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Testing back to back power transformers for use as an isolation transformer

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intabits:
I want/need an isolation transformer, but they are not especially cheap. And as I have many other power transformers lying around this place, I should use two of those in a back to back configuration if it will provide a viable result. But I was concerned about voltage "sag" at the output under load, and temperature rise.

So I tested three handy pairs of transformers, using light bulbs as a dummy load, and with a variac to adjust the input voltage for a 240V output at the load. I was going to do power and efficiency calculations, but found the temperature rises to be quite small, and so just went with the "Input-voltage-for-240V-output" as the metric to compare them with.

Here are the transformer pairs used, with estimated VA ratings of 90VA, 150VA, and 2KVA, with weights of 2Kg, 4.5Kg and 22Kg.



Those very large ones served as a reference as what the very best I could come up with would be.

Here are the results of the testing:-



I have decided to make a general purpose isolation transformer box using the 150VA pair.

And I'll attach handles to the large ones, along with some connector arrangement to enable them to be quickly configured for any purpose, including as an isolation transformer, but still allowing them to be general purpose high power units.

Video:-

Cliff Matthews:
Each trafo sags ~10% from NL to full load, so 2 in series doubles the problem. I'll try-out this cct one day..

bob91343:
Many isolation transformers include a slight stepup to provide full output under load.  Having said that, your solution looks good to me.

Stay within ratings and you will have good results.

David Hess:
I have a pair of 1500 watt transformers configured back to back and mounted to a 2x6 that work fine as an isolation transformer.  Since they support 120V/240V with split windings, the output side actually provides two 120 volt AC isolated outputs or isolated 240 volts AC if needed from a 120 volt AC line.

intabits:
The secondary output of the big ones is 130V with 240V input, and the primary has 200V, 220V & 240V tappings, so I can also get 120V if I use the 220V input. (and I just found a possible need for that). (And having three of the beasts allows for all sorts of configurations)

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