On 2/5/12 8:13 PM, jhowson4@xxxxxxxxxxx wrote:
Kevin, I do not know for certain at all. But I speculate that It might be possible. On a single phase motor, either 2 or 4 flats are generally used to remain symmetric to the driving coils depending on motor speed. So a three phase motor has 3 sets of driving coils each separated by 120 degrees (I think) So to me it would stand to reason that one could utilize flats separated by 120 degrees or some third multiple such as 3 at 120 apart , 6 at 60 apart, etc. Then again, the peaks for each phase occur at a different time, so the above statements could be complete nonsense for that reason. But I would be very interested in finding out if it were possible, that would be cool.
I haven't actually tried it or thought about it much, because it's fairly easy to come by a synchronous 3 phase motor. They're a lot more common than single phase synchronous motors because there's plenty of industrial processes that need a bunch of motors that are running at the same speed. Although these days, a good variable speed drive and an induction motor might be a better solution.
I would expect it would take only two flats, just like on the single phase motor. The net effect of the three phase windings is to create a rotating magnetic field, and for a 3600 RPM motor, that field has only 2 poles (if you put a permanent magnet in the middle of the field, there would be only one position it could be in at a time). For and 1800 RPM motor, there's 4 poles (N/S/N/S going around the circle), so your "rotor" could be a set of 2 or 4 bar magnets (hard to describe in words) that match.
In fact, that's what a synchronous motor is. In the "wound rotor" variety, the rotor field is created by a DC excitation. Or, in small motors, the rotor is a permanent magnet.
What an induction motor does is basically a transformer and motor all in one. There's no permanent magnet in the middle, rather, currents are induced in the rotor by the changing field of the stator, which creates the rotor's magnetic field. That magnetic field is, in turn, attracted by the stator's rotating field, so the rotor turns.
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