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Field Oriented Control Basics (preferably VISSIM context)

Arif12426
Prodigy 10 points


Hi guys,

I am currently trying to understand FOC using the VISSIM examples. I have some basic questions and would love to have some answers from experienced guys:

1) In AC motors what I simply understand that if I change the frequency I'll change the speed (rpm) of the motor and if I increase the voltage of the AC input (freq const) it will increase the torque..right?

2) In sensored FOC in Inverse Park Transform if I change the Q value voltage is changing causing torque change and D = 0...my question is how the theta angle will change and start commutation? Because when the motor is standstill the increase of Q only causes the Inverse Park Transform output to change a DC value (as the motor is not rotating). So is it the inrush of current causing minute movement which is sensed by the encoder and starts a cumulative action?

3) If so which direction the motor will turn? If I ask in a different way how do I make sure the motor turns to the right direction?

4) In VISSIM example pmsm32sim I have tried and it works without the current feedback. So what is the significance of current feedback here? As I already have Q axis PID limiting values?

5) Park Transform should give DC output of D and Q from Phase Current sensing. But in VISSIM simulation and in reality the D & Q output from the Park Transform block is noisy (AC) as the inputs itself. So my question is just theoretical or really used in practical circuits. or in the context of the VISSIM pmsm32sim what is the significance of using the Park transform in noisy current sensing?

  • 0
    Prodigy 70 points

     

    1) Generally speaking yes, so long as the torque load does not cause the slip between the driving phase field frequency and the motor rotational frequency to get too large.

    2) It is an optical absolute encoder that gives exact rotor position. The sensorless technique looks for a back EMF superimposed on the driving voltage from the PWM signal. The back EMF is only present when the rotor is moving, so startup of a sensorless motor is a problem. You can either use an encoder for startup, or put out a slow open loop field rotation to start the motor turning, then switch to sensorless when sufficient back EMF is detected.

    3) If you mean sensorless, then choose the open loop direction you want at startup time.

    4) the PMSM32sim example uses the Id, Iq currents to create an estimate of rotor position (thetaSMOpos). But it uses the actual rotor position for the control.  There is a plot to compare actual vs estimated rotor position.

    5) Yes, the feedback currents can be used in feedback for real sensorless motor control. However, you are correct that you need low noise, clean signals to get good response. That means you need to take care when designing the analog portion of your inverter board. The DRV8312-kit reference kit from TI shows such an design and VisSim runs a PMSM motor quite well in sensorless mode with it.