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Relationship between Fundamental PWM frequency and Rotor Speed

Patrick Fisher2
Intellectual 785 points

Hi all,

First of all, sorry if the title is misleading, hard to explain....

Basically, I put a scope on one phase of a 22pole motor running at 100 and 500RPM and want to know what the math is that relates the spike to the mechanical frequency.... PWM is set to 45kHZ, but there obviously is a frequency 'riding' (other way round I guess) on that to generate the rotating magnetic field which determines the rotor speed. I figured it should be easy enough to reverse-engineer the relationship from the frequency found from FFT given that its a synchronous motor, but I haven't been able to... FOC complicates things since it's no longer in discrete 60 degree 'segments'....

The first image is the motor at 100RPM, spike occurs at ~85-90Hz...

Second image is the motor at 500RPM, spike occurs at ~170-180Hz...

Thanks, if i'm not even in the ballpark with what I am trying to do, could you explain what actually does determine the frequency of the rotating magnetic field - regarding parameters in user.h and speed setting.

  • 0
    TI__Guru** 119680 points

    Hmmm.

    With a 22 pole motor,

    100 RPM * 22 poles / 120 = 18.333 Hz sine wave being applied (this is the speed of your stator flux, hence the speed of the rotor flux following)

    500 RPM * 22 poles / 120 = 91.666 Hz sine wave being applied

    Your 45 KHz PWM (22.22 usec)  is just how many "discrete voltage steps" make up the construction of this waveform.

    Take your 100 RPM,  that's 0.054 seconds for a full electrical cycle, * 45 KHz = 2454 PWM "voltage step" updates in the sinewave cycle.

     

     

  • 0 in reply to ChrisClearman
    Intellectual 785 points

    Thanks Chris,

    That was exactly my understanding before my scope sent me off on a tangent... 

    Regarding the FFT, am I mistaking to expect to see a spike in the spektrum at 18.33 & 91.66Hz? Fresh from a comms course so I may be thinking about this the wrong way - 45kHz carrier 18hz modulation - hence my thinking I could see  the stator flux using FFT - maybe all the harmonics make this too hard and I am just picking up mechanical vibrations?

    -  Maybe I could generate the pure 45kHz (50% duty) on a boosterpack and minus it to see the stator flux? If nothing else it would be an interesting exercise:)

  • 0 in reply to Patrick Fisher2
    TI__Guru** 119680 points

    Patrick,

    To be honest, I'm not sure. Out of my area of knowledge.

     

  • 0 in reply to ChrisClearman
    Intellectual 785 points

    No worries Chris, I'll let you know if I have any success scoping the stator flux...

  • 0 in reply to Patrick Fisher2
    Expert 1410 points

    Hi Patrick,

    What about the current waveform in time domain? The current is sinusoidal? Perhaps you have a high distorsion?

    Regards.

    Emmanuel