High Performance Control of AC Drives with Matlab / Simulink Models

3.5 Relationship between Carrier-based PWM and SVPWM 22

Although the two PWM schemes, carrier-based and SVPWM, appear different, there exists a similarity between the two approaches. In the SPWM approach, the three-phase modulating waves are compared with a triangular high frequency carrier to determine the switching instants of the three phases. The modulating wave of a given phase is the average leg voltage corresponding to that phase. The most commonly used modulating signals are sinusoidal waves. The triplen (multiple of three) frequency component can be added as a zero sequence component to the three-phase sinusoidal waves or modulating signals. Hence, the choice of the triplen frequency components to be added to the fundamental signals lead to different modulating waveforms with different harmonic spectrum. The choice of these triplen frequency components is thus a degree of freedom in the SPWM technique.

In the SVPWM technique, the voltage reference is provided in terms of a rotating space vector. The magnitude and the frequency of the fundamental component of the output are specified by the magnitude and frequency of the reference vector. The reference vector is sampled once in every switching period. The inverter is switched such that the average voltage vector is equal to the sampled reference vector in every switching period. The inverter switching is done according to the reference voltage vector location. Two active and two zero vectors are used in every switching ...