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Thread: 120 volt coil

  1. #1
    What do you guys get for resistance on a 120 volt coil for condenser fan motor contactor??? Unit is a carrier 50DL..not that that matters....Thanks in advance

  2. #2
    Join Date
    Aug 2003
    Well 120 volt Coil Help's? But You Need AMPS (or VA Of Contactor to help you out.Should be Marked on Contactor.)

    (TYP. DC CALC)
    E =VOLTS ~or~ (V = VOLTS) Divided
    I =AMPERES ~or~ (A = AMPERES)Equals
    R = OHMS ~or~ (R = RESISTANCE)

    (Single Phase AC YOU CALC) Diffrent Beast..
    Example...120v (E)coil Times 1 amp (I) Note: clamp amp meter around hot coil wire if Possable, For amp Totel) That will give you the(watts)Power - 'Then Dived this Power with ether the (1) AMPS/2 Square or (120v) volts/2 Square Dived with The Power 120 watt
    Should Equal 120 Ohms

    120v/2 120W
    ______ _______ = 120ohm
    120W 1AMP/2

    (Note..coil's Vary with Type,Manuf,Ware etc...also most small contactor's are in .1 to .8 amp Range...But if its not 0 ohm (Shorted) or SAY 1,000,000 (open)plus you should be ok?

    IF your asking what size of contactor (check FLA/Running amp/and HP size, Voltage Rateings etc...

    Sorry Been long time sense i used OHM LAW...

  3. #3
    when in doubt, change it out! Period. End of Report!!!

    You dont have time to do engineering calcs on a field repair job.
    That is work you do over a drawing table in some office in Chicago.
    When your working on some RTU, you have time to run for the right part and change it out.

    I have some boring stories of jobs where some guy replaced the bad coil with the wrong voltage coil.
    I got another one where the contactor didnt pull in. Come to find out the box we grabbed said 208v, not 115v on it.
    Shoulda paid closer attention to what we were doing.

    Had another contactor we replaced. Hit the switch and the contactor pulled in alright, for about one milisecond. Then it smoked.
    Guess what we did wrong on that one?

    There should be another 115volt coil on another RTU or something. Take a reading off that one and compare the two.
    Measure your voltage across the terminals when the voltage is applied. If you have significant voltage drop, perhaps you have a serious high load on your step down transformer.
    Or, yo may have a mechanically defective contactor itself. It may not be your hold in coil at all.

    "When in doubt, change it out."

  4. #4
    an "expert" Guest
    "When in doubt, change it out."
    Expect that to actually happen at times, but I would hope it's not the Rule...

  5. #5
    Join Date
    May 2002
    Baton Rouge, Louisiana
    Have another unit like it on the job or do they have 2 fan motors? I find comparing if you have another works best
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  6. #6
    Originally posted by joey791
    Have another unit like it on the job or do they have 2 fan motors? I find comparing if you have another works best
    brilliant minds think alike

  7. #7
    Originally posted by an "expert"
    "When in doubt, change it out."
    Expect that to actually happen at times, but I would hope it's not the Rule...

    Unfortunately ... it is a rule. An "unwriten rule" by contractors who dont know how to deal with their people. It is the rule of; when in doubt about this tech's performance .. change him out with another ....

    It is assumed we are disposable, expendable .... a throw labor force.

    A company will spend hours bringing you onboard and then with less notice than the sound of a shotgun blast ... your fired! Gone, outa there!!!

    And it's never an open discussion. Truth has nothing to do with whether or not you are kept.

    When in doubt, change them out.

  8. #8
    Join Date
    Nov 2003
    Rochester, New York
    "When in doubt, change it out."

    I tend to go by the practice"when in doubt,take your time,don't hurry and do dumb things and finally troubleshoot that is what they pay us for. Any monkey can swap parts until they get it right. IMHO

  9. #9
    Join Date
    Nov 2004
    North Florida/South Georgia

    when in doubt, change it out

    I think the point R12 made was missed. I've had a few bad mngrs. over the years. I once had to sit through a 30 min. session on proper troubleshooting procedures of a thermocouple. Give me a break! Who cares! At $80.00 per hr, some points aren't worth proving. We don't really know why the gentleman was asking about the coil, but this is how I usually handle this type of situation. Power to the coil? If yes & its not pulling in, the coil is of no use. If you suspect the coil is causing a short, process of elimination can solve the mystery everytime. I'm not trying to ruffle any feathers, its just the opinon of an old fart here in the trenches.
    All my leon freaked out!

  10. #10
    Join Date
    Apr 2004
    Originally posted by R12rules
    "When in doubt, change it out."
    I agree, especially when it is a low cost item. If it is an intermittent problem, the worst thing you can do is sit on your bucket for two hours trying to find the problem and not change anything. I try to check out the most likely causes, and if I suspect some particular part, if its fairly cheap to replace, I'll pull the trigger on that shotgun and replace it. If I don't do anything, then I have absolutely no chance of solving the problem. At least that way, I have some chance.

  11. #11
    Join Date
    Sep 2004
    I agree with lemb. If its intermittent you could spend days or weeks even befor it fails again. You can only check every thing so many times befor you say to your self the clock is running and what is the most common problem ive seen to make it not work. I some times take the chance and replace a part I dont like to do that but its all I can do and its a good guess but some times it bites me in the arse. I am not a part installer im just a service tech that wants to learn every thing.
    Free cooling and heating all at the wrong time of year.

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