I have a permanent magnet synchronous motor, without a datasheet for it. Could you kindly advise how do I find the number of poles without actually opening the motor up?

After some searching, I found possible two methods for doing this and would like to know if they are the right way of accessing the number of poles.

First one is just to use the sync speed equation (N = 120 * F/P). I rotate the motor with the help of another motor at a constant speed (N), both these motor's shafts are connected back to back. Probing the back-EMF on an oscilloscope, we get to know the frequency (F). By these two values can I calculate the number of poles (P)?

Secondly, the zero-cross method: I probe the phase to phase back-EMF, and while the motor is rotating, just check how many times the sine wave crosses zero in one motor rotation?

I have a permanent magnet synchronous motor, without a datasheet for it. Could you kindly advise how do I find the number of poles without actually opening the motor up?

After some searching, I found possible two methods for doing this and would like to know if they are the right way of accessing the number of poles.

First one is just to use the sync speed equation (N = 120 * F/P). I rotate the motor with the help of another motor at a constant speed (N), both these motor's shafts are connected back to back. Probing the back-EMF on an oscilloscope, we get to know the frequency (F). By these two values can I calculate the number of poles (P)?

Secondly, the zero-cross method: I probe the phase to phase back-EMF, and while the motor is rotating, just check how many times the sine wave crosses zero in one motor rotation?

Update: Another notable method, found on youtube, : Link

I have a permanent magnet synchronous motor, without a datasheet for it. Could you kindly advise how do i find the number of poles without actually opening the motor up?

After some searching, i found possible two methods for doing this and would like to know if they are the right way of accessing the number of poles.

First one , is just to use the sync speed equation. (N = 120 * F/P). I rotate the motor with the help of another motor at a constant speed(N), both these motor's shafts are connected back to back. Probing the back-emf on an oscilloscope, we get to know the frequency(F). By these two values can i calculate the number of Poles(P)?

Secondly, The zero cross method, i probe the phase to phase back emf , and while the motor is rotating  , just check how many times the Sine wave crosses zero in one motor rotation?

Thanks,

I have a permanent magnet synchronous motor, without a datasheet for it. Could you kindly advise how do I find the number of poles without actually opening the motor up?

After some searching, I found possible two methods for doing this and would like to know if they are the right way of accessing the number of poles.

First one is just to use the sync speed equation (N = 120 * F/P). I rotate the motor with the help of another motor at a constant speed  (N), both these motor's shafts are connected back to back. Probing the back-EMF on an oscilloscope, we get to know the frequency  (F). By these two values can I calculate the number of poles (P)?

Secondly, the zero-cross method: I probe the phase to phase back-EMF, and while the motor is rotating, just check how many times the sine wave crosses zero in one motor rotation?