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DRV10983Q1EVM: Abnormal Back EMF

Ganesan Manikandan7
Intellectual 285 points
Part Number: DRV10983Q1EVM

Hello Sir/Madam, 

I am trying to spin a 6V BLDC with the DRV10983Q1-EVM by connecting the three phases to U,V,W and supplying 7V. I have programmed phase resistance and Kt value by:

1. Measuring no load resistance between two phases and dividing it by 2

2. Kt= [6/(40,000 rpm/60/2 pole pairs)]  ~ 18 mV/Hz . I have tweaked the value of Kt by +- 30% from this value to get it up and spinning. 

However, I see a back EMF error Code 2 as the system moves to closed loop. This however does not stop motor operation as I have disabled lock detect on the software GUI. I would like to understand what am I doing wrong and if this would affect the torque curves and characterization of the motor?

Please see attached image above.

Thanks,

Ganesan M.

  • 0
    TI__Mastermind 18900 points

    Hey Ganesan,

    If the driver does not trigger any other fault during operation after disabling the fault detection for the BEMF then there could be problem with the programmed Kt. As a result, let’s make sure that verify your measured Kt value with a different method highlighted in the tuning guide.

    Take your motor and manually spin it with your fingers when the driver is turned off. In addition, probe any of the phase voltages in reference to ground. You should see an induced sinusoidal type voltage waveform (known as the Back EMF) appear on the oscilloscope. Then measure the voltage of the waveform, peak to peak, and the frequency of the waveform. Then, using the equation below, you should be able to find Kt.

    Kt = (V_pk-pk/2)*(1/f_BEMF)

    BEMF abnormal checks the programmed Kt and actively measures the Kt (in the same way mentioned above) and compares them. If they are not within a threshold for a certain amount of time, then the fault will trigger. Usually, this is helpful when the motor is stuck and the BEMF isn't being generated because the motor isn't moving. But this could be a false trigger based on your description. Look at section 8.4.8.3 in the datasheet for more information

    Measure Kt in the new method above, insert the new value, and see if BEMF abnormal continues to trigger. Let me know if anything else unexpected happens from here.

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Intellectual 285 points

    zwmd012012-256_torque-curve.pdfHi Cole, 

    Thanks for getting back to me. With some effort, I was able to measure the Kt value manually, and it turned out to be 9 mV/Hz. I am able to spin the motor without any faults on the GUI. However, I see the following concerns:

    1) The motor draws more current at no load than specified by the data sheet. At no load 140 RPM, I see around 600 mA while the datasheet calls out 250 mA. 

    2) It gets hot within minutes of spinning at the maximum no load RPM of ~140 

    3) It is extremely noisy, contradicting the noise numbers provided by the vendor.

    Please find the attached motor torque curves and noise specifications. I believe I am not driving the motor efficiently and that, consequently, is the reason for the noise and the heat. Please share your thoughts.

    Best,

    Ganesan M.ZWMD012012-256 Inspection.pdf

  • 0 in reply to Ganesan Manikandan7
    TI__Mastermind 18900 points

    Hey Ganesan,

    Looking at the datasheet/test doucment of the motor, it looks like the motor is rated for 6V DC. Is this a maximum spec? If so, the DRV10983Q1 may not be the device for you. The rated voltage for VCC on the 983Q1 can go up to 28V, which is far above the maximum for the motor. As a result, if you were running the motor at 100% duty cycle, and the DRV10983Q1 has a VCC of 28V, then the motor is effectively seeing 28V. This would explain the extra heat and current you are experiencing.

    Unfortunately, we don't have a device for the 6V range but the DRV10983Q1 can go down to 6.2V operation if the 6V is not a maximum spec.

    Please let me know if you have any other questions.

    -Cole

    edit: 6.2V minimum on 983Q1 not 8V