Subject : Power Electronics
Unit : AC to DC Converters
Operation of a three phase uncontrolled bridge rectifier supplying a capacitive load Circuit diagram & Waveforms
Operation of a three phase uncontrolled bridge rectifier with a capacitive load:
Fig: Operation of a 3 phase uncontrolled bridge rectifier with Capacitor load
- The top group diodes (D_{1}, D_{3}, D_{5}) form a “Maximum value circuit” and therefore the maximum of the phase voltages v_{an}, v_{bn}, v_{cn }appears at the positive dc bus.
- On the other hand, the bottom group diodes (D_{2}, D_{4}, D_{6}) form a “Minimum value circuit”. Therefore the minimum of the phase voltages v_{an}, v_{bn }and v_{cn }appears at the negative dc bus. Therefore, the output voltage waveform at any instant is equal to the maximum of the six line voltages v_{ab}, v_{bc}, v_{ca}, v_{ba}, v_{cb }and v_{ac }provided at least one diode from the top group and one from the bottom group conducts at that instant.
- None of the diodes will conducts, however if the output capacitor voltage is larger than the maximum line voltage. All the six operating modes of a 3 phase bridge rectifier namely, D_{1}D_{2}, D_{2}D_{3}, D_{3}D_{4}, D_{4}D_{5}, D_{5}D_{6 }and D_{6}D_{1 }appear in that order. An additional operating mode in which none of the diodes conduct appears in the conduction diagram.
- During these periods the output capacitor discharges through the load. As the capacitor voltage decreases its voltage becomes equal to the incoming line voltage. At this instant the appropriate diodes from both the top and the bottom group starts conducting and continuous to do so till the sum of the capacitor charging current and the load current becomes zero.
- i_{i }is discontinuous and contains large ripple. This is a major disadvantage of this converter. This ripple is also reflected in the input current of the rectifier. However, the displacement factor of the converter still remains unity. The current i_{i }can be made continuous by connecting an inductor of appropriate value between the rectifier and the capacitor. Analysis of such a converter is similar to a converter supplying an R – L – E load where the value of E is .