AC motors with more than three phases

We could build large ones (250-25,000 HP) but that's probably not what you want.

Where are you going to get more phases to run it with? There aren't many people making drives for more than three phases, and they would be custom.

I don't think you would get any more efficiency, since the same amount of steel and copper goes into the motor. If you run it from a drive, you can lose a phase and it will keep running, which a three-phase probably wouldn't (couldn't start it, anyway). Also, you can cut the current rating on the semiconductors (but then you need more of them).

I have some literature on the "Chorus" product, and IMHO, it's mostly fluff. They are using the fifth (or other) harmonic to get extra torque, but I think on any normal system the benifits would be outweighed by the extra complexity.

See "Disturbance-Free Operation of a Multiphase Current-Regulated Motor Drive with an Opened Phase," Jen-Ren Fu and Thomas A. Lipo, IEEE Trans. on Industry Applications, V30, No. 5, Sept./Oct. 1994.

Regards, Allen

Allen,

Thanks for the pointer to the article. Next time I'm at the university library I'll try to dig it up.

About generating the extra phases, I guess I'd need an inverter to go along with it. Your point about the expense is noted.

Thanks again.

I think a hundred years ago there were systems with many more phases. Does anyone know what Tesla's original had? With time it was discovered that three was enough and more just added expense.

Stepping motors are just a fancy form of synchronous motors. If a stepping motor with a dozen individual addressable poles runs continuously does that make it a twelve phase motor?

Walter.

: > There is a company (Chorus Motors) who claims to have one (naturally : > no protos are available) and they claim it is the best thing since : > sliced bread -- higher efficiency, better startup torque etc. : >

: > Anyway I was just wondering if anyone has used a motor with more than : > three phases or knows where to get one? : : Where are you going to get more phases to run it with? There aren't : many people making drives for more than three phases, and they would : be custom. : : Regards, : Allen : snipped-for-privacy@LSUSA.com

Actually, normal North American 120 / 240 power is really two phase. The two lines are 120 VAC, 180 deg out of phase. That makes 240 line to line. What is usually referred to as two phase, 208 is really two out of three phases. Six phase is easy to come by. You take three transformers connected in Star to a three phase system and put a center tap on each. Connect all the center taps to ground. Now you have a six phase system with all phases separated by 60 degrees. To get a twelve phase system you take another three transformers and connect them in Delta and connect the secondaries in the same way as the original three. Now you have twelve phases all 30 degrees apart.

But I have no idea what it is good for. The typical electrical motor is 90% efficient already. That leaves very little room for improvement. It could be used to make an extremely low ripple DC supply, I guess.

Walter.

The only time I've seen 12-phase power used was at an aluminum refining plant where I had a job interview fresh out of college. This was in 1984, and they had just converted from mercury vapor to solid state rectifiers, supplying about 90,000 amps to the electrolysis process.

A few years ago, I had a project with six VFD-controlled pumps where the odd drives had delta-delta isolation transformers and the even drives had delta-wye isolation transformers. My PLC program had to keep a balance between odd and even pumps running, so that the phase shift could reduce harmonics in the 480 volt service.

Mike