Space vector based synchronized PWM algorithm for three level voltage source inverters: Principles and application to V/f drives

This paper describes the synchronized PWM techniques based on space vector approach for three level voltage source inverters. The objective is to improve the output waveform of the inverters used in high power applications, where the switching frequency is very low. This is achieved by maintaining the synchronization and half wave symmetry, quarter wave symmetry, three-phase symmetry in the PWM waveforms. The principles of achieving synchronization and symmetries are presented. The proposed algorithm is implemented on TMS320F240 based digital controller and applied to V/f drive. The experimental waveforms of the inverter output voltage and motor no load current for different operating conditions of the drive are presented. The V_WTHD of the line voltage and I_THD of the no load current of the motor are computed from the experimental data. The performance measure in terms of the V_WTHD of the line voltage is computed for the entire operating region of the drive for the proposed algorithm and the synchronized SPWM technique. The comparative results show that the proposed method has improved THD over SPWM method. Another significant feature of the proposed algorithm is that the symmetry and synchronization leads to self-balancing of the D.C. bus capacitor voltages.