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# Operating a transformer at half the voltage? Answered

What could be the effects of the output when operating a transformer at half its rated voltage, specifically to the amperage?

as two examples:

If I had a transformer rated for an input of 240v and an output of 20A at 24v and I was to supply the primary with 120v, what would happen to the output?

If I had a variac (autotransformer, variable transformer, ect.) and it was rated for an input of 240v with an output of 0-260v at 11A, what would happen to the output ratings at only 120v?

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Transformers work by raising or dropping voltage in a ratio. In your case, the transformer in question is a 10-1 step down transformer because it steps 240v down to 24v. What would happen if you run it at 120v is the output would drop to 12v. I don't believe the amperage rating would change, but the output might based on Ohm's law. If voltage goes down while resistance load remains the same, the current will also drop. The output of the variac should also be halved based on the input voltage. This is based on what I remember from my degree in electronics from almost 40 years ago so be sure to test these assertions before connecting it to anything important. A multimeter will tell you the output voltage when connected to 120v. I hope this helps.

For an ideal transformer the input power is equivalent to the output power. Real transformers tent towards a small reduction in power at the output (that's why they get warm).

So if your 240 to 24v transformer has an output of 24V at 20A or 480W the input current would be 480/240 = 2A . Your transformer will be wound with suitable wire to pass these currents.

If you supply the transformer with 120V you will get a 12V output. 20A at 12V would be 240W so the input would be approx 240/120 = 2A.

Please note: The transformer will most likely have been designed to work at 240V 50Hz. I assume you will be operating the transformer at 120V 60Hz. This may affect the efficiency of the transformer.

Hope this helps.

A note about going from 50cps or Hz to 60Hz...

The 50Hz xfmr is designed to just avoid saturating the iron core ( bad thing ) which at the faster 60Hz is well above iron core saturation... I would say that you could increase the current by 6/5 or 24A, keeping in mind that the wire gauge, which doesn't care about this small frequency change, will get hotter and loose a little voltage due to RI losses.

FYI as the frequency increases transformers can use less iron and begin to weigh less... Which was the main reason WWII bomber aircraft adopted a 400cps for electronics like to run remote electric gun turrets...