Spectral analysis for non-sinusoidal PWM with variable switching frequency for harmonic reduction in inverters

Malik Bahnam Abbu Samter
Dr. Mohammed T. Lazim

Abstract: Pulse Width Modulation (PWM) is a very commonly used method in DC/AC conversion. Actually, any stream of pulses that have a high fundamental component can be assumed as PWM strategy. The most compatible technique of PWM for the use with variable speed control of induction motor is the Carrier Based PWM (CB-PWM). They are divided into two groups, Continuous PWM (CPWM) and Discontinuous PWM (DPWM). They all can be implemented using triangle intersection method and most of them can be implemented using direct digital method. This thesis projects on the CB-PWM with all its common strategies in both triangular intersection and direct digital implementation methods. Detailed analysis about CPWM and DPWM are involved, containing the derivations for mathematical equations to estimated switching periods for inverter in both direct digital as well as triangular intersection methods. Moreover, an analysis for switching losses and linearity range of operation was clarified. The performance of these methods was estimated with the aid of computer analysis for all the mentioned strategies in both triangle intersection and direct digital methods, thereby, the waveforms of voltage and current were analyzed to obtain the spectrum and the total harmonic distortion (THD) with different operation frequencies. It was noticed that, the least order harmonics are very small as compared to the fundamental, while at and near the carrier frequency there are high amplitude harmonics. This type of spectrum suits the induction motors and other inductive loads because it suppresses the high order harmonics. SPWM losses the linearity of control at 0.78 of modulation index, whereas THIPWM linear control range extends to 0.88. SVPWM and DPWM strategies remains linear till 0.907 of modulation index, and thus they are superior in linearity range to SPWM and to THIPWM. I Among all CPWM and DPWM methods, the THIPWM showed the best waveform quality, followed by SVPWM. Even though, SVPWM is preferred in most because several practical considerations. SVPWM is superior to the DPWM strategies and torque stability in waveform quality; it resulted in better THD and better waveform qualities. Another criterion is the switching losses. DPWM cease to switch for a complete 120o segment. Hence its_ inverter switches less than CPWM by 2/3 ratio. But this cannot express the reduction in switching losses alone, where it depends on the load power factor. Generally, all DPWM strategies provided close THD values, but GDPWM sometimes is the preferable even on SVPWM due to its low switching losses, where, it reduces the switching losses to 50