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Maximum commutation frequency?

David F.
Prodigy 205 points
Other Parts Discussed in Thread: DRV10866, DRV11873, DRV8312, DRV8313, DRV8301

I received eval kits for DRV10866 and DRV11873.  Hooking up the motor that I intend to use in my application (Maxon motors EC-14 flat, 6V nominal [part # 339251]), doesn't work correctly with either eval kit.

At low duty cycle, and low speed, the driver turns the motor as expected.  There seems to be a hard speed limit, or commutation rate, that the drivers can achieve.  I measured a maximum of 720Hz on the FG signal of the 10866, and 500Hz on the FG signal of the 11873.  This would equate to electrical commutation frequencies of 43,200 / minute, and 30,000 / minute, respectively, on the 2 drivers.

I tried varying supply voltage, and motor loading to see if the cut-out frequency would move.  Regardless of these changes, the drivers would cut out for a second or two, and then attempt to restart.  So the limit appears to be speed-related, not current or voltage related.  Is there anything that can be done to increase this limit?

If these are limits to the design, why are these numbers not stated in the datasheets?

Thank you,

David

  • 0
    TI__Mastermind 26065 points

    Hi David,

    Could you try DRV11873 with FS set to HIGH?

    It is true that there are hard speed limit to DRV10866 and DRV11873, please refer to the following limits.

    For stable running:

    DRV10866 FG<650Hz

    DRV11873 FG<470Hz @FS=0;

    DRV11873 FG<950Hz @FS=1;

    LOCK ON will assert after the limit is reached.

    Most of the 12v motors can be handled within the range, but you are right we should conside to include those limits in datasheets.

    There will be no external work around unless change the motor parameters to lower the speed or control the speed below the limit with PWM IN.

    Thanks.

    Wilson Zuo

    Motor Application Team

  • 0 in reply to Wilson Zuo
    Prodigy 205 points

    Wilson,

    Thank you for answering my questions.  I was surprised by the ratio of "electronic gearing" that my selected motor has.  From what I measure, it needs 20 electrical cycles per mechanical cycle.  So even a 950Hz limit corresponds to 2850 mechanical rpm (950*60/20), with this specific motor.

    I did try removing the FS jumper on the DRV11873 eval board, and it does increase the speed limit.  On my specific board, it worked to 990Hz.

    I think it is important to have this information on the datasheet.  For most 2 pole-pair traditional BLDC motors (internal rotor), these limits wouldn't cause a problem.  But with an external rotor BLDC motor, "electronic gearing" is usually in play, and these limits will often cause issues (at typical fan/blower/chopper speeds).

    Thanks again for getting this answered so quickly.

    David

  • 0 in reply to David F.
    TI__Mastermind 26065 points

    Hi David,

     Astonishing,  you have a BLDC motor with 40 poles/20 pole pairs. That is the most one I have ever heard. Could you share any information about the application?

    Also we have DRV8313 EVM which combines with a MSP430 MCU to do the sensorless control. For higher current, we have C2000 + DRV8301 or C2000 + DRV8312 kits.

    You can check whether those are fit for you.

    Thanks.

    Wilson

     

  • 0 in reply to Wilson Zuo
    Prodigy 15 points

    Hi Wilson,  I am using the same Maxon EC-14 flat sensorless BLDC motor for my application and am looking for a simple IC controller.  I was looking at your DRV11873.  I ran into the same problem as David F with the previous integrated chip I tried (a Toshiba TB6588FG), where the motor speed was limited to 12,000 rpm by a maximum commutation frequency of 800Hz.  I realised that the maximum commutation frequency had to be divided by  the number of pole pairs (for the EC14 Flat this is 4 pole pairs) thus giving a maximum speed of 800 *60/4 = 12,000 rpm.  I want to run this motor up to 20,000 rpm.  The data sheet for the DRV11873 states 

    The motor driver MOSFETs will operate at
    constant switching frequency 125 kHz when the FS pin is pulled high and 62.5 kHz when the FS pin is pulled
    low.

    my question is how do you get the maximum commutation frequency of 

    DRV11873 FG<470Hz @FS=0;

    DRV11873 FG<950Hz @FS=1;

    from this info and are these values correct.  If so my maximum motor speed would be limited to 950*60/4 = 14,250 rpm with this driver

    Do you have any alternative simple integrated drivers which might be suitable?

    The other drivers you mentioned seem to require additional miro-controllers and in-depth programming.

    regards,  Ben 

  • 0 in reply to Ben Jones
    TI__Mastermind 26065 points

    Hi Ben

    The maximum commutation frequency of DRV11873 is a designed internal logic limit. It is not related to the output switching frequency. FS controls the logic clock double or half, so the limit is about to 1:2 when FS=0 : FS=1.

    DRV8313 kit is not need to do a deep programming if you just want to run the motor like DRV11873. What we need to do is to latch the parameters from GUI to the MCU. You can find the source code about the parameters pass down from GUI to MCU. The main control algorithm of the instaspin is not open source but you also don't need to modify.

    Thanks.

    Wilson

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