Switching signal generation for a three phase inverter using SVPWM

[Kalpa Pathum Welivitigoda]

Hi, I am developing a three phase 2 level inverter using SVPWM. I am using the power control PWM modules of the PIC18F4431 microcontroller and I am using SVPWM using reduced computation method as presented in "Design and Implementation of Space Vector PWM Inverter Based on a Low Cost Microcontroller" paper.

For the time being I do not sense any external three phase instantaneous values for the algorithm, instead I have an array of of 900 samples from a pure sin wave. My clock speed in the PIC is 8MHz and my PWM frequency is 10kHz (The IGBT pack I am using can only go up to 20kHz). The output signal should be a 50Hz sinusoidal waveform.

I have got two questions,

1) How to select sampling time so that the output voltage waveform is of 50Hz?
2) How to select the optimum switching frequency for the inverter?

[1] https://link.springer.com/article/10.1007/s13369-012-0464-2#page-1

 

[andig]

1. A lookup table constructs your output waveform. Say, taking a 180° table of 36 points you can have the table increment (Via an Index Counter) such that the whole 36 are repeated every 20mS=50Hz. Now if you offset the count with 12 and 24 (120°) you get the other two phases. Generally the increment of the table index will come from a timer interrupt so varying that offset gives you a variable frequency drive.

2. It depends upon your switching devices and power levels. Commercially available IGBTs with power level of 75Amps/1200V can be hard switched to about 20KHz then switching losses increases. However MOSFETs (Not the SiC ones, boy they are expensive) offer lower voltage level but higher frequency can be hard switched. For motor drives with 3rd Harmonic Injection for Vboost, I would not recommend a PWM Freq more than 20Khz.