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DRV8350R: Cannot control at low speed

ArTzy
Genius 13939 points
Part Number: DRV8350R

Hi Experts,

Good day. Seeking your assistance on this query:

I made a motor driver with the schematic below. When I write 100% power to the power pin, the motor is powered at a high speed (Im assuming 100% power), so that is no problem.

The issue is that I can not control the low speeds of the motor. When I write 10% PWM to the driver after I have the motor already spinning, the motor completely stops instead of going slowly (I even tried spinning it by hand to help it start if that was the issue but to no avail).

When I write anything below 40% after the motor stops spinning, the motor will still not turn on (again, even when I start spinning it by hand to help it).

In summary, I cannot make the motor go below 40% power from a standstill (when I try to do so the motor just stops), and I cannot make the motor slow down to under 10% power from an already spinning state (the motor stops spinning below 10% pwm duty cycle even if it was spinning already).

Tried fix:
I thought the PWM write frequency to the driver was messing it up so I scanned every frequency from 100,000 to 0 in increments of 1000 and found that 19,000 was the frequency with the lowest PWM that was able to turn the motor when going from already spinning (around 10% power/PWM duty cycle). The other frequencies did not turn the motor at all, or alternated between turning the motor and not turning the motor.

I made 4 of these motor drivers and put them in parallel, receiving the same power and signal. At 19,000hz PWM, I decreased the power from 100% to 0% over one minute and found that some of the motors/drivers stopped about +-2% power from each other. In other words, there was a period of duty cycles where some of the motor drivers kept turning at the desired speed while others completely stopped. Im not sure if this has anything to do with the stopping at low input duty cycle issue, and may be just manufacturing/assembly error.

I know that brushless motors and brushless motor drivers, in general, are able to be operated at slow speeds. Is this a chip limitation or is there a fix?

Could I have mixed up the hall sensor pins or motor pins to be out of order?

Regards,
Archie A.

  • 0
    TI__Mastermind 26046 points

    Hi  Archie,

    I'm assuming you're using 1x PWM mode correctly by setting the MODE pin to the right configuration, correct? 
    When the motor spins, is it vibrating or make noise? Do you see large fluctuations in supply current? If so, the Hall connections could be out of order. 

    If the motor isn't spinning at low duty cycles, there is not enough current to make the motor spin at steady state. This means you're either not providing enough current from the supply, there is large impedance in the motor current paths, or something is causing there to be less current than required (such as cycle-by-cycle overcurrent protection). 

    To test these things, can you use a scope and capture waveforms of PWM, GHx, GLx, and SHx to see if the gate drive and motor phase outputs are expected at low duty cycles?

    Could you also try looking at SHA, SHB, and SHC simultaneously to see if you are getting okay waveforms for trapezoidal commutation? This will confirm if your Hall sensors states are correct.  

    A couple of other things to try are to check your VDS and IDRIVE settings, we recommend lower IDRIVE values based on the Qgd of your FETs and VDS settings calculated correctly based on motor peak current (E2E FAQ on IDRIVE). Usually VDS should be set to a value between ID continuous drain current rating of the FET and the motor peak current in your application (I_D > VDS > I_overcurrent * Rdson). 

    Thanks,
    Aaron