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Thread: Transformer

  1. #1
    Join Date
    Feb 2005
    Posts
    344
    What would make a control (24vac)transformer primary (230vac)open up?

    This unit is tripping a 3.2 amp overload installed in the secondary circuit when a call for heat occurs.

    I know there must be a short in the circuitry. I am curious why the primary would open instead of the secondary.

    Any thoughts?

    Best regards...

  2. #2
    Join Date
    May 2003
    Location
    Dixie
    Posts
    1,344
    if the 3 amp is tripping then wouldn't it be the secondary instead of the primary?...

  3. #3
    Join Date
    Feb 2005
    Posts
    344

    frigetater

    I did a poor job of making this clear. On several occasions the 3.2 amp breaker tripped. It is in the secondary side.

    One time the primary winding burned.

    I was wondering why the primary opened instead of tripping the breaker in the secondary.

  4. #4
    Join Date
    Jun 2003
    Location
    Florida
    Posts
    9,548
    If the primary burned it should have thrown the breaker/fuse in the panel/disconnect. Time for a new transformer and find that short. Was the multi-tap wiring on the primary side correct? I've seen people leave it wired from the factory instead of correcting for actual voltage.
    If everything was always done "by the book"....the book would never change.

  5. #5
    Join Date
    Feb 2005
    Posts
    344

    smokin68

    Good point. I don't remember if the tech mentioned checking for proper primary wiring.

    Thanks, I'll let you know what we find.

    Best regards...

  6. #6
    Join Date
    Oct 2005
    Location
    Fairfax, VA
    Posts
    111
    Heat buildup?
    If the secondary breaker is tripping you have some wiring problems. Are there any solenoid valves in the system? Taking the coil off a SV will pull big amps.

  7. #7
    Join Date
    Jan 2001
    Location
    Nebraska
    Posts
    319
    Transformers that step voltages up or down have different size wire in the primary and secondary windings.

    For example a 120 to 24 volt step-down transformer rated at 40VA will have the wire sized in both the primary and secondary to handle the VA rating. If the primary of this transformer is sized to handle 40VA the windings must be capable of carrying .333 amps (VA/V 40/120 =.3333).

    In the secondary at rating load the winding must be able to carry 1.666 Amps at rated load.(VA/V 40/24 = 1.66 amps)


    The basic relationship of this transformer is 120/24 or a 5:1 ratio, so if the current load on the secondary is 1.66 amps this is reflected in to the primary as 1/5 of that load or 1.66/5= .333amps.

    Since this is a step down ratio the primary will have approximately 5 x more turns than the secondary but will be of a lighter gauge wire. If you have a load exceeding 3 amps in the secondary (must have if the fuse blows) 1/5 of the current flowing in the secondary will flow in the primary. Since both windings are overloaded it is just a horse race to see which burns out first. It is not a all unusual to see the primary go before the secondary even though the overload is in the secondary side.

  8. #8
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    Tomr

    No need for my post as you stated it well. Tom says the truth

    Richard

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