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Hi Anton

Here are a few answers provided by one of our RedR Experts:

1/ As a general rule you can run single phase loads off a 3 phase generator but as noted it is better to try and balance the loads across the phases (it’s the basis of all grid connected systems, just they have lots of different loads , see also 9/ below.

2/ Remember that the power rating for a three phase generator is the total for the three (single) phases. This means that it supplies only 1/3 of that rating for each single phase. So a 15KVA generator can supply only 5 KVA for each of the single phases. You can’t cheat and have say 10KVA on one phase and nothing on the others – you’ll burn out the windings if it doesn’t trip on overload.

3/ As well as generator KVA the actual KW loads that the generator can supply will be 20% less. The 15KVA generator can only supply 12 kW (as a total of three phases) or max of 4 KW on each phase.

4/ The problems being talked about (undervoltage etc) seem to be indicating that the generator is being overloaded on the phase with the air conditioning load/s, not that the neutral point its drifting (it doesn’t.)

5/ Another cause of undervoltage trips could be the very short duration inrush current you get at the instant motors start. So generators have to be sized to be 2 to 3 times larger than the running load to provide enough capacity for this startup.

6/ Changing the undervoltage trip is not going to damage the generator, but the connected load may draw higher current and if the connected load has no overload protection this could cause damage or decrease service life.

7/ Ultimately a generator will stall if the load is to large for the load it is connected to, so if you set the undervoltage to low you may end up with a stalled generator.

8/ The overvoltage trip may just be the “slow” response from the generator regulating voltage device and/or engine governor when the high load turns off . This is more likely if the generator is not large enough.

9/ It’s very hard to distribute loads across three phases on small installations with only a few loads you really need to use a single phase generator, particularly if this is for long term use. They are available up to about 25kVA but may not be universally available.

I hope this helps to answer your question. Please let us know what you do and if you need more information you can post another question here on KnowledgePoint.

Harriette