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DRV8308 speed control over-current during high to low speed transistions

Peter Jarman
Prodigy 50 points
Other Parts Discussed in Thread: DRV8303, DRV8308

I am using the DRV8303 EVM to drive a 4 pole three phase motor, and I have the speed control loop reliably locking the rotor speed to input frequencies between 30Hz and 195Hz (relating to speed between 900rom and 5850rpm). When I increase input frequency in a single step, the speed increases as expected, but when I decrease speed, the motor takes a large pulse of current, in many cases shutting down the power supply, before restarting at the lower speed. With some gain and filter settings, I can get it to switch to the lower speed without shutting down the PSU but it always takes a large current during the transition and the motor always 'jerks' powerfully, before the final speed is achieved.

Do you have any idea what is causing this and how I can stop it.   

  • 0
    TI__Mastermind 20985 points
    Peter, here are some ideas I recommend looking into:

    1. Probe the clock input to check how cleanly it changes frequencies.
    2. Reduce DRV8308 register SPDGAIN to see if that improves the behavior.
    3. Use a frequency ramp instead of a step-change.
    4. Possibly increase bulk capacitance to stabilize the current transients and prevent your PSU from shutting down.

    The DRV8308 is continually comparing motor speed with the reference clock, so if there's a large instant error in the two, I can see that current may spike.

    Best regards,
    RE