IMPROVEMENT OF THE ENERGY PERFORMANCE OF ASYMMETRIC EXCITORS OF SYNCHRONOUS MACHINES IN VOLTAGE FORCING MODES

Abstract

The main mode of operation of synchronous machines in electric drives of industrial plants is the mode in which a static load moment close to the nominal is applied to the shaft. If the synchronous machine works as a generator, when consumers of comparable power are connected to the stator to stabilize the output voltage of the generator, forcing of the excitation voltage is applied by influencing the control angles of the thyristors of the controlled rectifier. In the nominal mode, the control angles of the thyristors of the exciter are significant, which makes it possible to ensure a sufficient multiplicity of the voltage during forcing. But with significant thyristor opening angles, the controlled rectifier consumes significant reactive power, which reduces the power factor of the rectifier bridge. In addition, forcing the excitation voltage is necessary for synchronous motors when a significant load is applied to the shaft and when voltage drops in the power supply network.
 It is widely used to connect capacitors in parallel to the windings of the power transformer, or to use two zero diodes that are connected in series and connected to the output of the exciter to improve its energy performance. A significant improvement in the power factor can be achieved by using forced capacitive switching nodes. However, the improvement of energy indicators by turning on zero diodes and capacitors leads to a significant change in the harmonic composition of the excitation voltage. In the work, a research of the harmonic composition of the excitation voltage of synchronous machines with asymmetric bridge rectifiers using artificial commutation circuits was carried out. 
 To research the harmonic composition, experimentally obtained dependences of the forced excitation voltage, which are decomposed into a trigonometric series, are used. Calculated amplitude spectra of the excitation voltage during natural and forced switching of thyristors. The above calculations show that the coefficient of ripples of the output voltage of the thyristor exciter when using a rectifier with forced commutation is more than two times less (5.4 %) than the coefficient of ripples of the output voltage of a rectifier with natural commutation (11.2 %). The conducted experiments and the results of their analysis proved the expediency of using the developed exciter with forced capacitive commutation.