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Original question:

DRV10983: Inductive AVS disabled, but still Voltage Surge is observed at 24V.

DRV10983: Speed Command Buffer always non zero !

Himanshu Rasal
Prodigy 45 points
Part Number: DRV10983

Hi,

I am using a DRV10983 to drive a 24V DC motor. It was observed that the speed command buffer at 0x1B and 0x1C are non zero even when zero speed command is given and motor has stopped.
This is seen when Mechanical AVS has been enabled but does not appear when the Inductive AVS is enabled solely. 

1. What is the cause of this non zero register value and will this cause any issue in run time ?
2. Why is this observed only when Mechanical AVS is enabled ?

Logs for your reference.

                 TIME               ||     Speed Command || Speed Command Buffer || RPM_Read

[2021-09-30 16:03:43.104]                    S00                        B245                           R428
[2021-09-30 16:03:44.114]                    S00                        B232                           R343
[2021-09-30 16:03:45.103]                    S00                        B220                           R286
[2021-09-30 16:03:46.105]                    S00                        B207                           R240
[2021-09-30 16:03:47.120]                    S00                        B195                           R205
[2021-09-30 16:03:48.111]                    S00                         B182                           R178
[2021-09-30 16:03:49.115]                    S00                        B169                            R153
[2021-09-30 16:03:50.113]                    S00                        B157                            R133
[2021-09-30 16:03:51.116]                    S00                        B144                            R116
[2021-09-30 16:03:52.125]                    S00                        B132                            R100
[2021-09-30 16:03:53.114]                    S00                        B119                             R86
[2021-09-30 16:03:54.121]                    S00                        B106                            R74
[2021-09-30 16:03:55.113]                    S00                        B94                              R63
[2021-09-30 16:03:56.125]                    S00                        B81                              R50
[2021-09-30 16:03:57.134]                    S00                        B69                              R40
[2021-09-30 16:03:58.125]                    S00                        B56                              R33
[2021-09-30 16:03:59.134]                    S00                        B44                              R21
[2021-09-30 16:04:00.140]                    S00                        B31                              R16
[2021-09-30 16:04:01.133]                    S00                        B18                              R14

//Fan has stopped completely here


[2021-09-30 16:04:02.140]                    S00                        B7                                R14
[2021-09-30 16:04:03.125]                    S00                        B7                                R14
[2021-09-30 16:04:04.135]                    S00                        B7                                R14
[2021-09-30 16:04:05.143]                    S00                        B7                                R14
[2021-09-30 16:04:06.135]                    S00                        B7                                R14
[2021-09-30 16:04:07.146]                    S00                        B7                                R14
[2021-09-30 16:04:08.134]                    S00                        B7                                R14
[2021-09-30 16:04:09.143]                    S00                        B7                                R14
[2021-09-30 16:04:10.153]                    S00                        B7                                R14
[2021-09-30 16:04:11.145]                    S00                        B7                                R14
[2021-09-30 16:04:12.154]                    S00                        B7                                R13
[2021-09-30 16:04:13.154]                    S00                        B7                                R14
[2021-09-30 16:04:14.153]                    S00                        B7                                R13
[2021-09-30 16:04:15.161]                    S00                        B7                                R13
[2021-09-30 16:04:16.152]                    S00                        B7                                R13
[2021-09-30 16:04:17.160]                    S00                        B7                                R13
[2021-09-30 16:04:18.163]                    S00                        B7                                R14
[2021-09-30 16:04:19.155]                    S00                        B7                                R14

  • 0
    TI__Mastermind 24415 points

    Hi Himanshu,

    If the speed command suddenly drops such that the BEMF voltage generated by the motor is greater than the voltage that is applied to the motor, then the mechanical energy of the motor is returned to the power supply and the VCC voltage surges. The mechanical AVS function works to prevent this from happening. The DRV10983 buffers the speed command value and limits the resulting output voltage, UMIN, so that it is not less than the BEMF voltage of the motor. The BEMF voltage in the mechanical AVS function is determined using the programmed value for the Kt of the motor along with the speed. Below figure shows the criteria used by the mechanical AVS function.

    Inductive AVS does not work in DRV10983. 

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Prodigy 45 points

    Hi Vishnu,

    Thank you for the swift reply. Yes I understand the functionality, but since when I give speed command zero the Vu should eventually turn out as 0V and the buffer also should turn out 0%. But it was observed that the buffer as well as the RPM read were non zero even when the motor had stopped rotating.

    The motor was at halt but the buffer still kept showing PWM period percentages and the RPM read was still non zero, noting that the BEMF when the motor is at halt should be zero.

    I did not understand this behavior ! 

  • 0 in reply to Himanshu Rasal
    TI__Mastermind 24415 points

    Himanshu,

    Looks like the programmed Kt value might be too high. Can you reduce the programmed Kt and check if it improves? If you see the difference between Speed Command and Speed Cmd Buffer getting closer, continue to reduce the Kt, until they are the same.

    Also, for how long do you see the speed command buffer and rpm read back non-zero value after the fan reaches stationary state? 

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    TI__Mastermind 26046 points

    HI Himanshu,

    Do you need any additional help?

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    TI__Mastermind 24415 points

    Hi Himanshu,

    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