Posted by MarkB_MI on December 14, 2008 at 13:06:45 from (216.234.125.55):
In Reply to: Electrical posted by tlak on December 14, 2008 at 11:09:58:
All AC power generated in the US is three-phase. Single phase power is just one leg of three phase.
Polyphase power was invented by Nicola Tesla, and made large-scale AC power generation practical. Although it is possible to have more than three phases, there's no practical reason to do so. The advantage of three phase over single phase is that it provides much more constant power, so big generators and motors don't vibrate themselves to pieces. With single phase power, instantaneous power changes at 120 cycles per second. In a three phase device, each phase is 120 degrees out of phase with the other two; this overlapping means that power is nearly constant and almost eliminates vibration.
The two phases, or legs, in a single phase 240 circuit are 180 degrees out of phase with each other, but we still call it "single phase".
If you look at any AC transmission line, you will almost always see three conductors: Three phases and a neutral. (Sometimes you'll see five conductors because there's two neutrals.) If you see a transmission line with only two conductors, it is probably DC.
There are actually two configurations for three phase: "wye" (or "Y") and "delta". A wye-wound motor has one end of each winding tied to a common neutral. A delta motor has no neutral, each end of each winding is connected to the other two windings.
If you want to see what a three phase generator looks like, just take apart your car's alternator.
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