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DRV10975EVM: Recommended motor specifications

Brad FIRTH
Prodigy 30 points
Part Number: DRV10975EVM
Other Parts Discussed in Thread: DRV11873EVM,

I am evaluating TI motor drive ICs for a low power cooling fan application, we do not have the motors yet so I am experimenting with motors from consumer drones with the motors free-running (no load).

First motor GARTT Z2205S, 14 poles, 3.5mV/Hz and 32mΩ measured. Similar size to intended application but much higher current rating (and more poles).

Second motor BetaFPV 11000kV (for very small drone), 12 poles, 1.2mV/Hz and 0.12Ω measured. - electrically this is closest to intended application.

 

First attempts: DRV11873evm with Schottky diodes fitted and a range of resistance values (to form the common connection) – neither motor turns, some jerking on start-up but times out and stops.

 

DRV10975evm configured over i2c with motor resistance and k

Motor 1 – doesn’t start unless k is increased to about 15mV/Hz then, once running, reduced to 4mV/Hz as the current increases towards current limit. Will not transition to closed loop operation (stalls).

Motor 2 – starts most of the time, but often times out and tries again, usually starts by second attempt. Transitions to closed loop control and can get up to about 8500 rpm (should do 10k).

 

My experience so far does not fill me with confidence, so – is there a subset of motors that these chips can drive successfully and if so, what specification should I be aiming for in the motor?

We have some flexibility to alter the motor properties at this stage. We want to run a small cooling fan at up to 10000 rpm.

Thanks

  • 0
    TI__Mastermind 19720 points

    Hi Brad,

    Our device will spin almost all motors that within the Table 10 spec table on the datasheet. 

    I suggest that you follow the tuning guide very closely to ensure that you have all the correct parameters to start up your motor reliably.http://www.ti.com/lit/ug/slou395d/slou395d.pdf


    Regards,

    Michael W.

  • 0 in reply to Michael.Walker
    Prodigy 30 points

    Hi Michael,

    I did follow the tuning guide, it instructs you to measure the phase resistance which I did (with a 4 point measurement method).  The measured dc resistance is close to the limits of what the driver can do at 32mOhm (64mOhm/2) and if I enter this into the gui it never starts.  

     I guessed that the evm has some track to pad and tracking resistance that is not negligible at this level (maybe the Mosfet on resistance needs to be allowed for as well?) and so I wound this figure up to 150mohm.  At this point the motor starts and is controllable – 150mOhm seems to be about the right value from trial and error – not sure how I can measure the value in circuit to confirm the theory.  I guess there is a bit of skin effect that increases the motor resistance at the pwm frequency but some of the extra must come from the EVM - any idea what has to be allowed for?  Mosfets/bondwires or just tracks and external connections?

    I did find that the control loop is sensitive to supply voltage which may cause us problems running the fan directly from the battery (12V nominal) rather than a regulated supply.  - a few volts change causes the motor to stall.  Do you know why this is and is there a way around it?  I didn't try very slowly changing the voltage - maybe it is ok with a gradual supply change.  

    Regards

      

    -----------------------------------------------------------------------

    Brad Firth

    Consultant Engineer

    Roke Manor Research Limited

    Tel: +44 (0)1794 833199

    brad.firth@roke.co.uk

  • 0 in reply to Brad FIRTH
    TI__Mastermind 19720 points

    Hi Brad,

    I have experienced the same thing as you on bench as well where I have to increase or decrease the resistance to start the motor properly. I am not quite sure why this happens, I do not think it is because skin effect or because of the on resistance of the MOSFET. 

    Is the start up sensitive to supply voltage or is the motor sensitive to supply voltage when it is running? The device does sample the VCC many times per second to know the supply voltage so it should not be susceptible to changing supply voltage when the motor is already running.

    Regards,

    Michael

  • 0 in reply to Michael.Walker
    Prodigy 30 points

    Hi Michael,

    I might have cycled the supply to quickly re supply variations.

    Anyway, the  control loop seems to depend quite critically on the resistance parameter, typically, I have to get it within +/- 10% of the sweet spot to get stable operation.  In the equipment, it may be vulnerable to changes in resistance as the stator heats up (copper resistivity has a significant tempco).

    I can't see a way around this apart sending different parameters as the system heats up.  It is a shame the chip doesn't measure the resistance it sees during the start up phase since it appears to require a different resistance programmed to that which can be measured outside the package. 

    The back emf constant on the other hand appears to be a more stable parameter and the value taken from the scope can be programmed up and it seems to run with it as measured.

    Brad

  • 0 in reply to Brad FIRTH
    TI__Mastermind 19720 points

    Hi Brad,

    I am sorry to hear that our chip cannot give what you need. I dont know of any way to tune for changing motor resistance. 

    Regards,

    Michael W.