This method may be called as wave-stepping, in which pulses of different widths and heights are added to produce a resultant stepped wave with reduced harmonic content. Figure 1(a) to shows two stepped-wave inverters fed from a common d.c. supply. These inverters are connected to a common load through transformers having turns ratio of $1 : 3$ and $1 : 1,$ respectively.

The inverter 1 is so operated that its output voltage is $E_{L_{1}},$ as shown in Fig.1(b). The output-voltage level is either zero or positive during the first half-cycle. During second half-cycle, the output voltage would be either zero or negative. This type of modulation in which the output voltage has only two levels during any half-cycle, is called as two-level modulation.

The inverter 2 is so operated that its output voltage is $E_{L_{2}},$ as shown in Fig.1(c). It is observed front $E_{L 2}$ waveform that the level of output voltage is positive, zero and negative during the first half-cycle. Therefore, inverter 2 is operated with three-level modulation.

The resultant output voltage waveform from a series combination of inverter 1 and inverter 2 is obtained by superimposing the waveforms of Fig.1(b and c), as shown in Fig.1(d). The load- voltage waveform of Fig.1(d) shows that the amplitude of output voltage is $4E_{\mathrm{dc}}$ and waveform has four steps.

Fourier analysis of Fig.1(d) would give harmonics whose amplitudes would depend upon the values of $P_{1}$ , $P_{2}, P_{3}, P_{4}$ and amplitude of $E_{L} .$ By a proper choice of these parameters, third, fifth and seventh harmonics can be reduced considerably and the fundamental component optimized. It is noted that the three-level modulation of inverter 2 helps achieving the required wave-stepping of the resultant output voltage waveform.