Power Factor Vs. Motor Poles 3

[CB21093]

Working on a project requiring very high motor efficiency. In reviewing motor selections, I notice with changes in the number of poles in asynchronous motors, PF changes. Can anyone explain why?

 

[electricpete]

For the same horsepower, a lower-speed motor has bigger radius. That means larger rotor circumference. That means longer airgap (not deeper airgap... longer around the circumference).

Magnetizing a longer circumference to the same flux density will require more magnetizing amperes.

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[edison123]

Low pf in low speed machines is due to more flux leakages between the poles.

 

[7anoter4]

In my opinion both gentlemen are right. The rotor diameter is bigger if the poles number is bigger [as electricpete stated] .But as the magnetic flux will be almost the same (depending more or less upon the voltage), the magnetic flux density - measured in Gauss [or Wb/sqr.m=Tesla] will be different.
The rotor diameter will increase with the factor depending on p^ (2/3) that means for p=2 [4 poles] will be 2^ (2/3) =
1.587 times the diameter of 2 poles motor [roughly]. But the magnetic flux density will increase 2 time [as the air gap length per poles will be 1/2].So, ultimately, the flux density will increase 2/1.578= 1.267 times [roughly, of course!].
That means the saturation of the magnetic core [mainly in the teeth region]will be more pronounced in return, so the magnetizing amperes will increase[ as electricpete said].But in the same time also the notch leakage reactance will increase[ as edison123 said].The influence of the magnetizing current on the pf is evidently more important than the leakage reactance.
Best Regards

 

[sreid]

Tell us about your application. Induction motors are inharently inefficient since power is required to flux the rotor.

 

[CB21093]

GREAT answers all - thanks! I'll try to stay of the EE's hair on this one!