Direct-Conversion Electric Propulsion System

Abstract

The use of an AC electric drive with a frequency converter is a common practice in the world shipbuilding industry. The use of alternating current allows one to reduce the dimensions of the engine and improve the operational characteristics of the system as a whole which led to the almost complete replacement of DC systems in the design of new ships. As the basis of an electromotive system, autonomous voltage inverters with pulse-width modulation and an intermediate DC link are most often used. The operation of such installations is associated with the occurrence of significant electromagnetic emissions and, in the absence of a filter, imposes increased requirements for the motor insulation. The improvement of the quality of the output voltage and currents is restricted by the limit of the switching speed of the transistors on the one hand, and the increase in heat release on the other hand. Systems with direct frequency conversion are also used. Their advantages include a low switching frequency of IGBT transistors, no intermediate conversion and associated losses, as well as piecewise sinusoidal modulation of output currents and voltages with a lower distortion of the output voltage. In comparison with PWM modulators, the advantage of cycloconverters consisting in a voltage quality is offset by less flexibility in controlling the harmonic composition of currents, and the complexity of further improving the conversion characteristics. The proposed work is devoted to the development of the technical characteristics of electromotive systems with direct frequency conversion, and solves the problem of further improving their characteristics.