jgk@osc.COM (Joe Keane) (04/06/91)
Almost all `serious' power is three-phase, because that's the easiest to work with. In a power system the generators and transmission or sub-transmission lines are almost always three-phase. Also all motors of any substantial size are three-phase. It's only when you get down to a few houses that you use a single phase. I think Nikola Tesla deserves a lot of credit for coming up with the three-phase alternating-current system we use today. The neat part is that although the voltages are constantly changing, the total amount of power transmitted is constant. Now, to the original subject, converting single-phase to three-phase power. The executive summary is that it's a pain in the ass, and you shouldn't do it unless you have a good reason. It's a lot easier to just get the power company to run a couple more wires. There are basically two ways to do it, and neither one is easy. One way is to use some sort of rotating machinery. The simplest is just a motor-generator set. The real way people do it is more complicated to explain. The general idea is that you wire up a rotating machine so that it presents a low impendance to negative-sequence and zero-sequence power, while not affecting positive-sequence power. If you understand what this means then you probably already know what i'm talking about. Then you can feed power into one phase and have it come out balanced. This is like a phase balancer in reverse. The other way is to convert it to DC and back. You rectify and filter the incoming phase, and then invert back to three phases. Although this has the advantage of being solid-state, it's still pretty expensive. Why would you want to do such a thing? One example is electric trains. Clearly you can only get a single phase from a catenary or power rail. But you may still want to use three-phase motors, just because they're so nice. So this is a good application for phase conversion. -- Joe Keane, not a real EE jgk@osc.com (...!uunet!stratus!osc!jgk)