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Most motors with more than 2 poles I see (4 poles, 6 poles etc) use a run capacitor and a use the auxiliary winding all the time. Some even employ starting capacitors but they always have a run capacitor.

But why does 2 pole motors only use the auxiliary winding for starting? I haven't seen any single phase 2 pole motor that use a run capacitor.

user2934303
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  • A 2 pole motor cannot have an auxiliary winding by definition else it would have more than 2 poles. – Andy aka Aug 16 '20 at 13:50
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    @Andyaka That's not how it works, the auxiliary winding doesn't change the amount of poles. In a 4 pole motor for example both the main and the auxiliary wingdings produce 4 poles, they're just out of phase so the resulting 4 pole magnetic field rotates. In the same fashion you can absolutely have 2 groups of wingdings producing a 2 pole rotating field, like the old 2 phase motors had in the 2 phase electrical systems that existed in early 20th century, before 3 phase system became the most common (and you can still make them, of course, all you need are 2 phases with a 90º difference). – user2934303 Aug 16 '20 at 14:03

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The choice of starting scheme for a single-phase motor is primarily based on the desired torque vs speed curve. More specifically the scheme and capacitor value determine the available starting torque (zero speed, locked rotor), accelerating torque (0 to 80%+ speed) and peak torque. The torque required depends on the load characteristics. The torque vs. speed diagrams shown below illustrate the differences among the schemes.

The number of poles does not have a direct bearing on the scheme used. The number of poles determines the motor's synchronous operating speed and thus the normal operating RPM.

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Figures adapted from Fitzgerald, Kingsley, Umans, Electric Machinery 4th ed