Can a bucking transformer get "high on its own supply"?

Question

There's a lot of industrial 277V in North America that's a few volts too hot to support level 2 EV charging. It just needs to be knocked down to roughly 240V, and that's not that hard to do with a small bucking transformer, thusly, outputting 241V (close enough).

For instance at 48A, the bucking transformer is 36V x 48A = 1.7 kVA. Not a big transformer. So this is pretty achievable.

^{simulate this circuit – Schematic created using CircuitLab}

Note that the current economics are a little weird here - even though 48A is flowing through the outer ring circuit, giving 48A @ 241V (11.5 kW), and the transformer is drawing 6.2 amps, the 277V draw isn't 48A + 6.2A = 54.2A... it's actually 48A - 6.2A = 41.8A.

However, 277V-36V transformers are far less common than 240V-36V transformers. Is it possible to feed the bucking transformer from its own output? Like this

^{simulate this circuit}

Answer 1

You are essentially creating an autotransformer. You can make such an autotransformer by wiring together the primary and secondary windings of a step-down transformer, and it will work fine providing you observe all the voltage and current specs for your transformer. Note that an autotransformer is not isolating. Make sure you don't need an isolating transformer in your application.

One other point worth mentioning. In your schematic, you place polarity dots on your transformer. In practice, it is often not obvious which way to connect a primary lead to a secondary lead. The easiest solution, AFAIK, is to test one way, and if that gives the wrong transformer ratio, swap either the primary lead, or the secondary lead. But it is important in general to do this testing at less than the normal operating voltage. Not only is it safer for you, but it is also safer for the transformer.