45 Frequency Converters 2.3.1 Uncontrolled rectifi ers Uncontrolled rectifi ers consist of diodes as shown in Fig. 2.4 How diodes work. Fig. 2.4 How diodes work A diode allows current to fl ow in one direction only: from the anode (A) to the cathode (K). The current is blocked if it attempts to fl ow from the cathode to the anode. It is not possible to control the current strength, as is the case with some other semiconductors. An AC voltage across a diode is converted into a pulsating DC voltage. If a three-phase AC voltage is supplied to an uncontrolled three-phase rectifi er, the DC voltage will pulsate continuously. Fig. 2.5 Uncontrolled rectifi er (B6-diode bridge) Fig.
2.5 Uncontrolled rectifi er (B6-diode bridge)shows an uncontrolled three-phase rectifi er consisting of two groups of diodes. One group consists of diodes D 1 , D 3 and D 5 . The other group consists of diodes D 4 , D 6 and D 2 . Each diode conducts for one- third of the period T (120°). In both groups, the diodes conduct in sequence. Periods during which both groups conduct are off set in relation to each other by one sixth of the period T (60°). Diode group D 1,3,5 conducts the positive voltage. When the voltage of phase L 1 reaches the positive peak value, terminal (A) takes on the value of phase L 1 .
Reverse voltages of the magnitude U L1-2 and U L1-3 are present across the other two diodes.