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UPS transformer with 3 wires altogether

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max.wwwang:
Hi,

I torn town another Eaton UPS today. I'm surprised it uses a transformer with only 3 wires altogether. Refer to the photos, they are Red, Brown & Blue. I measured the resistance between all of them. It's about 12 Ohm Brown-Red and Brown-Blue, about 3 Ohm Red-Blue. I guess it's an ”autotransformer" with only one winding. I hooked Brown/Red with mains 240V AC and tested voltage between Red/Blue I got about 32V AC. The voltage between Brown/Blue is about 270V.

From this, I think Brown/Blue is the full winding, and Red is in the middle. If I hook Brown/Blue to 24V AC [this should be 240V AC], I would be able to get 28V AC between Reb/Blue and 212V between Brown/Red. Is my understanding correct? I can easily have another test.

My main question, though, is - what's the point of not using a transformer with two separate windings which provides electrical isolation between the primary and secondary? Also, is this transformer good to be used in a power supply? (Its high capacity looks useful)

Thanks.

max.wwwang:
More photos.

andy3055:
Your reasoning seems correct on the first part. As for feeding 24 volts between Brn/Blu, you will not get anything more than that between any of the taps because those are the 2 ends of the complete winding. If you are feeding 240 volts between Brn/Blu, you will get 211.76 volts from Brn/Blu. I believe you missed a '0' there.

As for why they used an auto-transformer in this case, it is because it is cheap to produce and it is not supposed to provide mains isolation in this case anyway. And so, for that very reason, this will not be a good candidate for a bench power supply.

max.wwwang:

--- Quote from: andy3055 on February 15, 2020, 07:59:46 am ---Your reasoning seems correct on the first part. As for feeding 24 volts between Brn/Blu, you will not get anything more than that between any of the taps because those are the 2 ends of the complete winding. If you are feeding 240 volts between Brn/Blu, you will get 211.76 volts from Brn/Blu. I believe you missed a '0' there.

As for why they used an auto-transformer in this case, it is because it is cheap to produce and it is not supposed to provide mains isolation in this case anyway. And so, for that very reason, this will not be a good candidate for a bench power supply.

--- End quote ---

Yes, I missed a 0; it should be 240V AC. And another test confirmed this. Pity this good beast is not suitable for a DIY bench top power supply!

andy3055:
You can always work out the turns ratio and rewind a secondary with a smaller wire to suit your need discarding the smaller winding from the existing to make room. It is not that hard. If you go that route, make sure to measure/note the voltage from the smaller section you are discarding and count the number of turns so that you can work out the turns ratio.

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