Calculating RPM from BEMF frequency of BLDC motor

[Tomsik]

Hello,
I'm a beginner in driving motors and I need to calculate RPM (rotation speed) from frequency BEMF signal on 1-phase of a 12N14P motor when it'is possible.

I measured 1.6kHz repeating signal (sinus) at 1-phase of BLDC motor.
Can I calculate RPM from these parameters?

Thank you 

[mikerj]

You would need to know the number of poles in the motor to derive RPM from that figure.

[mzzj]

https://www.rcgroups.com/forums/showthread.php?2779762-How-many-poles-on-a-motor

[Tomsik]

According to the shared thread I think I have 12 poles of stator, it means 6 pair poles.

[Tomsik]

Please help me 

[janoc]

It is explained in that rcgroups article - the number of poles corresponds to the number of voltage cycles per rotation (logical, the voltage changes as many times as many magnets pass in front of the coil).

So take the double of measured frequency, divide by the number of poles and multiply by 60 to get RPM value. However, this is going to be a fairly noisy value, depending on how exactly are you measuring the BEMF.

BTW, you need to count poles of rotor (magnets!) not the stator poles. 12N14P is a 14 pole motor (the number before the P - again, did you read that rcgroups posting? 12N - 12 stator windings, 14 magnets), so:

2*1600/14 * 60 = 13714 rpm, if I am calculating it correctly.

[Tomsik]

Thank you for answer.
Why do you multiply RPM by two?

[janoc]

Thank you for answer.
Why do you multiply RPM by two?

I think that has to do with the fact that each magnet passes over the coil you are using to sense the BEMF twice during each rotation (once over each "end").