# Phase shifting one leg of a 3 phase 5kw generator

#### MrSoftware

Joined Oct 29, 2013
2,144
This is just a curious question, triggered by me seeing some army surplus 3 phase 5kw generators on youtube. Apparently they give you (3) 120V legs each at 120 degrees, so if you need 120V single phase then no problem, just use one leg. But assume you want 240V. You can use a step-up transformer on a single leg, or you can use 2 legs and get 208V (because the 2 legs are 120 degrees out of phase). But say you want 240V and you want to use 2 legs for more power. What would it take to shift one leg by 70 degrees while maintaining 120V?

#### panic mode

Joined Oct 10, 2011
2,524
phase shift in generators is accomplished by geometry (physical displacement of windings).
so without moving coil or rewinding i don't see a way to do it. but if you provide specific model and some pictures, there may be a way to hack something....

#### crutschow

Joined Mar 14, 2008
33,343
The easiest is probably to use a step-up autotransformer on the 208V to get to 240V.

Here is a discussion of some other options that give a better (but not exact) balance of the load among the three phases. .

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#### MisterBill2

Joined Jan 23, 2018
16,521
Over the years I have seen three phase power of different arrangements, and I have often just connected a 240 volt load across two legs of a 3-phase supply, and never had any problems at all. The only caution is that neither side is "neutral", and so all parts of the feed are able to deliver a shock. So avoid using a white wire for either side.
And one more thought: look at the output of a 3-phase transformer in the DELTA configuration and see that relative to one end the other end is 180 degrees out of phase. Strange to consider but totally true.
That is the beauty of 3-phase power.

#### MrSoftware

Joined Oct 29, 2013
2,144
This is the one that caught my interest:

I just stumbled across this same guy explaining how to convert it (mechanically) to single phase:

But I'm still curious about electrical solutions, such as what @crutschow posted, just for curiosity sake.

#### MisterBill2

Joined Jan 23, 2018
16,521
This is just a curious question, triggered by me seeing some army surplus 3 phase 5kw generators on youtube. Apparently they give you (3) 120V legs each at 120 degrees, so if you need 120V single phase then no problem, just use one leg. But assume you want 240V. You can use a step-up transformer on a single leg, or you can use 2 legs and get 208V (because the 2 legs are 120 degrees out of phase). But say you want 240V and you want to use 2 legs for more power. What would it take to shift one leg by 70 degrees while maintaining 120V?
OK, now in reference to the original question, there are two different arrangements for the distribution of power. The more current one, which has been around for a lot of years, maybe 50 or more, is effectively three phases in a "Y" arrangement, with a neutral at the center junction of the "Y". Each line then is 120 volts from neutral. And all three lines are the same 120 volts above neutral, with 208 volts between lines, due to the 120 degrees phase difference.
There is also another 3-phse standard found in a lot of locations, which has 240 volts between phases, and has for two of the three 120 volts to a neutral, and a lower voltage, around 100 volts, from the third phase to neutral. In a single phase panel such as in most houses that third phase is not present, and so the two lines with 240 volts between and 120 volts to neutral are used, along with the neutral. I have dealt with both versions, and most likely the surplus version generators are of the "Y" type, where the voltages may not be exactly 208 volts and 120 volts. Most equipment that uses the higher voltage is rated 208/230 volts, or even 208/220 volts. There are very few applications that I can think of that would need the full 240 volt single phase.