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MCF8329EVM: What is the expected behaviour if the value entered for inductance is lower than the actual inductance? What if it's higher?

John Potts
Prodigy 30 points
Part Number: MCF8329EVM
Other Parts Discussed in Thread: MCF8329A

Tool/software:

I've been having some trouble getting a motor to behave as expected with the EVM, and discovered a discrepancy today. The datasheet for the motor lists the inductance as 0.43mH at 1kHz, but I recently purchased an LCR meter and am reading around 0.15mH at 1kHz across any two of the phases. I've been having trouble with various aspects of getting the motor working (frequent BEMF faults, MPET has a lot of trouble completing, motor only reaches 90% of top speed, noise frequently causing the gate driver to reset during spinup until Chsd_lss capacitors were added), but when I tried the new value and ran MPET, the motor stopped while spinning up and something seems to have burnt out. The EVM still starts and is recognized by motor studio, but any time I try to start MPET or just start the motor spinning, I get an immediate OCP_VDS fault. The gate driver also immediately heats up to around 50C as soon as power is plugged in.

So, I'm wondering which is more likely: 

  1. the manufacturer's datasheet is inaccurate, the previous poor behaviour was caused in part by the incorrect inductance value, and the damage while starting MPET was unrelated, or
  2. the LCR meter's reading is incorrect, the previous poor behaviour is caused by something else, and the EVM will burn itself out if you set the inductance too low.

I'm also going to follow up with the motor manufacturer, but any clarity on how the MCF8329A gate driver chip behaves when a parameter is set incorrectly would be appreciated.

  • 0
    TI__Expert 6011 points

    Hi John,

    Thanks for reaching out. 

    John Potts said:
    The datasheet for the motor lists the inductance as 0.43mH at 1kHz, but I recently purchased an LCR meter and am reading around 0.15mH at 1kHz across any two of the phases

    Could you verify if you are measuring the right inductance values? Typically phase inductance is what the MCF8329 requires. You can double check the datasheet if the phase inductance is what is mentioned. Refer to this e2e FAQ for more details.

    John Potts said:
    the LCR meter's reading is incorrect, the previous poor behaviour is caused by something else, and the EVM will burn itself out if you set the inductance too low.

    The incorrect inductance value must not have such drastic effects. Any chance the current levels set were too high? What is your fault configurations?

    Regards,

    Sachin S

  • 0 in reply to Sachin S
    Prodigy 30 points

    Hello Sachin,

    Sachin S said:
    Could you verify if you are measuring the right inductance values? Typically phase inductance is what the MCF8329 requires. You can double check the datasheet if the phase inductance is what is mentioned. Refer to this e2e FAQ for more details.

    What do you mean by "if you are measuring the right inductance values", ie what incorrect value could I be measuring? On the motor there are 3 phase wires and 5 wires for the hall sensor, and I'm not measuring the inductance of any of the hall sensor wires. As described on the MCF8329A datasheet and in that FAQ, I've measured the inductance across two phases, and then divided by two to get the per-phase inductance. The results were the same no matter which two phases I measured across, and there is no accessible center tap that I could measure against.

    The motor datasheet does not say whether the listed value is per-phase or phase-phase. However, the listed resistance is described as "terminal resistance", leading me to believe that the values listed on the datasheet are phase-phase. Because of this, I was previously using a value of 0.215mH for the inductance.

    Sachin S said:
    The incorrect inductance value must not have such drastic effects. Any chance the current levels set were too high? What is your fault configurations?

    The fault configurations are as follows:

    FAULT_CONFIG1

    • SATURATION_FLAGS_EN: Disable
    • IPD_FREQ_FAULT_EN: Enable
    • IPD_TIMEOUT_FAULT_EN: Enable
    • MTR_LCK_MODE: Fault automatically cleared after LCK_RETRY time
    • LCK_RETRY: 2 s
    • LOCK_ILIMIT_DEG: 5 ms
    • LOCK_ILIMIT_MODE: Fault automatically cleared after LCK_RETRY time
    • LOCK_ILIMIT: 60%
    • HW_LOCK_ILIMIT: 65%
    • ILIMIT: 30%

    FAULT_CONFIG2:

    • AUTO_RETRY_TIMES: No Limit
    • MAX_VM_MODE: Automatic clear if voltage in bounds
    • MAX_VM_MOTOR: No Limit
    • MIN_VM_MODE: Automatic clear if voltage in bounds
    • MIN_VM_MOTOR: No Limit
    • VM_UV_OV_HYS: 1V for UV and 2V for OV
    • HW_LOCK_ILIMIT_DEG: 2 us
    • HW_LOCK_ILIMIT_MODE: Hardware limit lock detection causes latched fault; nfault active; Gate driver is tristated
    • NO_MTR_THR: 1%
    • ABNORMAL_BEMF_THR: 70%
    • LOCK_ABN_SPEED: 130%
    • LOCK3_EN: Enable
    • LOCK2_EN: Enable
    • LOCK1_EN: Enable

    I'm using the default 1mOhm shunt resistor that comes installed on the EVM, a CSA_GAIN of 40V/V, and a BASE_CURRENT value of 0x400. Furthermore, I have ILIMIT and OL_ILIMIT both set to 30%. From my calculations these set a BASE_CURRENT  of 37.5A, a current limit of 11.25A, and a locking current limit of 22.5A.

    I would hope that setting an incorrect inductance value would not have such drastic effects, but I would like to know what effects it should have. How does the behaviour of the internal control loop change when different inductance values are entered? 

  • 0 in reply to John Potts
    TI__Expert 6011 points

    Hello John,

    John Potts said:
    What do you mean by "if you are measuring the right inductance values",

    Wanted to confirm that we are measuring per-phase inductance and not the phase to phase values. I think you got this right.

    John Potts said:
    How does the behaviour of the internal control loop change when different inductance values are entered? 

    If the inductance value is too far from the actual value, the BEMF estimation and current control would not work as intended (you may see a non sinusoidal, glitchy phase current waveforms)

    Your FAULT configurations look ok. Only suggestion is to enable SATURATION_FLAGS_EN to flag any saturation of current/speed loops.

    Thanks and regards,

    Sachin S