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  1. #1

    Causes of a failed 24V transformer and contractor coils???

    Beside lightning and damaged (shorted or grounded control) wires what could cause a failure on a control transformer?

    The unit is a split 10T HP with heat strips. Twice this unit's has control transformer and heat strip contractor coils have burned out? The unit is a 3P 240 HP. There are a total of four units at this facility and this is the only one that is blowing the control transformer? The transformer is a 75VA. The thermostat is located around 30 feet away. The 24 volt circuit is fused correctly.

    Any probable causes would be welcomed.

    Thanks,

  2. #2
    Join Date
    Mar 2013
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    Springville, NY
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    2,134
    i forget what manufacturer it was, and i've only worked on one of these units, a 3 phase rooftop unit...

    the oem transformers exploded with regularity causing melted copper to fly everywhere.

    i want to say carrier...but i could be wrong. whatever manufacturer it was had a bad batch of transformers.

    could be what's happening with yours. but hopefully it's something easier to diagnose.

    too high of a load on the transformer would give it some trouble.
    ~~
    Political Correctness - the Language of Wussies

  3. #3
    Join Date
    Jul 2007
    Location
    south bay california
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    132
    burned out? like caught on fire?
    voltage drop would be my first suspect. When the voltage goes down, the current goes up, and so does the heat. If it's a coil for a load, like the strip heat, maybe when the strip heat contactor pulls in the strip heat causes a voltage drop.
    throw an amp clamp on the entering power, operate the strip heat and see if you over amp. read the entering volts across all phases before, and while the strip heat is running.

  4. #4
    Join Date
    Mar 2007
    Location
    South Texas
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    203
    Read your incoming power and make sure the right voltage tap is being used, if it is set for 240 and you only have 208 your secondary voltage will be too low and will cause this issue see it alot

  5. #5
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    I think the transformer problem being share above is about a Trane rooftop unit.

    There are two main causes of transformer failure.

    1) An overload (too much current draw) on the secondary.
    This can be anything form a short to gr4ound of control wiring, a device failure, etc.

    2) A turn to turn short in the transformer. I'm not sure if an autotransformer (three leads) is more likely to have a failure than a two-winding unit, but I am currently awaiting parts to fix this damage:

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    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  6. #6
    Join Date
    Jan 2012
    Location
    lake of the ozarks,MO
    Posts
    279
    try to inspect all the wiring for possible shorts to ground. if the insulation is worn on the defrost sensor and the wind blows onto copper or water melts onto wires. intermittant low voltage shorts drive me crazy.

  7. #7
    Join Date
    May 2013
    Location
    Kelowna BC
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    Are you replacing just the coils on contractors or whole thing? If a contactor doesn't fully close it will draw substantially more current.

    Eg. Have seen damaged aux contacts mounted to contactors cause this issue a number of times.

  8. #8
    Join Date
    Nov 2006
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    Southeastern Pa
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    Quote Originally Posted by Lahrs View Post
    Are you replacing just the coils on contractors or whole thing? If a contactor doesn't fully close it will draw substantially more current.

    Eg. Have seen damaged aux contacts mounted to contactors cause this issue a number of times.
    That's a good thing to point out. The "pull in" current is grerater than the "holding" current.

    The moving part, called the armature (just like the spinning center of a wound rotor motor, because it moves) must be fully within the magnetic field of the contactor coil in order to focus enough counter emf to limit the control current. Think about what happens if you remove a solenoid coil from its valve, and then energize it. The coil becomes toasted, because there is not enough counter emf within the coil without that metal center to throttle down the current.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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