PWM is an acronym for Pulse Width Modulation and varies the switching pulse width of transistors.　This method creates many pulses within a single cycle of the output waveform and outputs the average voltage of all pulse width to as a sine wave.
The following shows the principle of voltage adjustment by the PWM inverter:
The figure compares the modulated wave of a triangular waveform and the signal wave of a sine wave.
The state where the signal wave is larger than the triangular wave is regarded as the positive logic and switches the transistor.
By this switching, the phase-to-phase voltage is output voltage in which the average voltage of pulses modulated into a sine wave.
This control allows for controlling the frequency and voltage by the inverter. Figure 1