Sinusoidal Pulse Width Modulation (SPWM) Control
Sinusoidal Pulse Width Modulation (SPWM):
Fig: Sinusoidal pulse width modulation control
- Various types of modifications in PWM techniques have been proposed. One important method is sinusoidal pulse width modulation (SPWM) control, the pulse widths are generated by comparing a triangular reference voltage vr of amplitude Ar and frequency fr, with a carrier half sinusoidal voltage vc of variable amplitude Ac and frequency 2fs.
- The sinusoidal voltage vc is in phase with the input phase voltage vs and has twice the supply frequency fs. The widths of the pulses (and the output voltage) are varied by changing the amplitude Ar or the modulation index M from 0 to 1. The modulation index, M is Ac/Ar.
- It may be noted that the width of the pulses obtained are variable. The width are smaller at the centre of the carrier signal (sinusoidal), and increases as one goes to the start and end of the above signal.
- It may be noted that, in the earlier case (multiple PWM control), the pulse widths are uniform (equal).
- In this type of control, the displacement factor is unity, and the power factor is improved. The lower order harmonics one eliminated or reduced. For example, width four pulses per half cycle, the lowest order harmonic is the fifth, and so on.
- Different modifications have been suggested to take one such example, as the pulse width one small in the centre, the carrier signal is modified to care of this. The triangular waveforms are kept same, upto some point from the start and end of the cycle, and then the pulse widths can be made uniform.
- So, the power factor is improved with various control methods. For different PWM methods used, the harmonic components of the voltage waveforms are also decreased or eliminated.