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6-step pwm for ACI motor

Mikhail40741
Prodigy 50 points

For some specific research, i have the task to implement open loop v/f control of ACI motor with 6-step pwm. I used dmclib as base for my v/f module and counter module providing frequency control. As for PWM generation, i intended to use the bldcpwm code, however this module uses commutation scheme, where at any time only two gates used, with high gates being controlled by PWM, and low switches being fully opened. Classic 6-step scheme, which i need,  implies 3-switch operation

As rough example, inverter scheme:

1 3 5

4 6 2

where 1 is phase A high gate, 4 phase A low gate and so on.

First state: gates 1, 5 and 6 are opened

Second state: 1, 6 and 2 are opened. Next image explains it better:

Its not really clear to me, what to do if i want to control output voltage level. For now i came up with next decision: inside every step, control same side gates with pwm and leave other side gate opened. Code example for first state:

if (p->State==0) // 1,5,6
    {
        EPwm1Regs.AQCSFRC.bit.CSFA = 0;
        EPwm1Regs.AQCTLA.bit.CAU = 2;
        EPwm1Regs.AQCTLA.bit.ZRO = 1;
        EPwm1Regs.CMPA.half.CMPA = (int)(CMPVal); // 1A - pwm
        EPwm1Regs.AQCSFRC.bit.CSFB = 1; // 1B - off

        EPwm2Regs.AQCSFRC.bit.CSFA = 1; // 2A - off
        EPwm2Regs.AQCSFRC.bit.CSFB = 2; // 2B - on

        EPwm3Regs.AQCSFRC.bit.CSFA = 0;
        EPwm3Regs.AQCTLA.bit.CAU = 2;
        EPwm3Regs.AQCTLA.bit.ZRO = 1;
        EPwm3Regs.CMPA.half.CMPA = (int)(CMPVal); // 3A - pwm
        EPwm3Regs.AQCSFRC.bit.CSFB = 1; // 3B - off
    }

I didnt do real tests with machine yet, so it is unknown if i am right, or totally wrong, so i would appreciate any opinions and advices.

 

 

  • 0
    Genius 5170 points

    Hello ... as far as I know a traditional 6-pulse inverter can vary only the output frequency ... the magnitude of the output voltage can be adjusted only by controlling the DC bus voltage. If PWM or any other switching techniques are introduced then it becomes a PWM inverter and no longer should be called a six-pulse inverter. The inrtoduction of switching will completely change the harmonic signature of the inverter. How about you setup your system with a front-end DC/DC converter (to change the DC bus voltage) feeding a traditional six-pulse inverter - this way you should have the capability of generating variable frequency and variable magnitude output voltage.

  • 0 in reply to Arefeen
    Prodigy 50 points

    Hello. Thank you for clarification. Unfortunately, apart from controller, target system will have only ac/dc converter, 3-phase inverter and motor itself. So i need to do v/f control with 6-step pwm generation with all i got. I only can hope this will work, as the research isn't related to motor itself, and all i need is to get a spin.

  • 0 in reply to Mikhail40741
    Genius 5170 points

    hello .. this most likely will work as AC motor will operate normally as long as it gets a balanced three phase voltages. However, your system will not look like a traditional six-pulse inverter ... it will be a variation of a standard PWM inverter.