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Transformer current Rating

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Joined: 31 Mar 2007
Posts: 22

PostPosted: Wed May 02, 2007 10:58 am    Post subject: Transformer current Rating Reply with quote

I pulled a transformer from a UPS, and was wondering approximatly how much current it is rated for (even a real rough estimate is all I'm looking for). It is about 5"X5"X5". The output voltage is rated at 31V (CT, and voltages are marked). The wires coming from it are #10. (I really wish I could take a picture of it, but my dad took his digital camera for the weekend).

Any guesses are welcome.... If you want to know something a little more specific, just ask
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Joined: 10 Aug 2005
Posts: 20
Location: Oakland, California

PostPosted: Wed May 02, 2007 5:07 pm    Post subject: Reply with quote

The POWER rating of a transformer of a given physical configuration is reasonably related to its core dimensions, assuming reasonable utilization of core capabilities. Check some catalogs like DigiKey or Mouser or some transformer manufacturer's sites to see what power transformers of the same core dimensions as yours are rated at, then compute the current output for the voltage of your transformer.

Another method of determining transformer capability is to load it down with a variable load, very gradually raising the output current until transformer gets warm, but not so hot it is uncomfortable to touch. Both of these techniques work best for a transformer with a single secondary winding, since with multiple windings you don't know how the power dissipation was intended by the manufacturer to be divided between the various windings.

You can create a variable load with other transformers and/or Variacs (variable autotransformers) and an array of light bulbs or a space heater. Use your imagination, but be careful of current and power ratings of the items you use for the load.

To avoid burning out the transformer under test once you approach the current limits you must limit the increase output current to small increments and let the temperature stabilize for an hour or so before continuing because it will take a long time for the heat being generated deep within the windings to completely saturate (thermally) the transformer.

Remember that this type of test gives you an indication of power capability in the environment of the test. If the transformer will be in a confined space in actual use, it's power output must be derated or the test must be done in the intended environment and ambient temperature.

I am not sure of this, but I'm guessing that a thermally saturated core surface temperature of about 125 degrees F would be quite acceptable. See if you can find specs on that. This is comfortable to touch. 140 degrees F is too hot to touch for long and too hot for an ordinary transformer for extended periods.

This is all guesswork. I am only a hobbiest, not a transformer engineer.

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