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DRV10975 tuning: current probe recommendation

Other Parts Discussed in Thread: DRV10983, DRV10975, DRV10963, DRV10964

The Tuning Guide seems to rely heavily on current measurements.  Am I correct in assuming that I really should have a current probe?

I bought one for this purpose, but evidently the bandwidth is not high enough.  My current probe has DC to 100 kHz bandwidth.  Am I correct that this bandwidth is inadequate?   What would you recommend for bandwidth?  Many probes have a minimum and a maximum to their bandwidth specification.  Please let me know the recommended min and max.  Thanks

  • Hi Gary,

    Our experts have been notified and should reply soon.
  • Hi Gary,

    100 kHz should be good enough for testing motor. Since motor current measured on phases will be in electrical Hz, 100 Khz should give you adequate data to tune motor

    Regards,

    Krushal 

  • Thank you. I guess I'm not properly interpreting what I'm seeing. This is with a prototype motor that we are developing. I see a low frequency wave form (about 2 Hz roughly; the motor is turning at about 60 rpm ... rough numbers) with a higher frequency impressed on it, about 200 Hz component. But the whole thing has some distortion, so I thought I was not resolving some high frequency. I think I was fooled by the appearance of the 200 Hz signal on a very low scope time base. Also, the waveform does not look like the waveform in the tuning guide. I see a 2 Hz wave with P-P amplitude of 1 amp. The 200 Hz component is "on top of" that having a P-P amplitude of about 0.5 amp. I can't download an image from the scope right now. I'll post a picture when I'm able to.
  • Hi Gary,

    Can you please share your waveforms and parameters used to run test. What is intended speed of operation ? 2 hz seems slow for DRV10983 to control motor.
    Higher frequency might be the PWM switching frequency (around 25 kHz) imposed on current waveform.
    Can you also verify that parameters like motor resistance and motor Kt are set correctly.

    Regards,
    Krushal
  • Thanks for your reply.  It will be a day or two before I can respond ... I'm not ignoring you!

  • I have an old 'scope that would not recognize a USB drive that was "too large".  It took me a while to find a 128 MB drive.

    Here are some traces of my motor operating at 10 Hz open loop.   I can see an overall  sinusoid, and higher frequency (~300 Hz) "ripple".  If I speed up the time base on my scope one more notch, the traces look like garbage.  I didn't post one of those assuming that it's some kind of timing/aliasing problem with the scope.  If I continue to turn up the time base, I can see a 40 kHz "ripple".     Note that this is getting close to the 100 kHz bandwidth of my current probe.

    If you can interpret the "ripples" I would be grateful.  And if  you could suggest what I might try to better tune the parameters, that would be great, also.  I haven't yet got closed loop operation running.  

    10 Hz operation ...

    10 Hz rotation, 300 Hz ripple

    40 kHz ripple

  • Hi Gary,

    Apologizes for late reply. Ripples are coming from PWM swirching frequency which can be either 25 or 50 kHz based on configured value.

    Can you please share motor parameters like inductance, resistance, BEMF constant, it seems inductance of motor is low and it causing this ripples.

    On 'Advanced Setting' page of GUI can you check double PWM frequency to increase switching frequency 

    What happens when you run motor at higher speed ?

    Regards,

    Krushal 

  • This is a small axial flux DC motor, a prototype for a reaction wheel.  The required torque should be low ... around 20-50  mNm depending on the bearings.  The coils are air-core.  Three pole.

    The inductance per phase should be 8 micro Henry (by calculation, not measurement).   The phase resistance is about 0.3 Ohm.   Voltage:  24 V.   BEMF const is about 5-8 mV/Hz according to the procedure in the tuning guide, but the motor spins with setting of R = .2 Ohm,  and Kt = 70 mV/Hz.

    I'm running open loop.   I have to give the motor a spin with my hand to get it going.   If I increase the speed via the Open To Closed Loop Threshold, the motor might speed up, but more often it stops spinning.  On stopping, sometimes the software cuts out the voltage and recycles the start-up sequence, and sometimes the voltage continues, but the motor fails to spin.    In order to get it spinning at a higher speed I typically have to reduce the value of Phase Resistance.   The motor spins at 2.4 Hz with R set to 0.5 Ohm,  and at ~10 Hz the setting has to be around 0.174 Ohm.   I think the highest speed I've gotten it to is 10.4 Hz.   Higher than that, I can't find a set of parameters that works; usually the motor stops but the voltage continues.  

    If the resistance value is set too high, I sometimes hear a noise from the motor that sounds like friction ... I kind of "scraping" noise ... which goes away if I reduce the resistance setting.

    Also:  I've never gotten things working with Open Loop Current set to 1.6 A.   0.8 A works, but at 1.6 A the waveforms start, but then they cut out after a few seconds.

    Hope all that helps.   I really appreciate any help or advice you can give me in interpreting what's going on, and what might improve things.  We do have a second generation motor being drawn.  It will probably have more windings in the coils.  currently each phase has three coils, and each coil has ten loops.  

    Thanks, Gary

  • Hi Gary,

    Is this BLDC or DC motor ?, DRV10975 can be used with only 3 phase BLDC motors.

    The phase current shows chattering because of low electrical time constant (8u/0.3ohms), recommended minimum operating electrical constant is 100 usec (Section 10.2.1 Design Requirements in datasheet).

    Whats is intended maximum speed for this application ?; looking at BEMF constant and operating voltage maximum electrical speed seems to be 4500-5000 Hz. This speed is too high for DRV10975 since DRV10975 samples current and controls every PWM cycle, maximum speed recommended is 1000 Hz.  I would recommend to spin motor at lower voltage or use lower voltage version for 3 phase BLDC driver DRV10963 or DRV10964

    Since new motor will have higher turns, I am expecting inductance to increase and it will help DRV10975 drive motor effectively.

    Regards,

    Krushal