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MCF8329A: Troubleshooting LOCK_ILIMIT faults, discrepancy with BUS_CURRENT

Ian Garcia
Prodigy 60 points
Part Number: MCF8329A
Other Parts Discussed in Thread: MCF8329EVM, MCF8316A

I'm testing the MCF8329EVM in an application with pretty high load that engages quickly and often getting LOCK_ILIMIT faults. My motor is rated for peak currents of 78A, my BASE_CURRENT is set to 0x1000 = 150A, and LOCK_ILIMIT is 65% = 97.5A

I'm monitoring BUS_CURRENT through the GUI and never seeing it go above 11A even during tests where the motor faults. I've monitored current out of the power supply with a current probe and oscilloscope and the values are definitely higher, peaks of ~20A, but that's still nowhere near the LOCK_ILIMIT setting.

According to the datasheet LOCK_ILIMIT is based off of phase current, which I presume must be different than bus current. How can I monitor this value? Either through the GUI or with an oscilloscope. I could try attaching probe around one of the phases but I have no way to predict which phase will be active during the fault and only have one current probe. Also why does BUS_CURRENT disagree with what I'm measuring coming out of the power supply? Is it just heavily averaged?

  • 0
    TI__Expert 8840 points

    Hi Ian,

    I assume you are using the default 1mΩ external current shunt resistor that's populated on the EVM by default, and CSA_GAIN = 10V/V?

    Could you please also help me check what your configurations are for the following settings?

    • OL_ILIMIT
    • ILIMIT
    • ALIGN_OR_SLOW_CURRENT_LIMIT
    • HW_LOCK_ILIMIT

    You should be able to monitor the phase current by using clamp current meter on one of the motor phase wires. While you would only be monitor one phase current, it'd still give a good idea of what stage of the motor operation (startup, open loop, or closed loop, etc) the fault occurs and can help narrow down the problem.

    Regarding the BUS_CURRENT, it might be due to that with full scale current = 150A, lower current sensing may not be very accurate due to noise being amplified at lower current level. Also, the BUS_CURRENT is estimated by the device, taking into account the phase currents and other system currents in active mode, therefore, there are also some inherent inaccuracies.

    Regards,
    Eric C.

  • 0 in reply to Eric Chen
    Prodigy 60 points

    Yes using the 1mOhm resistor and CSA_GAIN = 10V/V

    OL_ILIMIT = 5%

    ILIMIT = 60%

    ALIGN_OR_SLOW_CURRENT_LIMIT = 10%

    HW_LOCK_ILIMIT = 80%

    I also increased LOCK_ILIMIT_DEG to 5ms. I initially was getting HW_LOCK_ILIMIT faults but they seem to have gone away after doing this. Abnormal BEMF faults are disabled, I wasn't able to get them to go away even at the lowest setting of 40%. The MCF8316A Tuning Guide mentioned that this can happen due to load dynamics so I wasn't very surprised.

    I'm pretty confident the motor is operating in closed loop. It's allowed to stabilize at its target speed before the load is applied. With no load (just belt drive and spindle attached) there's no issues. It starts up smoothly and maintains its target very well. I have two additional current probes on order so I'll try getting a scope capture of all the phases and the fault line early next week and report back.

  • 0 in reply to Ian Garcia
    TI__Mastermind 19825 points

    Hi Ian,

    Today is a holiday for the US, but we will aim to provide a response later this week.

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 60 points

    Hi I think help is no longer needed. After adjusting some PID terms and increasing LOCK_ILIMIT_DEG I'm no longer getting the faults. I misspoke earlier, should have said HW_LOCK_ILIMIT_DEG