46 Frequency Converters The same principle applies to diode group D 4,6,2 . Here terminal (B) takes on the negative phase voltage. If, at a given time, L 3 reaches the negative threshold value, diode D 6 conducts. The other two diodes are subject to reverse voltages of the magnitude U L3-1 and U L3-2 . The DC output voltage of the uncontrolled rectifi er is constant and represents the diff erence between the voltages of the two diode groups. The average value of the pulsating DC voltage is approximately 1.31 to 1.41 times the mains voltage with a three-phase supply or approximately 0.9 to 1.2 times the AC voltage in the case of a single-phase supply.
The current consumption of the diodes is not sinusoidal. Consequently, uncontrolled rectifi ers generate mains interference. To counteract this, FCs with B12 and B18 rectifi ers are increasingly used. B12 and B18 rectifi ers comprise 12 or 18 diodes respectively, organised in groups of 6. 2.3.2 Semi-controlled Rectifi ers In the case of semi-controlled rectifi ers, a thyristor group takes the place of one of the diode groups (for example, D 4,6,2 as shown in Fig. 2.5 – Uncontrolled rectifi er (B6-diode bridge). The thyristors are also referred to as silicon controlled rectifi ers (SCR). SCRs are found in many applications in electronics, and in particular for power control.
By controlling the fi ring times of the thyristors, it is possible to limit the inrush current of the units and achieve soft-charging of the capacitors in the intermediate circuit. The output voltage of these rectifi ers is identical to that produced by uncontrolled rectifi ers. Typically, semi-controlled rectifi ers are found in FCs of power size 37 kW and greater. + Fig. 2.6 How thyristors work