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?

  • 1
    \$\begingroup\$ I don't understand the difference between your two methods. Your second method just sounds like the thing you'd do while looking at the oscilloscope? \$\endgroup\$ Commented Jul 31, 2023 at 0:34
  • \$\begingroup\$ Yes , while the motor is rotating i check the number of zero crossings on the oscilloscope for one rotation of the motor shaft., But i don't know if this is actually correct. \$\endgroup\$
    – AK47
    Commented Jul 31, 2023 at 3:09

2 Answers 2


Yes, you can do that. Both methods are the same thing, essentially. For many motors you don't even need a second motor back-to-back. If you can spin the shaft by hand, just do roughly one revolution.

For most motors you need neither much nor constant speed to get a useful result on the scope. You are only interested in the number of complete sine waves you see on the scope screen after one shaft revolution. That's directly your number of pole pairs.

  • \$\begingroup\$ Thanks , this cleared it up for me. \$\endgroup\$
    – AK47
    Commented Aug 1, 2023 at 11:13

From my experience I would say that both methods are correct and in essence they are the same method. I'm not sure if in the second method you are asuming you are controlling your own motor or you also have an auxiliary motor, despite that, I know two other methods.

  1. You can supply the motor with some constant DC voltage of low magnitude (always check the current consumption to not burn the coils) to magnetically energize the stator coils. Then proceed to manually rotate to shaft and you will see that it won't move smothly and that it snaps into place on some particular positions. Chose one of these positions, rotate the shaft and count how many snaps there are in a full rotation. If there is one snap, you have one pole pair. I you have two snaps , you have two pole pairs and so on.

  2. This second method is less risky and requires less electrical equipment and if you are in a hurry it could be of some help.

It consistis of rotating the motor without an auxiliry one by attaching a weight to the shaft an let the force of gravity to produce a constant rotation while measuring with an oscilloscope the induced voltage in one of the phases.

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When I used this method I filmed both the motor and the oscilloscope and so I could measure how many periods did the the induced voltage sine wave form completed when the rotor shaft did completed one rotation.

You can put a flag and count how many times the motor has rotated or you could put marks on the rope with the distance to each mark being the perimeter of the shaft. And compare the nomber of shaft

I know its not the best method but you can have at least an idea of


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