47 Frequency Converters Referring to Fig. 2.6 How thyristors work, when α is between 0° and 90°, the thyristor circuit is used as a rectifi er. When the α value is between 90° and 300° the thyristor circuit is used as an inverter. 2.3.3 Fully-controlled Rectifi ers Fully-controlled rectifi ers involve the use of thyristors. As with a diode, a thyristor permits the current to fl ow from the anode (A) to the cathode (K) only. However, the diff erence is that the thyristor has a third terminal known as the gate (G). When the gate is triggered by a signal, the thyristor will conduct. Once current starts fl owing through the thyristor, it will continue conducting until the current drops to zero.
The current cannot be interrupted by sending a signal to the gate. Thyristors are used in rectifi ers. The signal sent to the gate is known as the α control signal of the thyristor. α is a time delay, which is specifi ed in degrees. The degree value indicates the delay between the voltage zero crossing and the time when the thyristor is triggered. Fig. 2.7 Fully-controlled three-phase rectifi er Fully-controlled three-phase rectifi ers can be broken down into two groups of thyristors: T1, T3 and T5, on the one hand, and T4, T6 and T2 on the other.
With fully controlled rectifi ers, α is calculated from the moment when the corresponding diode in an uncontrolled rectifi er would normally begin to conduct, that is, 30° after the voltage zero crossing. In all other respects, controlled rectifi ers behave like uncontrolled rectifi ers. The amplitude of the rectifi ed voltage can be varied by controlling α. Fully-controlled rectifi ers supply a DC voltage with an average value U, where U = 1.35 x mains voltage x cos α.