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European AC problem is just a phase

Jan 1, 2003

Question: We have a shore power system typical of many American-built boats — 120/240-volt, 50-amp service — which consists of a ground, a neutral and two 120-volt hot circuits that are 180° out of phase. We have some 120-volt appliances onboard, which are run from one or the other of those 120-volt circuits, but we also have a 240-volt deep-freeze and 240-volt heating/air conditioning that is powered by combining both 120-volt circuits to create 240-volt twin-phase electricity.

Here in Europe, the dockside power is either 230-volt single-phase or 380-volt three-phase. We have a transformer that converts the 230 to 120 volts, but I cannot figure out how to get two 120 out-of-phase legs. If I set up two transformers on the dock, both plugged into 230-volt European power, they create two in-phase legs. Therefore, when I wire those two legs to the two hot wires in our shore-power circuit, all the 110-volt appliances are powered properly, but the voltage between the two hot legs at the 240-volt appliances is zero, since the incoming 110 circuits are in phase.

The line frequency here is 50 Hz instead of 60 Hz, but fortunately, the appliances we need to run are built to operate properly on either.

I've thought of trying to use two out-of-phase legs of a 380-volt circuit, since each phase is 230 volts, but they are 120° out of phase, not 180°, and I don't know how that would work.

Willy Ritch-Smith and Maggie Gardiner are voyaging in the Mediterranean with their two daughters, aboard their 57-foot Dudley Dix-designed steel sloop.

Answer: The question you pose is very interesting. Are you running a standard manufacture isolation transformer aboard that is rated for 50/60-Hz operation? (If it's not rated for 50/60 Hz, you may have to derate the output by 20 percent.) If so, the fact that the European shore power is single-phase, rather than two-phase, should not matter.

The equipment you are operating, if it is designed to operate on 220 VAC, 50 or 60 Hz, should operate properly whether the 220 VAC is single- or two-phase. The problem arises in tapping the proper voltage from your transformer. If you are utilizing a transformer and automatic voltage-sensing and -switching device, then it is stepping the voltage down for both of your 120-volt legs, which are now both in phase (this is really irrelevant, since only single-phase devices are operating).

Some rewiring will be necessary, no matter which way your panel is now supplied, because the transformer does not offer two phases where only one is available ashore, which is the case here. You should be able to tap 230 from the transformer's secondary winding (most isolation transformers provide you with access lugs for no-voltage reduction output), running it directly to the breakers for the 220-VAC devices (while observing all American Boat and Yacht Council shore-power safety standards). There is no practical way to derive two phases from one, for an application such as this. In fact, the reason these appliances are usually designed to operate on 50 or 60 Hz is for applications such as this.

I've reviewed this scenario with Harris Allen, a technical advisor for Mastervolt, a manufacturer of isolation transformers and switch gear. He has confirmed that the 220-VAC gear should work well on European 230-VAC, single-phase. n

Steve C. D'Antonio is a marine writer and photographer and the boatyard manager at Zimmerman Marine in Mathews, Va.