Thus varying a, the fundamental output voltage can be controlled.
Constant a operation gives a crude output waveform with rich harmonic content. The dotted lines in Fig. 3b and c show a square wave. If the square wave can be modified to look more like a sine wave, the harmonics would be reduced. For this reason a is modulated as shown in Fig. 3d. Now, the six-stepped dotted line is more like a sinewave with fewer harmonics. The more pulses there are with different a's, the less are the harmonics.