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Thread: Transformer Impedance

  1. #1
    Join Date
    Feb 2009
    Allentown, PA

    Transformer Impedance

    I understand that transformer losses are a combination of core losses and I2R line losses.

    I believe transformer impedance is the combination of inductive impedance (No Loss) and I2R line losses.

    If you have a transformer with a high % impedance does that mean anything to you in terms of the efficiency of the transformer? Is a transformer with a 7.5% impedance common?

    I have a customer with a Dry Type GE 480 Delta - 240 Delta 75KVA 7.5%Z transformer which is operating too hot to even put your hand on the top center. The installation is only a year old and it is be operating at this temperature since installation. The transformer feeds a single machine, which runs continuously and I am sure it is fairly loaded. Kind of surprised to see 7.5% impedance. I am used to impedances more in the range of 4-6%.

  2. #2
    Join Date
    Feb 2005
    Charlotte, NC
    7.5% is common for one that small. You could have an overload, harmonic issue, installation issue, bad transfromer. Check loading and harmonics, if that is good test the transformer.

  3. #3
    Join Date
    Feb 2003
    The rated temperature rise of the windings of some dry type transformers is 150C over a 40 ambient. This can result in the enclosure being too hot to touch.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  4. #4
    Join Date
    Feb 2003
    In a transformer, impedance (%Z), regulation and efficiency do not have the same inter-relationships that most people assume.
    For example, a change in %Z from 5.75 to 7.5, can be accomplished with almost no detrimental effect on its efficiency.

    The operating temperature of a fully loaded transformer can ignite cardboard.
    As Don alluded to, the inside of the transformer can be over 450F
    Last edited by jim dungar; 11-29-10 at 09:23 PM.
    Just because you can, doesn't mean you should.

  5. #5
    Join Date
    Jul 2006
    SE USA as far as you can go
    Transformer impedance listed on the nameplate is the series impedance.

    The impedance is a combination of Copper losses (I sqd R), eddy current losses, hysteresis losses, and leakage flux.

    The percent impedance given is the magnitude of the combination of vector quantities, and is associated with the base rating of the transformer.

    According to IEEE Std C57.12.01, Table 12 the allowable temperature rise of external parts, over ambient would be 65 deg C, regardless of internal temp ratings. So, with a 40 deg C ambient, the external temp should not be over 105 deg C. Keep in mind that is 221 deg F, and water boils at 100 deg C or 212 deg F under standard conditions at sea level (at one atmosphere of pressure).

    For a std general purpose dry type 75KVA the 7.5% does seem high. I might expect something between 1.5% and 2.6%. 7.5% is high for any type that size, but it is what it is, just means it's poorly designed

    If it is also making a lot of noise, then you could be exceeding the allowable dbA level, and is probably due to winding vibration. Heat could also be from non-linear loads if you have them.

    Sounds like you just have a cheaply made transformer.
    "Just because you're paranoid, doesn't mean they're not out to get you"

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