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I plan to use an autotransformer to step up from 120v to 240v for a 240v multiplus. I will sometimes have 240v (50 amp split-phase) shore power available and will bypass the autotransformer. Do I need to isolate the incoming 240v from the output of the autotransformer? When 240v power is present there will be no 120v connected to the input side of the autotransformer.
You should be okay doing that. However, if you have 240V on the transformer, the 120V (center tap) will be energized. You need a way to make those wires safe.
You could also elect to open the breaker on the unit when you're not using it, although I don't care for that plan too much.
Thanks again for the helpful reply. I hadnt thought about the center tap being energized, autotransformers are new to me. Perhaps a big 3 position rotary switch before the Multiplus to select between the autotransformer output or the 240v shore line.
Maybe draw a picture of your plan and post it here. What I was imagining you were going to do wouldn’t work with a switch there, but you might have a better idea.
Actually, the center tap should be at 0v. As long as it's floating or grounded you will have no issue. The center will be a created neutral. As referenced to either hot leg it will be 110v (since you mention 50hz I'm assuming 220v L-L). If by chance you run into a grid with a neutral (North America) that autotransformer will try to balance the two hot legs versus the neutral and there is the possibility of overloading it, especially if your circuit is shared by other users. (RV park or Marina is a common example)
1. The center tap will be at 120V with respect to either line. That means a set of feed wires running out of the transformer will be energized, at 120V.
2. He doesn’t mention 50Hz. And we are on a 60Hz system here.
3. Just for the OP’s reference, USA has been at 120V nominal for a long time. 110V is an anachronism.
4. When he is feeding 240V, he should have only three incoming conductors, including the safety ground.
