A pure sine wave is obtained by using the SPWM technique. In this control technique, 2 triangle waves of a high frequency (2kHz in this case, but it's usually higher) are modulated with a 50Hz sine wave to obtain a PWM signal whose pulse width varies sinusoidally. The SPWM signals are fed to MOSFETs that switch power to a center tapped transformer.
On the secondary, a sine wave is obtained by filtering the SPWM signal using a 50Hz bandpass filter and a 2kHz harmonic trap filter.
