h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

What is the difference between single and three phase electricity?
If you need more specifics, it relates to generators powering remote monitoring equipment. One produces three phase electricity, and the other produces single phase. The electrician working on them seems to understand it (I hope!), but has not been very forthcoming with a simple explanation.
Thank you for your time.

Normal electricity comes in two wires. The electricity comes out from one wire and goes back in the other wire.

In direct current, which you get from batteries, the current comes in a continuous stream from one and flows into the other.

In alternating current, which is the normal type used in mains power, the current is constantly changing, 50 times every second (60 in America) so that sometimes it comes out from one wire and sometimes from the other.

Three phase current has basically four wires. Power comes out from three and returns via the fourth. While each of the three wires produces alternating current, they are all "out of phase" with each other, so as one is coming out, another is going in and the third is just on the point of turning. When the three of these are added together, they almost exactly cancel each other out, so the fourth wire is hardly needed at all and is sometimes omitted.

Three phase current is the norm for power transmission and for driving large motors. Industries usually get their power in three phase form. Power is delivered to housing estates in three phase form, but it is then spilt up, only one phase being delivered to each house.

Does that make any sense? It's hard to explain without diagrams.

I cannot thank you enough! that is the clearest explanation I have heard to date. If you know of any websites the explore this subject further, please post the url here.
Thank you.

I understood it too. But I would like somebody to explain to me why it was invented, what is it good for?

Two reasons for 3-phase electricity:

1. You can send one third of the power out on each of three lines and if managed properly so that the three loads are equal, you don't need any return wire. This reduces the number of wires greatly. This is why three-phase is used for all high voltage transmission.

2. A three phase motor is a very simple thing because the three phases can power three different coils in the motor, giving a smoothly rotating magnetic field with constant torque. By comparison a single phase motor gives a torque which pulses on and off 50 (or 60) times a second, causing strain on all the motor parts.