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DRV10974: 14-pole motor-comtroller

Jonathan Navor
Genius 9475 points
Part Number: DRV10974
Other Parts Discussed in Thread: MCF8316A

Hi Team, 

Good day! 

I have posted the customer's below and kindl plesae help us check.

I am using your DRV10974 to drive a 14-pole motor. The motor spins well but the speed resolution is too low (0.2%). Which controller would be suitable?

This answers the question about the 14-pole motor. However my concern is the speed-control. The DRV10974 has an internal 9 bit ADC. It converts the PWM-signal into an analog voltage which is then converted into a digital signal – via the 9-bit ADC – which then controls the motor-speed. Correct? When I increase the PWM-value in e.g. 0.1% steps either nothing happens or the speed jumps by 0.2% steps. For our application – chopper-motor for gas-analyzer - we need a very constant speed @ approx. 1100 rpm. Therefore we regulate the speed by adjusting the PWM in 0.1% steps.

 

The behavior of the system is as follows

 

The output voltage changes by approx. 0.025% after changing the PWM by 0.1%, i.e. change of 0.2% in motor-speed.

This doesn’t sound much, but 2% change of the output voltage is equivalent to 3000 ppm gas-concentration. Therefore we see a step of 30 ppm if the channels can’t be compensated by the reference-channel. In the example the channels are quite synchronous which helps. Still we would prefer to have lower jumps, i.e. higher speed-resolution. Do you have a solution?

Best regards,

Jonathan

  • 0
    TI__Mastermind 23045 points

    Hey Jonathan,

    Thanks for the question and for posting on the MD forum. The PWM signal is filtered using a low pass filter, once this is done the ADC samples to collect the amplitude of the sine wave that is created after the filter. So you assessment above is correct. Have you tried using the analog mode in the device?

    Instead of inputting a PWM signal and have the device determine the voltage from the duty cycle, an analog voltage is input into the device ranging from 1.8V to .27V. I have not tested this to see if it has a tighter resolution than the PWM mode but you can figure out what your output voltage will be based on the formula provided in the datasheet.

    You can calculate the voltage that provides the 1100 RPM needed for the gas analyzer and provide the analog voltage. I am not sure how tight the resolution is on this but it is worth a shot to see if it is better than using the PWM  mode. 

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    TI__Genius 9475 points

    Hi Isaac, 

    Thank you for your response. However our customer is looking for different controller that would be suitable for better speed resolution to drive a 14-pole motor. Kindly help to suggest. 

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Mastermind 23045 points

    Hey Jonathan, 

    I dont think any of the DRV10x devices are going to offer a tighter resolution than what is required for this application. Have you tried looking into the MCF8316A? The MCx are the newest integrated control devices that the team is working on. MCT devices offer trapezoidal control while the MCF devices offer field oriented control. Below is a small excerpt of the MCF8316A datasheet and it looks like there are more bits available for the PWM Input resolution but it is dependent on the PWM frequency used. The analog mode voltage resolution is also smaller than it is for DRV10974. FG output provides mechanical rotation information for up to 30-pole motors so this will work fine for a 14-pole motor.

     

    You can get the EVM here: www.ti.com/.../MCF8316AEVM

    I hope this helps!

    Best,

    Isaac