Harmonic loss comparison of modulating function type PWM techniques for VSI-fed induction motors

High frequency operation of modern power semiconductor devices have permitted higher number of switchings per cycle. PWM techniques based on minimization of some undesirable effect of harmonics (harmonic elimination, loss minimization, minimization of speed and torque ripples, etc.) are now restricted to limited applications. Modulating function (MF) type techniques (for which fundamental output voltage is proportional to modulation index) do not need such large tables, and therefore, they are suitable for widespread applications. Search of best modulating signal amongst several competing alternatives has led to the selection of seven of the best known MF techniques for comparison. For all these techniques, four loss factors (adopted from literature) are plotted against fundamental frequency/over constant volts per hertz range. Three of these loss factors are used to extract useful information about total harmonic losses through the method of loss equivalents. On the basis of this information, the different techniques are expressed in the increasing order of their harmonic losses for certain important frequencies. The fourth loss factor indicates the maximum peak current in the switching devices, and therefore, is used for the comparison on this basis. Studies for the seven and ten switchings per quarter cycle show the superiority of two of the techniques over the others. For deciding the better of the two techniques, the comparison need to be extended to higher number of switchings per cycle