When we talk about three phase power, most people glaze over and leave any discussion to the experts. We’re here to show you that it need not be that difficult.
Take a single phase power system. You can calculate the power you need easily, by multiplying the voltage (230V in Europe) by the current draw from your load (in Amps) and you’ve got the VA rating. If you are unsure of power factor (as most of us will be without special equipment) multiply this number by about 1.4 and choose an Uninterruptible Power Supply above this value. You’ll have no problems. But what about a three phase system?
A three phase UPS is, in its simplest form, three identical single phase UPS Systems stuck together, and you cannot overload any one of these systems.
Imagine you have a three phase load, of which you have split into several load sections – for example, the lighting circuit, the electrical distribution circuit and the cooling circuit. Each of which is a single phase load. You switch everything on, take your ammeter and measure the current flowing in each phase, and get, for example, 15A, 25A, 40A. So what size UPS do you need?
You could say that our current draw is 15+25+40 which is 80 and multiply this by 230 which gives us 18,400VA. So we need a three phase UPS rated at, say 20KVA. This is wrong, and it is wrong because you need to remember the three phase UPS is three – identically rated – single phase UPS Systems.
What you need to do is to take the maximum current draw, which in this case is 40A, and work this out as a single phase UPS. So we get 40×230 = 9200VA. Then multiply this by 3. The actual size of UPS we need is not 20KVA but actually 30KVA (9200×3=27,600).
Modern three phase UPS Systems can cope with 100% unbalanced loads, that is one phase is producing all the power and the others are supplying zero, but they cannot borrow power from one phase to the other.