DRV10983

Hi Sebastian,

There are several other things you can check and fine tune to improve performance.
- Try playing with value of Kt- increasing or decreasing value of Kt has effect on maximum speed and current drawn from power supply
- Try reducing control coefficient setting to lower value and check whether it improves performance
- Make sure resistance value is configured as ½ of measured phase to phase resistance

Regards,

Krushal

Dear Krushal,

I am afraid this won´t do the trick. I have experimented with all of these parameters:

Kt is specified at 35.8 mV/Hz, measuring it (as good as I can) gives approximately 40 mV/Hz, calculation according to the "Tuning Guide" (SLOU395C) gives 35.8 mV/Hz and 35.9 mV/Hz for the two suggested methods. As far as the options in the Kt register of the DRV10983 are concerned this all comes down to 36.6 mV/Hz or 40.3 mV/Hz. In experiments with the motor spinning at 18 thousand rpm and 40 thousand rpm I have varied Kt well below and above this range from 10 to 100 mV/Hz. Below 20 and above 100 the motor will not start properly anymore. In between this the current consumption at the two tested rotational speeds only varied insignificantly (relative to the accuracy of the DC current read out of the lab supply).

The control coefficient does not help either: Reducing it to 0.75 will not change the behavior noticeably (I have not tested load change behavior in this experiment) and further decreasing it to 0.5 or 0.25 yields reduced speed at the 40 thousand rpm setting. Enabling all error handling flags at this point makes the motor stop and restart, so I would guess this is not a wise choice of control coefficient. The current consumption at this point is less obviously, but not less than it would be with control coefficient set to 1.0 and the speed setting trimmed to the resulting speed of the described working point.

The motor resistance is specified as 1.3 Ohm (phase to phase), so that should make 0.65 Ohm for the phase resistance. I cannot measure phase to center in the motor, but phase to phase values give me between 1.25 and 1.4 Ohm between different specimen of the motor I have here. From the options the DRV10983 motor resistance register has the value would best be set to 0.619 Ohm and the motor will work with this configuration. Varying the  value between 0.387...0.851 Ohm will not significantly change current consumption at 18 and 40 thousand rpm. It will however change start up behavior with values below or above this range keeping the controller from safely reaching 40 thousand rpm at all.

So now where do we go from here? Again the motor spins nicely, BUT it consumes far to much current and produces significant amounts of heat: 15 minutes at 40 thousand rpm at no load and ambient temperature at 24 degree Celsius heats the motor enclosure to almost 120 degree Celsius ( this is only 10 Kelvin from maximum specified for the motor´s windings!). Using a Toshiba TB6575FNG controller the same conditions yield significantly less current consumptions and most importantly "only" 78 degree Celsius. This will make to motor much more shaky and gives a lot of noise, so I would very much prefer to use the TI part over the Toshiba, BUT the excess power consumption is a big issue here.

Do you have an explanation and even better a solution to my problem?

Best regards,

Sebastian.

Hi Sebastian,

Apologizes for delay in response

DRV10983 commutes motor based on zero crossing of motor Back emf. So when current is aligned correctly to back emf signal, motor is driven optimally.

Can you share parameters related to motor like phase inductance, pole pairs, inertia

Can you also provide waveforms of phase currents and phase voltage during motor commutation at different speeds with trapezoidal drive and DRV10983 to help understand problem better

Regards

Krushal

Dear Krushal,

Thanks for the response. Don´t worry too much about the delay, but I need to solve this soon, since I have to make a design decision on this within the next 2 weeks. Again the driver works smoothly, I a just very worried about the much higher current consumption than the reference design with a Toshiba driver.

Currently I am working with a motor that is overkill for the application, but our customer insists on using it. So the driver will only have to manage a small portion of the motors power. In detail the DRV10983's maximum current drive capability 3 A peak and 2 A continuous is perfect for the application even though the motor can in theory take much more. The parameters you requested are as follows:

- pole pairs:1
- phase inductance: 68 uH
- phase to phase resistance: 1.3 Ohm
- torque constant: 5.7 mNm/A
- speed constant: 1675 rpm/V equivalent to 35.8 mV/Hz
- stall speed: 40 thousand rpm
- nominal speed 36529 rpm
- max continuous: 2.45 A, 13.5 mNm

I had a colleague take a screenshot of the phase voltages (see attached - blue red and green are U, V and W phase voltages measured with reference to ground of the motor), but he had trouble measuring the phase currents. What do you suggest for a good phase current measurement setup without a dedicated current probe?

Best regards,

Sebastian.

Hi Sebastian,

A common practice other than current probes is shunt resistor and differential opamp. Current can be measured by connecting low value resistance (typical mΩ) in series with motor phase and then measuring differential voltage using differential amplifier across resistance.

Capturing phase current, bus current and phase voltage on single plot will be useful here.

I see one issue with motor is that it’s electrical time constant (L/R) is very less (~52 µSec). Recommended electrical time constant for DRV10983 is greater than 100 µSec. Built in back emf observer might have some phase lag in estimating zero crossing and hence more current consumption because of lower electrical time constant.

Regards

Krushal