How to calculate the Ke?

[gtop.1]

I refer here：https://wiki.stmicroelectronics.cn/stm32mcu/wiki/STM32MotorControl:How_To_manually_configure_the_motor_parameters 

Below is the profiler's result. Ke=9.59 Vrms/kRPM.

Below is my owm test.

Vpp=13.6v，pole pairs=5，fbemf=83.33Hz. Ke=(13.6*5*1000)/(2√2*83.33*60)=4.8085 Vrms/kRPM.

I wonder if I chose the wrong frequency？Is it between point A and point B or pointA and pointC?Thanks.

[cedric H]

Hello @gtop.1,

The frequency you chose is correct. I would be more in favor of definition issue regarding the pole pairs number to explain your factor 2 difference between the profiler output and your computation. 

Regards

Cedric

[gtop.1]

My motor ' poles is 10 (pole pairs is 5). Do you mean that calculate the Ke should use the poles not pole pairs?

[cedric H]

If you look at this documentation  How To manually configure the motor parameters - stm32mcu

Paragraph  2 Pole pair (number) parameter .

It is stated :

• The number of rotor stable positions in one mechanical turn represents the number of pole pairs.

So if you have 10 stable positions, then your pole pairs number must be 10.

Regards

Cedric

[gtop.1]

Unfortunately, it have 5 stable positions.This confuses me and I don't know what the problem is.

[gtop.1]

Is there any other possibility？

[cedric H]

Could you describe accurately how did you generate the oscilloscope dump ? 

How did you spin your motor for instance ? 

Cedric

[cedric H]

Hello @gtop.1,

No news ?

how did you spin your motor to generate your waveform, did you plug it to any electronic other than the scope ?

Cedric

[YanV]

Hi,

I have the exact same challenge.
I use this method to find that my motor have 5 poles pairs: How To manually configure the motor parameters - stm32mcu 
Then I use an external motor to turn my test unit to 403 rpm to measure the Vp-p of 4.02V and a frequency of 66.7Hz.
When using the equation I got a Ke of 1.78 Vrms/kRPM

Which mean that if I turn the motor at 403 rpm I shoud get a rms voltage of : 1.78*403/1000 = 0.717Vrms or 2.03Vpp... wich mean I have half the real voltage. (of 4.02V stated above)

Are you sure that the equation do not miss a factor '2' somewhere?

[Gael A]

Hello YanV,

At 403rpm, a 5 pole pairs motor should have a Bemf frequency of ~33.6Hz, not 66.7Hz (403/60*5 = 33.6).
Are you sure that you have the correct pole pair number ?
Another way to check your motor's pole pair number is to make it run in Open Loop at a defined electrical speed and count the number of seconds for the motor to make a turn.

Example : I make my motor run at 0.1Hz electrical (10s for an electrical turn), and it takes 20 secs to make a turn => pp = N_el / N_mec = T_mec / T_el = 20/10 = 2