poles number of induction motors?

[Wheretostart]

when using the equation Ns=120f/p to calculate the number of poles, if the calculation result is something like 6.x, why the result should be size down to 6, in stead of 8? what about 7.x,? still 6 or 8? any rule behind? thanks

 

[user 26087]

The speed of induction motor is Ns(1-s) where "s" is slip. So if you use the induction motor speed to calculate poles you would round down. Also the poles is an even integer so if an odd number is shown that should be a red flag for wrong answer.

 

[Wheretostart]

Thank you very much! It makes perfect sense.