I sympathise with James May. He says he doesn’t believe in electricity.
He has a point. External combustion, I can understand it. You light a fire, it heats water, that turns to steam which expands with sufficient force to make an up and down thing go up and down which in turn makes a turning around thing turn around and off you go. Internal combustion too. You have a suck-squeeze-bang-blow process (if you have four strokes, otherwise it’s simultaneous suck-and-squeeze followed by bang merging into blow) which also makes an up and down thing go up and down and so on. Electricity though involves a mysterious invisible something travelling through solid metal and into a thing which it makes turn around, light up, heat up or do complicated sums.
Anyway, if we just suspend disbelief for a while we can consider that electricity can come in two forms. It can just go from one place to another in a steady stream like water or it can go in one direction and then almost immediately turn back on itself and go the other way. That would be alternating current or AC and it’s what you get in your domestic supply.
AC doesn’t just suddenly change direction though, it does so somewhat gradually and it can be shown as a graph like this, complete with complicated abbreviations which needn’t concern us (they certainly don’t concern me as I haven’t an idea what they mean):
So your electricity is going from maximum voltage in one direction through zero to maximum in the other direction. Believers in electricity call this single phase and it’s good for most of us. Not so good for more demanding users though because larger motors don’t like the fluctuations.
Enter three phase. Now you get three supplies of electrons, each of which follows a curve just like the one above. Single phase is (where I am) 230v so three phase should obviously be 230*3 or 690v. Except that it’s not. It’s 400v.
There’s a reason for that. Each of the three phases follows the same kind of curve, but they don’t overlap. Each is deliberately arranged to be one third of a cycle or 120deg apart, so we get this:
At any point on the horizontal axis (time) the total voltage is the total of the three phases. This, though, is never three or even two maxima. It’s always less than that. If you apply really complicated sums involving difficult things like square roots and the calculus which I was never able to understand you discover that (assuming that each phase is equal) the total voltage is the individual voltage, 230 in my case, multiplied by the square root of three which gives 400. So there we have it.
The real point though is that three phase has far less voltage variation than single phase - it never reaches zero - and large motors run much better on it.
If you find yourself of an American persuasion you get your domestic supply as two phases of about 120v each. Combine them and you actually do get 240v. Why? Because the two phases are 180deg apart and peaks coincide (albeit in opposite directions) so the maximum is indeed twice the individual voltage.