Does anybody know of any companies that sell AC motors and controllers
built with more than three phases?
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?
We could build large ones (250-25,000 HP) but that's probably not what
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
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.
Allen Windhorn (507) 345-2782 FAX (507) 345-2805
Kato Engineering (Though I do not speak for Kato)
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.
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.
The only time I've seen 12-phase power used was at an aluminum refining
where I had a job interview fresh out of college. This was in 1984, and
had just converted from mercury vapor to solid state rectifiers,
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
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.
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?
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