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TMDXIDDK379D: Limiting speed issue with FOC for torque control

Cherry Zhou
Mastermind 22235 points
Part Number: TMDXIDDK379D

Hi team,

Here's an issue from the customer may need your help:

1) The FOC is in torque control mode, and it is expected that the speed will be limited if the speed goes to maximum when no load is present.  For example, half the maximum speed ,is it needed to turn on the speed loop?

Either time torque control or speed control, the Iq is eventually adjusted, and the Iq is reduced to reduce the speed, when Iq become smaller, the torque is reduced too. Is it necessary to adjust the Iq larger?  Do not increase the speed even if the Iq increases with the load?  

In general, servo motors sold on the market are all torque controlled and speed limited. The customer would like to know how to achieve that?

2) The speed of the PMSM motor is proportional to the frequency of the three-phase inverter, and the speed should be determined by the speed of the rotating vector. But looking at the FOC algorithm, which is ultimately used to adjust the Iq, does the size of the Iq only affect the duty cycle of SVPWM? With a small duty cycle, the speed is low, not the frequency?

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Guru** 114520 points
    Cherry Zhou said:
    is it needed to turn on the speed loop?

    That depends on the applications, it's an engineering topic. The user can implement the speed closed-loop for limiting the maximum speed even though the torque control mode is using.

    Cherry Zhou said:
    2) The speed of the PMSM motor is proportional to the frequency of the three-phase inverter, and the speed should be determined by the speed of the rotating vector. But looking at the FOC algorithm, which is ultimately used to adjust the Iq, does the size of the Iq only affect the duty cycle of SVPWM? With a small duty cycle, the speed is low, not the frequency?

    What do you mean the frequency at here? You might take a look at the application notes on ti.com or some textbooks about FOC if possible. The FOC will adjust the d and q-axis current to control the torque output of the motor, and also control the motor speed or frequency if the speed closed loop is enabled.

    Not fully understand your questions. Is customer using an example labs from TI? or their own software?

  • 0 in reply to Yanming Luo
    TI__Mastermind 22235 points

    Hi,

    Thanks for your help.

    Routine: IDDK_PM_Servo_F2837x-V2

    The drive board is not a TMDXIDDK379D, but has also been built with stm32.

    Yanming Luo said:
    What do you mean the frequency at here?

    The frequency they're referring to is the rotation frequency of the three-phase voltage finally generated after the foc, which is f in Speed=60f/p.

    The customer would like to know how speed is limited in torque control mode, specifically which variable is controlled inside the foc.

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Guru** 114520 points

    Generally, the maximum speed will be limited to the setting reference torque current, the motor and the load when the motor is working in torque control mode. If the users want to limit the speed also when using torque control mode, they have to add a speed closed-loop as well and just set the reference speed to the limitation speed and set the maximum output of the speed PI regulator to the reference torque current.