In this method of modulation, several pulses per half-cycle are used as in the case of multiple pulse width modulation. Instead of maintaining the width of all pulses the same as in the case of multiple-pulse modulation, the width of each pulse is varied proportional to the amplitude of a sine wave evaluated at the centre of the same pulse.

By comparing a sinusoidal reference signal with a triangular carrier wave of frequency, $f_c$ the gating signals are generated, as shown in Fig.1(a). The frequency of reference signal, $f_{r}$ determine the inverter output frequency, $f_{o}$ , and its peak amplitude, $E_{r}$ controls the modulation index, $M,$ and then in turn the RMS output voltage, $E_{L}$ .

The number of pulses per half-cycle depends on the carrier frequency. Within the constraint that two thyristors of the same arm $\left(T_{1}, T_{4}\right)$ cannot conduct at the same time, the instantaneous output voltage is shown in Fig.1(a). The same gating signals can be generated using unidirectional triangular carrier-wave as shown in Fig.1(b).

By varying the modulation index $M,$ the RMS output voltage can be varied. Itcan be observed that the area of each pulse corresponds approximately to the area under the sine-wave between the adjacent midpoints of OFF periods on the gating signals. If $P_{m}$ is the width of the $m^{th}$ th pulse, can be extended to find the rms output voltage.

$$E_{L}=E_{\mathrm{dc}}\left(\sum_{m=1}^{N_{p}} \frac{P_{m}}{\pi}\right)^{1 / 2}$$

Harmonic analysis of the output modulated voltage wave reveals that Sine PWM has the following important features:

(i) For modulation index less than one, the largest harmonic amplitudes in the output voltage are associated with harmonics of order $f_{c} / f_{r} \pm 1$ or $2 N_{p} \pm 1,$ where $N_{p}$ is the number of pulses per half-cycle. Thus, by increasing the number of pulses per half-cycle, the order of dominant harmonic frequency can be raised, which can then be filtered out easily. For $N_{p}=5,$ harmonics of the order of 9 and 11 become significant in the output voltage. It may be noted that the highest order of significant harmonic of modulated voltage-wave is centered around the carrier frequency, $f_{c}$.

(ii) For modulation index greater than one, lower order harmonics appear since for modulation index greater than one, pulse width is no longer a sinusoidal function of the angular position of the pulse.