To have a net output voltage with reduced harmonic content, output voltage from two or more inverters can be combined by means of transformers. The essential condition of this scheme is that the output voltage waveforms from the inverters must be similar but phase shifted from each other. Figure 1(a) shows a scheme for connecting two inverters and two transformers for harmonic elimination. Their output voltages, $E_{L_{1}}$ from inverter 1 and $E_{L_{2}}$ from inverter $2,$ are shown in Figure 1(b). As shown in this figure, waveform $E_{L_{2}}$ is taken to have a phase-shift of $\pi / 3$ radians with respect to $E_{L_{1}}$ waveform. By adding the vertical ordinates of $E_{L_{1}}$ and $E_{L_{2}},$ resultant output voltage $E_{L}$ is obtained. In this scheme, it is assumed that the transformers have a turns ratio of $1 : 1 .$

The absence of third harmonic in the output waveform $E_{L}$ can be explained by writing the Fourier-series for $E_{L_{1}}$ and $E_{L_{2}} .$

$\therefore \quad E_{L_{1}}=A_{1} \sin \omega t+A_{3} \sin 3 \ \omega t+A_{5} \sin 5 \ \omega t+\ldots$

Since the output of second inverter, $E_{L_{2}},$ is delayed by $\pi / 3$ .

$E_{L_{2}}=A_{1} \sin (\omega t-\pi / 3)+A_{3} \sin (\omega t-\pi / 3)+A_{5} \sin (\omega t-\pi / 3)+\ldots$

The resultant voltage $E_{L}$ is obtained by vector addition.

$E_{L}=E_{L_{1}}+E_{L_{2}}=\sqrt{3}\left[A_{1} \sin (\omega t-\pi / 6)+A_{5} \sin 5(\omega t+\pi / 6)+\ldots\right]$

From the above expression of $E_{L},$ it is observed that with the phase shifting of $\pi /3$ and combining voltages by transformer connection, it is possible to eliminate third harmonics. Along with third harmonics, the multiples of third harmonics, such as $9,12,$ are also eliminated. It should be noted that the resultant fundamental component is not twice the individual voltage, but it is $\sqrt{3} / 2$ $(=0.866)$ of that for individual output voltage and the effective output has been reduced by $(1-0.866)=13.4 \% .$ The main disadvantage of this method of harmonic reduction is the need for more number of inverters and transformers of similar ratings.