Method for controlling a three-phase inverter

Abstract

Inverters (5.1, 5.2) for feeding a three-phase load such as, for example, an asynchronous machine (6), are controlled by pulse width modulation, in such a manner that they supply output voltages (U.sub.5.1R, U.sub.5.2R) which are as closely sinusoidal as possible, having few harmonics per alternating-current phase (R, S, T). To achieve the pulse width modulation, a first auxiliary alternating voltage (U.sub.H1) of a sawtooth generator (1.1), on the one hand, and a second auxiliary alternating voltage (U.sub.H2) of a sawtooth generator (1.2), which is phase-shifted by 180.degree. with respect to the first one, on the other hand, is superimposed by a control signal transmitter (2) on a sinusoidal first superimposition alternating voltage (U.sub.StR, U.sub.StS, U.sub.StT) with the required frequency and phase angle in logic circuits (3.1, 3.2). Separate inverter groups (5.1, 5.2), the outputs of which are connected via a reactor coil (7), the center tab of which is connected to a stator winding (W.sub.R) of the asynchronous machine (6), are controlled in dependence on intersectons of the two superimposition voltages in each case. To reduce the turn-on and turn-off losses, the thyristors (T1, T1'; T2, T2') of the inverter are only switched over at intersections of the rising portion of the auxiliary alternating voltage (U.sub.H1, U.sub.H2). During this process, the inverter sections of the two phases (R, S, T), the firsat superimposition alternating voltage of which has been intersected are in each case switched at two successive intersectons while the inverter sections of the adjacent phases are switched over at the subsequent third intersecton. This allows the switching actions to be reduced by 1/3.