DRV10964EVM: Motor Speed

Hi ACGUY,

Thanks for posting your question in MD forum. We do not expect to see any change in the speed at 15 kHz input PWM frequency. Can you help me answer below questions.

1. Can you provide details on how this was tested? Did you increase the PWM frequency on the fly while the motor was spinning? 

2. How much is the difference in speed you see at 15 kHz versus other frequencies for the same duty cycle? Can you provide waveforms by plotting PWM duty cycle Vs speed at different PWM frequencies including 15 kHz?

Regards,

Vishnu

Hello Vishnu,

Thank you for reply,

1. Yes, I changed the input frequency during operate.
2.Please check the attached graph.

PWM duty is 90-% constant. The difference in motor speed change will be large when reduce the PWM duty.

Could you explain why this is happening?

Best regards,
ACGUY

Hi ACGUY,

Thanks for sharing the data. Did you give sufficient time for the motor speed to settle down before capturing this data? 

Regards,

Vishnu

Hello Vishnu,

After 1-min from change the PWM frequency, I measured each them. I think the motor speed was stable.
>> Did you give sufficient time for the motor speed to settle down before capturing this data?
Please tell me the intention for this question.
Do you think this device is affected by PWM frequency?
Does the motor speed change momentarily when the PWM frequency changes?

Regards,
ACGUY

Hi ACGUY,

When we change the PWM frequency, the PWM period changes and the device takes some time to read the duty cycle and that's the reason why I asked to wait before measuring the speed. Do you see the change in the speed when you change the PWM frequency after stopping the motor as opposed to changing the PWM frequency on the fly?

Regards,

Vishnu

Hi ACGUY,

Just wanted to follow up with you to see if you have any updates.

Regards,

Vishnu

Hello Vishnu,

The result wes same. Please try it you onEVM.

Regaids,
ACGUY

Hi ACGUY,

Just wanted to make sure we are on same page. So at 15 kHz, you measured the speed, stopped the motor and applied 30 kHz PWM frequency and measured the speed and you see both speeds to be different at same duty cycles? I will try this on the EVM to see if I can replicate the same issue. 

Regards,

Vishnu.

Hello Vishnu,

I tried it that on 15-kHz and 100kHz PWM frequency, the results ware the same as last time.

Regards,
ACGUY

Hi ACGUY,

Did you mean that the speed at 15 kHz and 100 kHz for the same duty cycle were the same?

Regards,

Vishnu.

Hello,

The motor speed has different on 15-kHz and 100-kHz input PWM frequency. Duty of input PWM is 90%(constant).This result is same as the last time.
If you have distrust on report from me, please check on actual with EVM.

Regards,

ACGUY

Hi ACGUY,

Noted. Thanks for sharing the details. I still didn't get a chance to replicate this issue on the EVM. Will check tomorrow and let you know my feedback.

Regards,

Vishnu

Hi ACGUY,

I did notice some drift in the speed when I changed the PWM frequency. Looks like the input PWM circuit is either miscalculating or when changing the input PWM frequency to the pin, the effective duty cycle is changing. I would recommend to set the PWM frequency to a fixed value.

Regards,

Vishnu.

Hi Vishnu,

Thank you for confirming.
Request;
1: Please teach me the correct formula for motor speed using Duty and PWM Frequency.
2: Please include the information to the datasheet.
3: Please allow me to tell the customer that the motor speed control by DRV10964 depends not only on the duty of the PWM signal but also on the frequency of the PWM signal. (I will tell "The cause and reason are unknown" too.)

Best regards,
ACGUY

Hi ACGUY,

I will close this for now, since I haven't heard a response back. If you want a follow-up, please feel free to reply to this thread or create a new thread.

Regards,

Vishnu