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Measure the LR time constant with DRV10983

Howard Zou
Genius 12365 points
Other Parts Discussed in Thread: DRV10983, DRV10975

The DRV10983 tuning guide says that the amplitude of the current ripple represents the LR time constant of the motor, but how can we know the exact time constant? For example, the current ripple in the tuning guide is 45mA, so the time constant is ? seconds.

Thanks for answering.

  • 0
    TI__Guru** 114905 points
    Hi Howard,

    Our experts have been notified and should reply soon.
  • 0 in reply to Rick Duncan
    TI__Expert 3010 points

    Hi Howard,

    Current ripple is used in formula described in DRV10983 and DRV10975 Tuning Guide in Section  5.1.5 Commutate Control Advance Mode and Setting  to calculate T Control Advance  : 20 μs × (Imax / Current Ripple) × (Open Loop Current × R) / VCC

    The procedure to fine tune T Control Advance setting is described in Section 5.4 Control Advance Time Fine Tune 

    Regards,

    Krushal 

  • 0 in reply to Krushal
    TI__Genius 12365 points

    Thank you Krushal, I've read that section.

    Can I consider that the current ripple is only used to calculate the Control Advance, and there is no equation to calculate the real LR constant time(Seconds) from current ripple(Ampere), right? We still have to measure L and R to get the constant time T=L/R.

    Regards,

    Krushal

  • 0 in reply to Howard Zou
    TI__Expert 3010 points
    Hi Howard,

    During Align state when motor is not spinning, essentially constant voltage (fixed duty cycle) is applied across LR of motor. So ripple current should give you good estimation of LR time constant. The duty cycle can be calculated using (Open loop current * R / VCC) and switching frequency is fixed to either 25 kHz or 50 kHz

    Regards,
    Krushal