Convertor control with selective reduction of line harmonics

In the paper, a new method of controlling the power of a d.c. series motor drive using the forced-commutation principle is proposed and investigated. In this method, it is possible to reduce any one unwanted harmonic and its multiples to a low value. However, only the reduction of triplen harmonics is investigated in the paper. A power circuit to realise the proposed method is also suggested. A slight modification of the proposed control circuit yields symmetrically pulsewidth modulated and sinusoidally modulated schemes with two chops per halfcycle. These two and the recently reported scheme using symmetrical pulsewidth modulation with a single chop per half-cycle are investigated and are compared with the proposed new method with regard to the line side harmonic components, power factor and the motor performance characteristics. Experimental verification of the basic principles involved is made. The experimental values closely agree with the theoretical results.