BLDC motor control at 60000 or more RPM

[prashant]

Posted on July 03, 2014 at 12:06

We are working on a Medical Respirator equipment, for which we need to control the BLDC motor up to 60,000 RPM.

To generate a speed of 60000 RPM,

Space Vector Modulation

PWM waveform main/base frequency shall be 4 KHZ.

To have a smooth ripple free current/torque we need carrier frequency (Space Vector Modulation) shall be around 100kHz.

Is it possible to generate this with coretex M4 or other controller

• 100 khz carrier frequency by SVM (Space Vector Modulation)

• Also at same time, it has to work on sensor less FOC as a close loop feedback for speed & to get optimum torque.

• For this it shall probably have current feedback from 2 motor coils.

• Also some other functions to satisfy respiratory system

Please provide input

Thanks Prashant

[Gigi]

Posted on July 14, 2014 at 11:07

Ciao

Prashant

Even if it is not usual and no so much tested, it is possible to set the carrier frequency of the PWM output to 100kHz. Due to the workload is not possible to execute the FOC each PWM period (max execution frequency of FOC BEMF sensorless is 50kHz using STM32F4xx) but you can setup the system to execute it each two or three PWM periods (two should gives the best allowed option). Ciao Gigi

[prashant]

Posted on August 12, 2014 at 11:39

Dear Gigi,

Thanks for reply. One more doubt, if we execute FOC after 2 PWM (assuming PWM frequency as 90Khz; so FOC frequency shall be 45 Khz), what shall be the CPU load? Thanks Prashant

[prashant]

Posted on August 12, 2014 at 13:22

Hi Gigi,

One more question on your reply. How did you arrive that 50khz FOC execution is achievable? Do you have any application note or any evidence that can help us in understanding the FOC algorithm execution timings and CPU load. It would be of great help if you can share this information. Thanks Prashant

[Dino COSTANZO]

Posted on August 12, 2014 at 17:05

Dear Prashant,

the 50kHz FOC is based on the fact that F4 completes the whole FOC + sensorless + 3shunt current reading in about 10us, and that the algorithm starts from the center of the PWM period. From this, you can work out easily the max foc rate of 50kHz.

On the other hand, we can say - for instance - that if the FOC frequency is 10 kHz then the cpu load is 10%.

Regards,

Dino

Hi Gigi,

One more question on your reply. How did you arrive that 50khz FOC execution is achievable? Do you have any application note or any evidence that can help us in understanding the FOC algorithm execution timings and CPU load. It would be of great help if you can share this information. Thanks Prashant

[Dino COSTANZO]

Posted on August 12, 2014 at 17:09

Dear Prashant,

the CPU load of STM32F4 at 45kHz FOC - 90 kHz PWM will be around 50% Regards, Dino

[prashant]

Posted on August 13, 2014 at 13:04

Hi Dino,

Thanks a lot for prompt & informative response. You have given very important information that sensorless FOC with 3 shunts is executed in 10uS. Please let us know what is the core frequency?(We assume it to be 180Mhz) It will be nice if you could share some ST document/ST weblink where this information is given; for our reference. Regards Prashant prashant.kharade@igate.com