208 volts transformer powered by 240 volts

[tshea]

Looked at a job. Customer has a 75kva 480-208/120 transformer being powered by 240 volt 3 phase. Taps were adjusted so output was about 480. Transformer was noisy, lots of humming.
Transformer is basically being backfed to produce a higher than available voltage. I suggested to customer to replace with a 240-480 transformer. I suspect the noise would be reduced. The more I talked with him, I discovered there is only one machine using the 480 power and it is not running at this time and probably won't for another few months. I suggested shutting the transformer down unless he needed the extra heat.
We'll see what he decides in a day or two.

 

[infinity]

Could applying 240 volts to a set of coils rated for 208 have something to do with it? Typically the taps are on the HV side.

 

[JFletcher]

Looked at a job. Customer has a 75kva 480-208/120 transformer being powered by 240 volt 3 phase. Taps were adjusted so output was about 480. Transformer was noisy, lots of humming.
Transformer is basically being backfed to produce a higher than available voltage. I suggested to customer to replace with a 240-480 transformer. I suspect the noise would be reduced. The more I talked with him, I discovered there is only one machine using the 480 power and it is not running at this time and probably won't for another few months. I suggested shutting the transformer down unless he needed the extra heat.
We'll see what he decides in a day or two.

240/208 = 15.4% over nominal, which would put out 553V on the 480V taps, or ~498V on the 432V taps (low as I think they go). Using a proper step up 240/480 xfmr would be better, as you suggested.

 

[Electric-Light]

Looked at a job. Customer has a 75kva 480-208/120 transformer being powered by 240 volt 3 phase. Taps were adjusted so output was about 480. Transformer was noisy, lots of humming.
Transformer is basically being backfed to produce a higher than available voltage. I suggested to customer to replace with a 240-480 transformer. I suspect the noise would be reduced. The more I talked with him, I discovered there is only one machine using the 480 power and it is not running at this time and probably won't for another few months. I suggested shutting the transformer down unless he needed the extra heat.
We'll see what he decides in a day or two.

The no load power loss goes up quite a bit as well when you do this. If the usage and controls allow unplugging and plugging only to use it, it should be fine to switch at the line side and turn it off after each use through a remote contactor.

Transformer can push more power per pound when you push the core harder. A microwave step up transformer has quite the no load loss, but it is fine for this application, because it is either off or fully loaded and do not need to worry about no load losses.