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Problem with sudden braking

Shiranga Ariyasinghe
Expert 1145 points
Other Parts Discussed in Thread: MOTORWARE

Hi Chris and all,

We are using motorware lab5a with resolver feedback. We have connected the inverter to a 330V battery pack. Once we apply a higher Iq_ref command (ex: 600A) with the load motor accelerate fast without any issue. But when we apply a zero Iq_ref (0A) command form higher Iq_ref command (600A)while the motor is rotating with the load, motor brake with oscillations. It feels like pulse braking. a very high voltage (500V) appears on the dc bus during this kind of braking event. After few oscillations it becomes normal deceleration. This did not occur in low Iq_ref command (ex:200A).

1) Is this normally expected?
2) If it is normal, What is the theory behind that?
3) How we could avoid this pulse braking?
4) Can we apply Zero Iq_ref command suddenly after high Iq_ref command (ex : 600A) with load condition.

Thank you.

  • 0
    TI__Guru** 116090 points
    1) Is this normally expected?
    A: It's normally, motor run into generation mode.

    2) If it is normal, What is the theory behind that?
    A: The machine was working as generator for the direction of torque is opposite to the motor running direction.

    3) How we could avoid this pulse braking?
    A: Reduce torque current slowly to keep both directions are the same.

    4) Can we apply Zero Iq_ref command suddenly after high Iq_ref command (ex : 600A) with load condition
    A: Yes, you can using this mode for the power energy will return to battery, and the resistance of batttery is very high to absorb these energy.
  • 0 in reply to Yanming Luo
    Expert 1145 points

    Thank you yanming,

    We highly appreciate your instant response.

    2) We do not request negative Iq current, so in theory the motor should not go generator mode or driving mode according to inverter four quadrant graph ( Torque should be zero, there is a speed. Then motor should be somewhere in speed axis).

    3) We will test that and update on slow reduction of Iq_ref.

    4) We totally disagree. Without giving a negative Iq_ref command to the motor controller, current should not be returned to the battery. This is the problem we have. According to theory when we apply zero Iq_ref, motor should rotate freely without going in to regen mode.

    Hope to hear from you.

    thank you

  • 0 in reply to Shiranga Ariyasinghe
    TI__Guru** 116090 points
    For PMSM, the motor will stop free without voltage boost if turned off all mosfets or set Iq_ref to zero and motor is not work in field weakening mode. In the same time, you need to set the minimum ouput of PID_Iq to zero. If not, the actual output torque is negative though the Iq_ref is zero.
    But the voltage will be boosted though set the Iq_ref to zero as above if motor is working in field weakening mode.
  • 0 in reply to Yanming Luo
    Expert 1145 points

    Thank you for the response Yanmin,

    I have few more doubts.

    Yanming said:
    you need to set the minimum ouput of PID_Iq to zero.

    Set minimum output of PID_iq mean, should I change outMin to zero as below?

    outMax = _IQsqrt(_IQmpy(maxVsMag,maxVsMag) - _IQmpy(CTRL_getVd_out_pu(handle),CTRL_getVd_out_pu(handle)));
    outMin = -outMax;
    outMin = 0;

    // set the minimum and maximum values
    PID_setMinMax(obj->pidHandle_Iq,outMin,outMax);

    Yanming said:
    But the voltage will be boosted though set the Iq_ref to zero as above if motor is working in field weakening mode

    We are doing torque control (lab5a). How can we go for field weakening mode with torque controlling? for a given constant Iq_ref command, does motor automatically reduce it's torque after base RPM to go for higher RPM? Or should we change the torque (Iq_ref) using controlling method ?

  • 0 in reply to Shiranga Ariyasinghe
    TI__Guru** 116090 points

    should I change outMin to zero as below?
    A: Right. Set outmin to zero.

    How can we go for field weakening mode with torque controlling?
    A: If Id_ref and Id_fdb are less than zero, motor is working in field weakening mode.

    for a given constant Iq_ref command, does motor automatically reduce it's torque after base RPM to go for higher RPM?
    A: No, constant Iq_ref command means constant torque output if no fwc, the motor can't run to high speed if the torque is not enough.

    Or should we change the torque (Iq_ref) using controlling method ?
    A: Yes, it depends on your application requirement, torque control is often used in automotive applications.

  • 0 in reply to Yanming Luo
    Expert 1145 points

    Thank you yanming,

    I think you didn't get my question :) . My question was as something like below. Let say I have a motor with a torque curves are as below for a given voltage(i got this image from web).

    Since we are doing torque control,  we are commanding Iq_ref (xx Amps) to achieve 75Nm (as red curve in graph). Then there must be an acceleration and angler speed will be increased. once motor reach to 4,000rpm (base speed)  with it's maximum voltage (voltage related to red graph) , motor goes to constant power region from constant torque region.  What will happen after that point?

    Because the motor achieved it's maximum rpm (voltage as well) which can spin with that amount of torque (75Nm related to red graph). Motor cannot increase it's rpm further with same amount of torque, but still we are giving same torque command Iq_ref (xx Amps). My question was, at that time should we add any software controller to increase motor speed (frequency) ?  Or will the motor go for further speeds (increase frequencies with it's max voltage)  by reducing it's actual torque output for the same Iq_ref (xx Amps)?

    Hope to hear from you.

  • 0 in reply to Shiranga Ariyasinghe
    TI__Guru** 116090 points
    Got your question. Generally, motor runs from constant torque region to constant power region, the real torque current will decrease and can't output the maximum reference current, the motor will go into filed weakening mode. So you need to control Id, set Id to negative reference value for PMSM, decrease Id_rate reference value for ACIM.
  • 0 in reply to Yanming Luo
    Expert 1145 points
    Thank you yanming,
    Got cleared a lot of things with your answer. But still have to clear some doubts. Before that we are using PMSM.
    If the motor can go filed weakening mode by reducing its real torque current to go higher rpms in constant power region, why would we reduce d-axis current to a negative value?
    Can I find tools and examples that can run end to end simulation model of foc? I have heard a of instaspin sim. Is that available?
  • 0 in reply to Shiranga Ariyasinghe
    TI__Guru** 116090 points
    The bemf constant (USER_MOTOR_RATED_FLUX) is too high to cause the dc bus voltage is not enough to enable motor run high speed, so you have to reduce the bemf constant by using field weakening control. You can find such control theory in most motor drive books or thesis.