Single-phase vs three-phase generator

Question

I am in India. We have 3 phase power coming in from the grid. They don't do single phase for anything over 3 kVA. 4 wires from the pole to a fuse box. 415V, 3 wires each going to a separate fuse and neutral. All appliances are single phase. They are distributed across all 3 phases. All sockets have a ground, neutral and live. Is this a 3 phase wye? That would mean 240V at each socket which sounds correct.

Now if you were to get a generator would you do a single phase or 3 phase generator? 3 phase is slightly cheaper. How would the single phase generator be connected in this scenario?

Answer 1

If you have a 3-phase generator, you need a 4-pole changeover switch to switch between the grid and the generator, switching all 3 phases and neutral. Never allow the generator and grid to be connected at the same time.

If you have a single phase generator that is large enough to drive all the loads, then set up a 4-pole changeover switch to change between the grid supply and the generator. Again, this should switch all the phases and the neutral.

Then connect the single phase of the generator to all three phases on the generator inputs of the changeover switch. So long as you only have single phase loads, the loads won't notice the difference.

Note: 3 phase systems often use an under-sized neutral, on the basis that the maximum current in the neutral can never be more than that of one phase. This won't apply if you're using a single phase generator. Make sure your neutral wiring can carry the full current from the generator.

Answer 2

How would the single phase generator be connected in this scenario?

If you have a few critical loads that you want to power with a generator during an outage, a single-phase generator would be ok if those loads are all on one phase or if they can conveniently re-connected to one phase.

However you connect the generator, you must have either contactors or a switch that can safely transfer the loads to the generator and assure that the generator does not connect to the incoming feeder. If you want to connect more than one phase to a single-phase generator, you need two or three transfer switches. If you get a three-phase generator, you need a three-phase transfer switch.

If you have a generator that is too small to power all of the loads that can potentially be connected, you must either provide a means to prevent connecting them or a means for disconnecting the generator if the connected loads are too high. Fuses will do that, but you need to be careful to coordinate the fuse characteristics with the generator capability. In addition, blowing fuses will be a nuisance and possibly result in problems due to shutting off loads unexpectedly.

If you are going to size the generator just to supply critical loads, it might be convenient to connect those loads to one phase and size the generator to power everything on that phase.

Depending on your exact distribution configuration and intentions, there may be other details to consider.

Answer 3

I would say, 'Go for the 3 phase generator' for the following reasons:

1. Absolutely no changes would be required to be made in the existing wiring.
2. Back-up power would also be ensured for 3 phase loads which may be installed in the future.