Charging of a filter capacitor in the intermediate circuit of a frequency converter

Abstract

A method and an apparatus for charging a filtering capacitor in the direct-voltage intermediate circuit of a voltage-controlled PWM frequency converter comprising a rectifier () connected to an alternating-current source (U, U, U), a direct-voltage intermediate circuit and an inverter unit ()/inverter units for feeding a multi-phase alternating voltage (U, U, U) of variable frequency into a load/loads (), said rectifier bridge having controlled semiconductor switches (V–V), especially thyristors, in one arm, preferably in the upper arm, and diodes (D–D) in the other arm, preferably the lower arm, and said direct-voltage intermediate circuit being provided with a capacitor unit (C) of a relatively high capacitance for filtering the voltage. The filtering capacitor is charged by means of the semiconductor switches of the rectifier bridge by adjusting their firing angle according to the measured voltage of the capacitor unit, and the firing angle is adjusted by forming the sum of the measured voltage (U) of the capacitor unit and a predetermined limit voltage (U) and comparing the said sum to the measured main voltages of the supply network.