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Overmodulation question

RLetizia
Intellectual 990 points
Other Parts Discussed in Thread: MOTORWARE

I’m testing a Marathon AC induction motor #56H17E5303, ½ HP, 4 pole, with the HV Kit.

Using Lab 10b, to reach rated speed (1800 RPM No Load) an over-modulation of 1.33 is needed.  Without over-modulation the no load speed is limited to around 1650 RPM.

But with over-modulation the motor becomes unstable with load.   Adding load distorts the current waveform and causes the motor to loose speed and sometimes stop.  The instability with load is less with less over-modulation, but then the top speed is correspondingly lower.   For example, with over-modulation set to 1.10 the motor runs OK at full load, but max speed is around 1690 RPM.

In a previous post Chris indicated that over-modulation works for most motors, but in a handful of cases current samples are too poor to keep stability.  He also indicated an updated better over-modulation technique should be in the next MotorWare release.

From my problem description, does it sound like my motor is one of the problem motors?

Any suggestions to make this motor deliver rated speed and torque?

Is there a schedule yet for the next release of MotorWare and will it have the over-modulation improvement?

Thanks.

  • 0
    TI__Guru** 118000 points
    Yes, we have the Over-Modulation update on the list for MW 15, so it should make it in. It will be August or September.

    Yes, it sounds like you are seeing this issue. The current samples you are getting aren't good enough hence the estimator is drifting and the angle for field alignment isn't good enough.

    You may want to double check the speed controller also. Typically as you reach top speeds you need to reduce the Kp for stability....but that doesn't quite fit what you are describing.

    Another thing to try is to limit to 1.0 modulation. Enable PowerWarp. Under no load you should be able to go MUCH faster than rated. If this is stable in the 1800-2000 RPM range then you know it is not your speed controller and you should just focus on the over-modulation issue...
  • 0 in reply to ChrisClearman
    Intellectual 990 points
    Hi Chris,
    Thanks for the suggestions and information.

    With the over-modulation limited to 1 and PowerWarp enabled the motor runs to 1900 RPM or higher. So it looks like the problem is with over-modulation with poor current sampling.

    I don't fully understand the Users Guide regarding 3-Shunt resistor current sampling and its limitations. Section 17.5 makes it sound like phase currents are selectively sampled depending on the drive states, but I don't see this implemented in MotorWare. The ISR always samples 3 currents, and it appears all 3 values are used in the control.

    Can you briefly explain why with 3-shunt resistors, sampling is harder when over-modulating, and why the "Sample Here" technique in section 17.5 isn't used or needed? Or maybe I am looking at this wrong.

    Thanks again.
  • 0 in reply to RLetizia
    TI__Guru** 118000 points
    " but I don't see this implemented in MotorWare."
    this is implemented in proj_lab10 where the svgen module is replaced by svgen_current

    "The ISR always samples 3 currents, "
    correct

    "and it appears all 3 values are used in the control."
    incorrect. only the first 2 are used until modulation index is large enough, then selectively used. the new svgen_current is similar, but has some different filtering and selection logic.

    "Can you briefly explain why with 3-shunt resistors, sampling is harder when over-modulating, and why the "Sample Here" technique in section 17.5 isn't used or needed? Or maybe I am looking at this wrong."

    When you are in high modulation the "on-time for the low-side" will be too small to get a valid sample. This will rotate between the phases.
  • 0 in reply to ChrisClearman
    Intellectual 990 points
    Sorry, I was looking at svgen for the wrong lab.
    Your explanation is great. Thanks.