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DRV8308: Acceleration and Startup Current in Closed-Loop Mode

Dustin Alinger
Prodigy 20 points
Part Number: DRV8308

Hi,

I'm using the DRV8308-EVM to drive a Maxon 339267 BLDC motor. I would like to use the closed-loop speed control mode to drive the motor at ~2250 RPM. The problem I'm having is that the motor is accelerating very quickly to full speed, resulting in a large startup current, as shown here:

The purple trace is the current into the eval board from the 12V supply and the yellow trace is the FGOUT pin. This startup current of 2A is not acceptable in my application. I could try ramping my commanded speed to get the acceleration profile I want, but I'm unable to find a single set of gains that work at both low speeds (~250 RPM) and my desired operating speed (~2250 RPM). My register settings are shown in the following two images:

Is it possible to find a set of controller parameters that give me a slow, well damped response to the step speed input so that I avoid the large startup current?

Thanks,

Dustin

  • 0
    TI__Genius 14487 points
    Hi Dustin,

    It's hard for me to recommend parameters for your system for the whole range of speeds. Usually finding the optimal settings is an experimental process depending on the motor and mechanical load. When you find good parameters for low speeds, what issues do you see at high speeds - and vice versa? Have you reviewed the tuning process in the user guide for the EVM?

    The reason for the large current is because the motor is essentially stalled at startup. You will see large currents until speed gets to a point where the back EMF finds equilibrium with the supply voltage. If possible, you could use a lower supply voltage for the step speed input condition.

    My second suggestion is to limit your startup currents is to use a larger sense resistor. The DRV8308 compares the current of the sense resistor to an internal reference, and it stops driving when the voltage on the sense pin exceeds the internal reference.

    My third suggestion is use a large bulk capacitor. The motor will draw the current from the capacitor instead of the supply during start-up.