Maximum RPM for 12-pole BLDC motor with BEMF sensing < 3500RPM?

[alevinthal]

Posted on March 24, 2010 at 02:03

Maximum RPM for 12-pole BLDC motor with BEMF sensing < 3500RPM?

[muratterzi2]

Posted on May 17, 2011 at 10:20

in case using hall interface rather than bemf sensing method you can get maximum available rpm s that your motor can  have. Bemf sensing need more computational time and this may get a delay on feedback loop that restrict maximal rpm.

[Gigi]

Posted on May 17, 2011 at 10:20

Let assume that the motor is 12 poles pairs (not 12 poles) then the mechanical to electrical ratio is 12 so we have

120 mechanical Hz becomes 1440 electrical Hz

180 mechanical Hz becomes 2160 electrical Hz

In the latter case we have a step time frequency of 2160 x 6 = 12960 Hz between two commutation event.

Then we can hypnotize that we have the sequence (Commutation, Demagnetization, Zero crossing, Commutation) each PWM cycle, we need to set 38.8kHz of PWM frequency.

First question is: can STM8 do the job?

Using the data reported in the validation document I see that it is required 14us between interrupt latency and execution time over a period of 25.72us. So it seems that the job can be performed using 54.4% of workload.

Second question is: these values are practically possible considering the stabilization time that depend on the power stage? And only one PWM period can be sufficient to achieve a good zero crossing detection?

I think that the electrical frequency of 680Hz (as explained in the thread you mention) can be more realistic.

Another suggestion, if we want to preserve the added value of the sensorless control, can be to analyze the possibility of a sinusoidal control using for instance a STM32 value line.

Sorry for the late answer.

Best regard

Gigi

[canta76]

Posted on August 25, 2011 at 01:24

Hi Gigi,

I've a BLDC model EMAX CF2822 like this one:

http://www.rcsmart.com.my/webshaper/store/viewProd.asp?pkProductItem=796

Somewhere I've read this motor is 12 poles (I don't know if it's 12 pole pairs or 6 pole pairs, I suppose 6).

Do you think it's possible to use the stm8 to drive this motor?

Best regards,

  Max

[muratterzi2]

Posted on August 25, 2011 at 12:27

Number of poles nor other motor parameters is not  restricted  by stms bldc motor control library. incase there where hall feddback you must define number of poles via related hall sensor code using header files.

basically you have three cable to drive your motor with three phase commutated scheme. according to power requirement you can adapt power stage.

[bille.stello]

Posted on August 29, 2011 at 09:32

Summarizing Murat and Gigi's replies

In case of Hall sensors -> no problems,

in case of sensor-less, it's possible to use STM8S if you use 38.8kHz PWM frequency and accept CPU load of about 55%

Regards,

Billino