The invention relates to an inverter system using a transformer to generate a sinusoidal wave by harmonic neutralization, and proposes to eliminate direct current offset in the transformer which would tend to saturate the same.
The invention is applicable to variable speed alternating current motor drives of the type which is controlled statically to provide a controlled variable frequency power supply from a direct current source. Such technique basically rests on controlled conduction of power rectifiers such as thyristors. A sinusoidal waveform is reconstructed from a predetermined number of square pulses of voltage having selected magnitudes. This technique called "harmonic neutralization" is described in U.S. Pat. No. 3,491,282 of T. M. Heinrich et al., entitled "Static Inverter Wherein A Plurality Of Square Waves Are So Summed As To Produce A Sinusoidal Output Wave." The technique consists in generating with inverters a plurality of pulses at the fundamental frequencies and of same amplitude spaced from each other between stages of inverters by the same electrical angle. The pulses are amplified by respective stage transformers having winding ratios which follow the cosine law from one transformer to the other. The secondary windings are connected in series so as to sum up the dimensioned pulses and a polyphase output waveform is obtained by vector addition of transformer secondary voltages. Adjustment of the conduction angle, e.g., of the width of the outputted dimensioned squarewaves appearing in the primaries of the dimensioning transformers is used for voltage control and it has been shown that, for the practical range of voltage control with such a method the quality of the constructed sinusoidal wave is not lost.
Since the thyristors are cyclically turned ON and OFF, the operation of the inverter normally causes a periodic and symmetrical change of the flux in the core of the transformer. In fact, discrepancies might occur in the timing of the power switches, in the characteristics of the components and SCR's of the inverter system. The inverter system includes a plurality of inverters each coupled at the output with a transformer, and such dissymmetries appear at the primary as direct current levels. Such direct current levels, or offset, in the transformer, will cause the latter to saturate after several cycles and coupling with the load will be lost. The result will be an overload of the inverter.
The object of the present invention is to provide for an automatic cancellation of any direct current offset appearing in the transformer at the output of an inverter.