For a 3-phase fully-controlled converter, output dc voltage is given as

Introduction:
A 3-phase controlled bridge rectifier is a circuit that converts AC (alternating current) voltage into DC (direct current) voltage. It consists of six controlled semiconductor devices, typically thyristors or diodes, arranged in a bridge configuration. The overlap angle refers to the time interval during which both the positive and negative half-cycles of the AC input voltage are applied to the load simultaneously.

Explanation:
When the overlap angle in a 3-phase controlled bridge rectifier is increased, the output DC voltage decreases. This can be understood through the following points:

1. Working principle: In a controlled bridge rectifier, the controlled semiconductor devices are triggered to conduct during specific intervals to control the output voltage. During each half-cycle of the AC input voltage, a pair of devices conducts alternately to rectify the AC voltage. The overlap angle determines the duration during which both devices of a pair are conducting simultaneously.

2. Effect of overlap angle: Increasing the overlap angle means that both devices of a pair conduct for a longer duration during each half-cycle. As a result, the effective conducting period of each device decreases, leading to a reduction in the overall conduction time. This reduces the average DC output voltage generated by the rectifier.

3. Voltage drop: During the overlap period, the conducting thyristors have a voltage drop across them, which reduces the effective voltage available for rectification. This leads to a decrease in the output DC voltage.

4. Waveform distortion: Increasing the overlap angle also results in waveform distortion of the rectified output voltage. This distortion introduces additional harmonics and reduces the overall quality of the rectified waveform. The distorted waveform further contributes to a decrease in the output DC voltage.

5. Load current: The decrease in the output DC voltage due to an increase in the overlap angle is more prominent when the load current is high. This is because a higher load current results in a larger voltage drop across the conducting devices, which further reduces the available voltage for rectification.

Therefore, with an increase in the overlap angle in a 3-phase controlled bridge rectifier, the output DC voltage decreases due to the reduced effective conduction time, increased voltage drop, waveform distortion, and load current effects.