3.2 Classification of PWM Schemes for Voltage Source Inverters
Different types of PWM techniques are available in the literature, but the most basic PWM is the SPWM. The high frequency carrier-wave is compared with the sinusoidal modulating signals to generate the appropriate gating signals for the inverters. The other PWM techniques are evolved from this basic PWM technique. SVPWM, although appearing different to the SPWM, has a strong implicit relationship with SPWM [1–5]. The gating time of each power switch is directly calculated from the analytical time equations in SVPWM 6. The power switches are then switched according to the predefined switching patterns. The achievable output in the case of SVPWM is higher when compared to the SPWM. The main aim of the modulation techniques is to attain the maximum voltage with the lowest Total Harmonic Distortion (THD) in the output voltages. The PWM techniques can be broadly classified as:
In discontinuous PWM techniques, the power switches are not switched at regular intervals. Some of the switches do not change states in a sampling period. This technique is employed to reduce the switching losses. The SVPWM method is modified to achieve discontinuous switching. A detailed discussion is provided in Section 4.6.