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Original question:

DRV10866EVM: Start-up sequence intermittent with high inertia

DRV10866: How to choose resistor to simulate center tap?

lloyd konneker
Expert 1790 points
Part Number: DRV10866
Other Parts Discussed in Thread: DRV10963,

Can you please explain the ramifications of choosing a value of resistor in the resistor net that simulates a common lead for a 3-wire BLDC as explained in section 8.1 of the datasheet?
There it says "To find the proper resistor value, start with a value of 10 kΩ and continue to decrease by 1 kΩ until the motor runs properly."  I don't understand the reasoning, and "properly" is too fuzzy.

For example, I have a motor that starts jerkily with 10k ohm resistors, but seems to start more smoothly with a 5k ohm resistor wye.

And I have another motor that sometimes starts smoothly, at other times it comes to rest at a dead spot, and won't start again (at least with the brief, low voltage pulses I give it.)  Before I change other parameters, I would like to understand this parameter.  For example, is 1k ohm the recommended minimum?

I also have considered using a DRV10963 but I think it requires more parts (say an MSP430) to provide a PWM signal (it won't start my motors with PWM tied high like the DRV10866 will.)

Maybe you could refer me to a book that explains 3-wire vs 4-wire and this simulation business.  Also called a pseudo center-tap?

This question might be related.  They seemed to have better luck with a 1k.  "startup sequence intermittent..."  e2e.ti.com/.../3012516

  • 0
    TI__Mastermind 19720 points

    Hi Lloyd,

    This device needs to have a way for all three phases of the motor voltages to be averaged so that it can compare this averaged voltage to the BEMF voltage and then decide to commutate. Normally a motor with a center tap that is accessible will already do this averaging. this is why the wye circuit is needed. The 1K is the minimum because if you go any lower it will stress the driver and take current away from the motor. I would recommend that you continue to lower this resistance and see if you can get one to work for you.

    Regards,

    Michael W.

  • 0 in reply to Michael.Walker
    Expert 1790 points

    OK.  I will experiment more.

    It escaped me that the COM pin is an input and doesn't carry any significant current to turn the motor.

    I still don't understand why too small a resistor would stress the driver.  It seems to me that a motor with a center tap would have very small resistance from the COM pin to a coil pin, but I suppose it would have high impedance.  Or maybe motors with a center tap have a wye network of resistors built in?

    Also, if the COM pin is just measuring a voltage, I don't understand why it would have any effect on the driving of the motor coils?  Some side effect I suppose.

    It will remain hand-waving magic to me until I have time to learn more.