheaper to get just the relay and cap then a "kit" from Carrier ,ect.
  • paul_79
    paul_79 Member Posts: 91
    start kit

    when i started out about 23 years ago there was a sheet with a formula ,( which had been around for a few year) i will have to look through my old papers and get  back to you.
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