Questions regarding an isolation transformer build

[flinx]

I am building an isolation transformer from the transformers pulled from 2 similar APC Backups UPS. Both units were 650s with rating of 410 watts. Currently I am testing it by connecting it to a dim bulb tester that is connected to a variac.

When first powered up the 100w bulb will flash say about medium brightness then settle down to very dim where you can see the filament glow a dull red. full power on the variac give me an slightly brighter orange glow.

I have left the setup running for more than a half hour this way and transformers never get warm.

Currently I am testing it without the dim bulb tester in the middle.

In the attached image you can see the way it is connected and the measurements from the first transformer. The second transformer has the same resistance measurements.

I have tried various combinations of connecting the 2 transformers together, white and yellow to white and yellow, white and purple to white and purple, yellow and purple to yellow and purple, yellow and white to white and yellow, etc. the best combination is the first one and the last one. the other ones output about 6 volts less.

Any suggestions on how to improve the circuit?

Edit:
After 1 hour the transformers both measure 99° f since they are class 130(b) I am guessing that is pretty good.

[Agent24]

Re: Questions regarding an isolation transformer build

I would expect that's about as good as you're going to get.

I would add a circuit breaker and fuse so you don't overload the transformers.