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PStechPaul

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About PStechPaul

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  1. I found an app note on the angular timer. I don't think it will be particularly useful for control of a 3 phase induction motor, especially if it does not implement a rotary shaft encoder. It seems to me that the PIC18F2331 series is the best legacy part to use, and otherwise the dsPIC33FJ32MCxx. The latter part may be preferable because it has support from the MCC or Harmony, but for now I want to keep it simple. http://ww1.microchip.com/downloads/en/AppNotes/90003143A.pdf
  2. I plan to use a look-up table for the PWM values, which may include the third harmonic. Then I will multiply those values based on speed. I have 60 points per cycle, which can be implemented with a 15 point table. One problem I see is that in order to have a full PWM cycle for each sample, the carrier frequency would need to be adjusted for various speeds. There may not be a problem if the PWM frequency is high enough that at least perhaps 10 PWM periods are used at maximum speed, which would mean that, for 60 points per cycle, the frequency for 60 Hz would be 60 * 60 * 10 = 36 kHz. There would still be up to a 10% or so error depending on the frequency. Otherwise, I could generate an interrupt on each PWM period to change the duty cycle, but that would mean a variable PWM frequency depending on speed. Perhaps the rectangular (modified sine) wave would be sufficient. In that case, the pulse width would remain constant, but the time between pulses would vary according to speed. There will be a maximum pulse width based on the inductance and saturation characteristics of the motor. I'm still trying to wrap my brain around these concepts, and maybe I'm over-thinking it. [edit] I had to look up HLT: http://ww1.microchip.com/downloads/en/AppNotes/90003122A.pdf
  3. How can I upload or choose an avatar? Also, how can I request notification of replies, after post has been submitted?
  4. Pretty cool stuff! Around 2004 I rewound a single phase 3/4 HP 120 VAC 4 pole induction motor, to make a 7 VAC, 12 pole,three phase motor. I used a 220V 3 HP VFD and two step-down transformers to run it at 240 Hz which was about 1800 RPM. My idea was to run a three phase motor directly on a 12 VDC battery, or perhaps a 48V battery pack clocked at 4x frequency to get 4x power. I also made a very rough VFD using a PIC16F684, with modified sine (rectangular) waves, and it did run, but after a bit the driver transistors blew out. I know a lot more now about how to properly choose and drive MOSFETs, and that's what I plan to do with this project. I added functionality to my spreadsheet that adds a percentage of 3rd harmonic to the synthesized waveform, and I can reproduce the increased output voltage in that way. This is 15% harmonic. I updated my file: http://enginuitysystems.com/pix/electronics/Three_Phase_Sine_Waves.ods
  5. I will probably use a PIC18F2331 or 2431, mostly because I already have a few on hand. But I might use a PIC16F18325 or similar device. I want to see, first, how simple I can make a three phase motor controller, even using rectangular "modified sine wave" drive, with basic V/f control. As I gain experience, I may choose something like the dsPIC33FJ16MC101 or dsPIC33FJ32MC102. This will be intended for use on small EVs like tractors.
  6. I have been trying to find a good way to produce three phase PWM sine waves, and I was getting stuck on the concept while looking at the phase-to-phase PWM signals. But then I realized that it just required generating three sine waves on the AH/AL, BH/BL, and CH/CL bridge pairs. Also, the PWM values need to be offset by 1/2 the peak value. So I made a spreadsheet that illustrates the waveforms of the individual bridges to ground, as well as the phase-to-phase voltages A-B, B-C, and C-A. Here is the spreadsheet and graphs: I plan to implement this on a PIC and will post source code and other details. Feel free to add suggestions and discussion. Three_Phase_Sine_Waves.ods http://enginuitysystems.com/pix/electronics/Three_Phase_Sine_Waves.ods
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