L6629 Speed/Torque control

[ignas]

Posted on February 27, 2015 at 14:42

Hello,

I'm trying to control 12V BLDC motor with 3 x 120� Hall sensors, using L6229PD eval board from ST. I'm controlling Speed by connecting 5V PWM signal (0-255) from microprocessor to FRW/REW pin (28) on CN4 connector. Visually everything seems OK - 0 one direction max speed, slowing down to zero when PWM comes close to mid point (128), and then reversing direction to maximum speed when reaching 255.BUT. Motor very quickly looses torque when speed starts to decrease. On 70-180 PWM values it is unusable for my application.How can I have high enough torque when decreasing speed ? Or how to control speed to not to loose the torque so drastically ? P.S. Onboard Speed/Torque switch - can't find any differences in those two modes. #eval6229pd #bldc-motor-control

[dgrimes]

Posted on March 16, 2015 at 23:06

Hello,

I am hoping to use the same function with this pin too. I know for another brand BLDC controller (not ST) the frequency of this signal is very important, too low frequency at the input pin makes low frequency for switching FETs and poor motor performance. The ST Design Tip DT0001 does not indicate anything about frequency, perhaps this is the same as their competitor part. What frequency were you testing? I had hoped 50-100Hz would be ok but competitor part needed >9kHz for good performance. D

[Enrico Poli]

Posted on April 08, 2015 at 13:06

Dears,

The PWM on the FRW\REV input controls the voltage applied to the motor phases, but in order to keep the speed under control you need a speed loop adjusting the PWM according to the motor load. When the load torque is increased the speed loop should increase the FWR\REV duty-cycle to keep the speed costant. Abut the PWM frequecy: it should be above 10 KHz in order to obtain good performance, otherwise the torque ripple makes the control performance poor. Regards Enrico