Novel algorithm for generating near optimal PWM waveforms for AC traction drives

A harmonic elimination optimised pulse-width modulation (PWM) scheme is an attractive alternative to both natural and regular sampled PWM schemes for voltage source inverter drives used in railway traction. However, its application has been limited by the fact that the exact switching angles cannot be calculated online by a microprocessor based waveform generator. The paper describes an algorithm for calculating the near optimal switching angles, which will permit a fast and efficient realisation in a microprocessor based controller. With this type of PWM scheme, it is possible to show analytically that components at the signalling frequencies are not generated in the DC supply input current, for existing power frequency track circuits. The switching angles calculated with the algorithm agree closely with the exact solutions over most of the range of the fundamental output.