Synthesis of six-step pulsewidth-modulated waveforms with selective harmonic elimination

A method for synthesizing single-phase and three-phase line-to-line PWM (pulse-width modulated) invertor waveforms which possess six-step symmetry is described. The waveforms are characterized by the angles which define the pulses in a 60° basis interval. A spectral analysis is performed to show that the single-phase six-step PWM waveform contains neither even nor triplen harmonics, and to derive the equations for the amplitudes of the fundamental component and the nonzero 6q±1 harmonics (q=1,2,3, etc.). A set of M=N+1 of these equations may be solved for the M angles which realize a given fundamental amplitude and eliminate N lower-order odd, nontriplen harmonics. It is found that a fundamental line-to-line amplitude of 1 pu is realizable, permitting 100% utilization of the DC bus voltage. The solution angles vary relatively linearly with respect to the required fundamental amplitude, permitting continuous voltage control and simultaneous harmonic elimination