Using A Three Phase Generator For Single Phase Loads

As an industrial generator rental business owner, I always cringe when a customer tells me that they want to strength a single phase load with one of my three phase generators. It is an inefficient operation for both the electricity producing generator in addition as the (typically diesel) engine. It is however, thoroughly possible and very easy for anyone with a measure of mechanical and mathematical aptitude to accomplish. Fair warning though, if you do your math incorrectly, you can easily end up causing expensive damage to the generator, and the engine can suffer in addition.

You cant buy a dedicated 1 phase generator that will produce more than about 10kw. It would be cost prohibitive to build and market because beyond that level, 3 phase machines are smaller and cheaper to make, and only a comparatively small number of people would need more than 10kw single phase anyway.

Sometimes the generator manufacturer will stamp a larger generator as 1 phase, but it is truly a 3 phase generator that has been rewired and derated to strength single phase loads. People who require more than 10kw usually have a three phase electrical service installed as it is much more efficient. Mostly Ive found the folks who require larger than 10kw single phase to be farmers who strength grain dryers or barns, and sometimes machine shops or small restaurants that need permanent strength.

THE GENERATOR MATH…

You NEED to derate the generator when powering single phase loads. Lets say the nameplate on the machine labels it as a 208 volt, 3 phase, 20 kw generator. You will need to subtract 1/3 of the nameplate kw to get a single phase rating. In this case 13.2 kw is your single phase output. Keep it below this level and youre golden.

THE DANGERS…

If you use more than 13.2 kw, the machine will nevertheless work great all the way up until the engine begins to labor. Thats going to happen at around 20 kw. The problem is that in order to produce single phase strength, only 2 of the 3 windings are used (thats why we derated by 1/3). Those 2 windings are now doing the work of 3. The generator will not closest fail, but heat will most definitely build up in the windings as more amps than rated are forced by them. This will rule to premature failure of the generator.

Assuming a diesel engine is used, there is no real danger operating at single phase, but diesel engines work best between 50% and 90% capacity. Anything below a 10kw load (on our example) will cause a condition called wet stacking to occur. This causes a loss of output ability, oil leakage around exhaust elements, oil burning, and sometimes soot coming out the exhaust. Use a load bank or increase the load to capacity to correct this condition.

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