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  1. #1
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    I worked on a trane heat pump today, That got me scraching my head.

    When I got there I found the breaker for the condenser was tripped.

    I reset after ohming out for ground. I checked capacitor UF with my meter. everything looks ok untill I checked out the amp draw.

    common wire 10.4 amps
    run - 8.54
    start- 8.54

    I had to checked this several times and rechecked and finally I got another meter and it was the same thing

    I ran it for a while but it didnt overheat or tripped the breaker.

    How can that be if the run and common wire is in series it should have the same amps and start winding on capacitor should only read around 2-3 amps

    Everthing else look ok. After I left and I thought about it. I should have ohmed out the compressor winding

    Have anybody come across that before?

    (also posted in hvac and pro forum)

  2. #2
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    I dont know.You have me confused, now.

    Change out the run capacitor.

  3. #3
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    Thread Starter
    I thought about changing the cap but I did not have a 60uf and my meter says it was ok. The unit was cooling and heating I tried it both cycle.

  4. #4
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    Dec 2004
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    Short on time so briefly -A compadre encountered a Tecumseh pot compressor years ago- single phase which had been miswired internally- I can't remember the characteristics of operation but it was found that when resistance readings
    across the windings were checked they were not correctly assigned to the normal terminals- I think they had crossed the run and start windings if memory serves. Wouldn't have believed it if I hadnt seen it myself.

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  6. #5
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    Jun 2004
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    If the unit is wired wrong I should have a high amp reading and compressor would go on overload. The unit is not that old.

    I am probably the first one to open this unit. exept of course the installer.

    I told them I was coming back to recheck. I will ohm out the compressor.

    I havent seen anything like it. Still scraching my head.

  7. #6
    Join Date
    Aug 2002
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    What did you check the run cap with?

    Did you test the amp draw on the run cap while under load?

    Do you have access to the factory specs for this compressor, electrically?

    Did you check the operating volts to run and common during loaded ops?

    Your voltage across run and start should be real high during loaded operations. What were those readings?


    Too bad you dont have an "O"-Scope for field use. This would tell you if your I was leading your E by so many degrees.

    Verrrrrryyyyy interesting...... got me

  8. #7
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    Hey Bob I checked the cap with my fluke 29 and fieldpiece 76. The second question is a trick question right.

    No access to factory specs yet.

    You bring up a good point on the voltage across the cap. I will check that when I go back.

    When I checked the amp draw on that compressor I think I short circuited the data bank in my head. I even forgot to ohm out the compressor.

    I'll be ready next time I go back.

    I think all I'm thinking about is my vacation next week to Hawaii. I cant wait to leave all this crap behind for couple weeks

  9. #8
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    I never assume everyone uses a Fluke to check caps. Not after that fiasco of a thread entitled: "How to check capacitors?"
    I got to see clearly that there are a lot of old wives tales still floating around out there about proper troubleshooting proceedures.

    Some of the guys who posted to that thread are waaaaaayyyyy out there!

    Sounds to me like ya just had a brainn fart and will not make that error next time out.


    Have fun in Hawaii

  10. #9
    Join Date
    Nov 2014
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    A compressor is a motor.
    A motor gives off counter EMF.
    Always use the reading from just the Common terminal, but check them all anyways.

    The current you are reading from the start winding is including that which is energizing the potential relay's coil. Don't expect these numbers to ever add up.

    Even if there is no potential relay (The white one that the start cap is plugged into) you will still have counter emf to deal with. The amp draw of compressor is always checked with the common winding on its own.

    Aside from all of that, you have the capacitor which is slowing down the voltage and applying its phase shift to keep the motor pushing strong. You will get amp draw readings like this for all PSC motors.

    Do not feel bad, this is one of the "left field-est" things of HVAC.

    Just remember, go off the common.

  11. #10
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    To determine which terminals are whut, just ohm out the windings to each other.
    Biggest number = Start to run
    Smallest number = run to common
    other number = start to common

  12. #11
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    Quote Originally Posted by wasup23 View Post
    A compressor is a motor.
    A motor gives off counter EMF.
    Always use the reading from just the Common terminal, but check them all anyways.

    The current you are reading from the start winding is including that which is energizing the potential relay's coil. Don't expect these numbers to ever add up.

    Even if there is no potential relay (The white one that the start cap is plugged into) you will still have counter emf to deal with. The amp draw of compressor is always checked with the common winding on its own.

    Aside from all of that, you have the capacitor which is slowing down the voltage and applying its phase shift to keep the motor pushing strong. You will get amp draw readings like this for all PSC motors.

    Do not feel bad, this is one of the "left field-est" things of HVAC.

    Just remember, go off the common.


    Hey, don't worry about it!!! The compressor died 10 yrs ago!!!!

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  14. #12
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    What size is it? 10 amps doesnt seem all that high. You cant acess the rla on a new system. Check for loose loose wires, loose and lugs at breaker and disconnect. Check total amps of the circuit ect....

    Sent from my SM-G550T2 using Tapatalk

  15. #13
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    Quote Originally Posted by Core_d View Post
    What size is it? 10 amps doesnt seem all that high. You cant acess the rla on a new system. Check for loose loose wires, loose and lugs at breaker and disconnect. Check total amps of the circuit ect....

    Sent from my SM-G550T2 using Tapatalk


    Yeah, i'm wondering if someone stuck a 60 mfd cap on a 2 ton. When it should have been around a 30. Maybe Shaka remembers.

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