Induction motor gone bipolar?

Wild_Bill wrote:

I got a reply from Regal Beloit regarding my wiring inquiry. I've uploaded the diagram here:

Interestingly enough, in the reply from the support representative, he said that this motor does not require a capacitor nor is it likely it will operate if one is used. So why is there a capacitor drawn in the diagram?

By the way, I reinstalled the repaired motor exactly the way it was previously installed, with the two capacitors, and it is working fine.

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David Farber
David Farber\'s Service Center
L.A., CA

I'll take exception to one part of Bill's otherwise great wording....

As you can see in the diagram linked in this thread; the running winding is always on [...when the motor is powered up, obviously...] but the starting winding is soon turned off.

The starting winding is on only for the ~~~100-200 ms it takes the motor to spin up. (I measured the time once in Machines Lab. Hi Prof Klingshirn!)

(You can work all this out with copious amounts of math but the following will do.)

What makes it all work is phase shift, both electrical and physical. The starting winding jerks the motor shaft slightly, then 90 degrees later [electrically] the running winding [offset physically around the path of the rotor] follows up with a second one. That one-two punch is what starts the motor in the correct direction. If you want to go the other way, flip the running winding around so the running jerk is the opposite side of the starting peak.

The capacitor also limits the starting current to some finite value; more capacitance in parallel is more current. Since it's used for only

200 ms, the starting winding can get away with much higher current than the running one. But hold the motor stalled; and smoke will soon arise...unless the breaker trips.

Now a shaded pole motor fudges in the phase shift without a cap, but gives far less torque. If you recall analog electric clocks; that's what they use.

Three phase makes it all trivial. You get 3 peaks for the three windings, each peak offset by 120 degrees [electrically] and the winding offset by

120 degrees physically; so you get ABCABCABCABC and the motor needs no starting winding nonsense at all....it just goes.

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