In a three-phase half-wave rectifier, each diode conducts for a duration of
Right Answer is:
Three-phase half-wave rectifier
Most of the industry operates on 3-phase power instead of a single phase. When it is necessary to change ac voltage to dc voltage, it is generally done with Phase rectifiers. There are two basic types of 3-phase rectifiers: the half-wave rectifier and the full-wave rectifier. A 3-phase, half-wave rectifier is shown in Figure. Since it is a half-wave rectifier, there are three diodes in the circuit, one for each phase. The cathode of each diode is connected to the neutral point through the load as shown in Figure.
Whenever a particular phase voltage is higher than the other two-phase voltages, the SCR of the corresponding phase will be forward biased and conducts as soon as the triggering pulse is applied. This converter is also known as a three-phase three-phase converter or three-phase M-3 converter.
With a resistive load, a three-phase half-wave controlled rectifier operates in two different modes of conduction such as
- Continuous conduction mode when firing angle a is less than 30°.
- Discontinuous conduction mode when firing angle a is greater than 30°.
Working of the three-phase half-wave rectifier
- The figure shows the voltage and current waveforms of the three-phase half-wave controlled rectifier when SCR is firing at ωt= π/6 with firing angle α = 0. If any firing pulse is applied in between ωt = 0 and π/6, thyristor T1 will not be conducted as it is reverse biased.
- When thyristor T1 is turned on at ωt = π/6 + α the R phase voltage VRN is applied across the load until the thyristor T2 is fired at ωt = 5π/6 + α. Since thyristor T2 is turned ON at ωt = 5π/6 + α, thyristorT1 will be reverse biased and turned OFF and the Y phase voltage VYN is applied across the load until the thyristor T3 is fired at ωt = 9π/6 + α. As thyristor T3 is turned ON at 9π/6 + α thyristor T2 will be reverse biased and turned OFF and then B phase voltage VBN is applied across the load until the thyristorT1 is fired at 13π/6 + α.
- Subsequently, the cycle will be repeated and each thyristor conducts for 2π/3 i.e 120° duration when the firing angle a is less than 30°. Hence each diode conducts for one-third (120°) of each 360° supply voltage cycle.