On reduction of harmonic reactive distortions and subharmonics of drives with VSI-fed induction motors controlled by direct torque and flux control methods

In many high power drives, in particular for traction, low frequency pulsation of line current is a major problem because of the requirement of compatibility with the supply and signalling system. A “flux trajectory corner folding technique” was proposed by the author for the direct self-control (DSC) to reduce the reactive stator current distortion Δ i_Sx by adding only a few extra inverter switching actions per one flux rotation period. It is shown in the paper that for drives for which the contents of input DC side current harmonics must be minimized forming the stator flux trajectory in such a way that it should remain between the narrowest circular band, is not a proper solution. A new way of forming the shape of this trajectory that allows elimination of dominating harmonics of the inverter output and drive input currents is given. It is shown that by using this technique the dominating line current harmonics can be reduced by at least 17 times in comparison to standard DSC. A disadvantage of both DSC and Takahashi´s DSC method is that they generate current subharmonics at the inverter output and input side. It is shown that amplitude of drive input current subharmonics can be greater than the amplitude of other harmonics. In the paper sources of subharmonics generated by these methods are shown. Techniques for reduction and elimination of these subharmonics are given. The proposed techniques are illustrated by results of simulation of a high power drive