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varying the voltage for 3-phase inverter
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hussamaldean:
Hi all,
I am currently doing a prototype of variable voltage variable frequency (for learning purposes) and I managed to generate the required pulses from arduino mega and also the frequency is variable.
now I am using regulated PS which set to 12VDC and I want to change the DC to the output of the inverter without changing the input voltage
is it done using duty cycle or there is something I am missing ?
regards
note: It took around three months of extensive study of timers in arduino and how to generate the SPWM signal :D
MagicSmoker:
Yes, for a straight up "V/Hz" (aka scalar) 3-ph. inverter you adjust the width of each pulse in the sinusoidal sequence in proportion to the desired RMS output voltage. Where most people get into trouble is with extremely narrow pulses near the zero crossings of the sine wave - just because the MCU can generate a 50ns wide pulse doesn't mean the bridge switches can reproduce it (especially IGBTs).

hussamaldean:

--- Quote from: MagicSmoker on May 22, 2019, 03:53:49 pm ---Yes, for a straight up "V/Hz" (aka scalar) 3-ph. inverter you adjust the width of each pulse in the sinusoidal sequence in proportion to the desired RMS output voltage. Where most people get into trouble is with extremely narrow pulses near the zero crossings of the sine wave - just because the MCU can generate a 50ns wide pulse doesn't mean the bridge switches can reproduce it (especially IGBTs).

--- End quote ---
I am using IR2104 with IRF510 mosfet
just adjusting the duty cycle?
schmitt trigger:
There are many ways to skin a cat, MagicSmoker has proposed the preferred solution, and briefly outlined its pitfalls.
But to follow on your idea that you want to change the DC output bus to the inverter, FOR LEARNING AND EXPERIMENTATION purposes I would suggest:

Make a front-end buck converter, whose output voltage would increases/decreases simultaneously and directly with the value of with the synthesized output frequency.
In other words the converter's duty cycle is proportional to the frequency you are synthesizing.



MagicSmoker:

--- Quote from: hussamaldean on May 22, 2019, 03:55:42 pm ---I am using IR2104 with IRF510 mosfet
just adjusting the duty cycle?

--- End quote ---

The parts you are using for the gate driver and bridge switches aren't really relevant because you change the duty cycle in software*. I am not a programmer, but the general idea to digitally synthesize a variable voltage, variable frequency sine wave is to store a series of sine constants in a lookup table** and then multiply all of the sine table values with another constant to get the on time for the bridge switch which is supposed to be active for that portion of the waveform. In fact, one of my favorite old-school web sites has (among other interesting things) a calculator that generates all the constants for a sine lookup table with however many bits you want: https://www.daycounter.com/Calculators/Sine-Generator-Calculator.phtml



* - it is entirely possible to do this strictly with analog circuitry, or mixed analog and digital logic, but doing it this way is either obsolete ot a lost art, depending on your perspective.

** - usually just the values from 0 to 90 degrees are stored.
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