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Transformer Current Output

Discussion in 'Power Electronics' started by Electromotive, May 6, 2020.

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  1. Electromotive

    Electromotive

    34
    3
    Feb 14, 2018
    Can anyone advise how I can determine (measure or calculate) the current output (in Amps) of a small 120vac/12vac transformer?
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Does it have a va rating anywhere marked..?
     
  3. Electromotive

    Electromotive

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    Feb 14, 2018
    No markings... That's the issue. thanks...Tom
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not easy to do. Try the steps described here.

    Edit:
    An idea for a more pragmatic approach:
    It seems that the standard EN 61558-1 allows a max. temperature of the transformer windings of 100 °C for material of class A1, more for higher quality materials. Since we do not know what materials your transformer is made of, 100 °C (~210 °F) seems to be a safe limit.
    You could determine the max. current by:
    - apply a reasonable load current (depending on the size of the transformer use 50 % of the rated current of a similarly sized transformer), by adding power resistors to the secondary side.
    - wait for e.g. 30 minutes and measure the temperature of the coil (measure at a few different points to get a valid reading).
    - If the temperature of the transformer is below 100 °C, increase the load current in increments of e.g. 10 % (or more if the transformer is really cold) by adding more resistors (in parallel). Again wait, then measure. Continue until you reach a winding temperature of ~100 °C. That should set the max. output current for reliable operation.
    If the temperature is above 100 °C, decrease the load and perform essentially the same steps as described before in the reverse direction (lower current).

    Add a fuse on the primary side with a current rating 1/10 of the load current (or a bit above) for overload protection.
    Be aware of the temperature of the environment while doing these measurements. If you want to operate the transformer in a closed housing, possibly badly vented, the environmental temperature of the transformer will be higher than during your initial measurements. Accordingly the winding temperature will also rise to a higher value. To avoid exceeding 100 °C, take this effect into account and lower the current accordingly,
     
    Last edited: May 6, 2020
    Electromotive likes this.
  5. bertus

    bertus Moderator

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    Nov 8, 2019
  6. Electromotive

    Electromotive

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    Feb 14, 2018
    Some very helpful information here. If the below doesn't mean anything to anyone here, I will try the Power resistor technique..Thank you.
    However: Removed the transformer from the enclosure and found this: LEI Logo in a circle, and 60LEIB-002?
    Anyone?
     
  7. Bluejets

    Bluejets

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    Oct 5, 2014
    Electromotive likes this.
  8. Electromotive

    Electromotive

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    Feb 14, 2018
    Bluejets: Thank you. This is great information. Except, mine doesn't have all that rust on it.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Another technique is to weigh the transformer. Generally speaking, transformers of a similar weight have a similar VA rating. This can vary for all sorts of reasons, but likely puts you in the ballpark. At the very least it provides a starting point for determining the load resistor(s) to try.
     
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