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whistler avatar image
whistler asked

Autotransformer: 120/240VAC variable input (single-phase); always 240VAC output (single-phase).

Hi everyone,

I have a sailboat that is entirely wired for 220VAC (single phase), including all appliances aboard. I am currently in Europe.

In January I will cross the Atlantic and I want to be able to accept 110VAC (single phase) from the shore and step it up to 240VAC (single phase) for use aboard. Hence, I'm looking at an Autotransformer.

I'm trying to understand exactly how this device works.

(1) can I give it any voltage (110VAC or 220VAC) and have it always gracefully output 240VAC regardless of input voltage?

or,

(2) do I need to implement a transfer switch, so that when I am in a country with 240VAC I bypass the Autotransformer entirely?

I have studied the manual and believe it's the latter, but as we're talking about mains voltage I would appreciate the confirmation. Option two would be much less convenient because I also have a 220VAC generator and so I will regularly switch input voltages from 110VAC to 220VAC and back, and at some point I am sure to make a mistake and forget to operate the transfer switch...

Thank you!

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Autotransformer
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3 Answers
rickc avatar image
rickc answered ·

It is the latter, which is not convenient. The autotransformer has no intelligence or switch to automatically determine the voltage. But beware, as you’ve hinted above, if you don’t check the shore power voltage first with a meter, you might inadvertently upconvert 220V to 440V! It always does a 2:1 step up when wired that way.

I would also carefully watch how you handle neutrals and grounds. I would also be paranoid and use an isolation transformer as well for galvanic isolation.

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whistler avatar image
whistler answered ·

Brilliant, thank you so much! Any thoughts on the best / safest way to implement this? I could either have:

(1) an AC transfer switch that bypasses the Autotransformer when 240VAC is available

(2) two separate shore power sockets with a sliding door in front so that only one can ever be exposed at once. One feeds the autotransformer, the other feeds the inverter directly

When you say "be careful how you handle neutrals and grounds", could you expand on this? I expect to take ground from the shore in either scenario, and also expect to tie my grounds to the negative bus on board (ground via the engine block to the sea, and the battery negatives). Is this correct? What are the common pitfalls for the neutral? (I do not expect to tie neutral to earth).

I am a fibreglass boat so I wasn't planning to fit a galvanic isolator... although I do have metal shafts and anodes in the water

Thanks again

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rickc avatar image
rickc answered ·

https://www.cruisersforum.com/forums/f14/neutral-to-ground-with-shore-power-262096.html

Lots of good commentary here, one of them used an Autotransformer and had corrosion.


Also, see https://www.victronenergy.com/upload/documents/Datasheet-Isolation-Transformers-EN.pdf

The 3600W version appears to do the automatic voltage switching for you, but I would ask your Victron dealer to confirm this.

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