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Comment by tripa | Hacker News Reader

This is the weirdest rationalization of three-phase power I've ever read. By your reasoning and disregarding cos phi issues, monophase power is also constant-zero power, since it's two wires anyway and one is always the opposite of the other.

The best explanation of why three I was ever given hinged on "it's the number-of-phases argmin for the needed volume of wiring".

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Three phase power is extraordinarily useful for electric motors. Mount three electromagnets radially and wire each up to a different phase, and you get a rotating magnetic field that spins on the axis. Electric generators produce three-phase power pretty naturally through the inverse process of spinning a magnet through the resistance provided by three groups of windings.

You can have a two phase power system[1], but two phase generators are less efficient and more expensive relative to three phase – for the exact same reason why power distribution is less efficient and more expensive relative to three phase.

[1] two phases in quadrature, which is very different from split phase, which is not two phase at all.

IIRC these designs don't naturally have torque at zero speed like three-phase induction motors do.

Yes. One of Tesla's more useful inventions was figuring out an simple way to get a synchronous motor started. Pre-Tesla schemes involved auxiliary starting motors and clutches. Tesla figured out how to run a synchronous motor in induction motor mode during starting.

To clarify, do you mean Tesla the person or Tesla the car company? I figure the answer should probably be obvious to me, but I'm not an electronics person and it's not.

Nikola Tesla, the late 19th / early 20th century inventor. Induction motors have been around for over a century.

Two-phase 90 degrees apart has zero-rpm torque, like a steam locomotive, the problem is that torque is variable. Three phase torque is constant.

You are right. Three-phase needs less copper wire for the same amount of power delivery, which is the real reason everyone uses it. It needs less copper because it never has "dead time" where the copper isn't delivering power (hence the desire for constant instantaneous power).

I tried to word my explanation as accurately as I could, but I had to leave out a lot of tricky details for people who aren't electrical engineers. You have to be careful not to mix up regular averages, RMS averages, and instantaneous values when you deal with AC. If you get them wrong, you come to incorrect conclusions, such as the idea that single-phase electricity has zero power. The average voltage is indeed zero, but you can't use regular averages when you calculate power delivery, only instantaneous voltage differences and currents. You get RMS averages when you integrate those instantaneous values over time, and that brings in the the cos(phi) issues you mention.

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