NEIEP Mechanics Practice Exam

What will happen to the DC voltage if a fuse blows in a three-phase, full-wave rectifier?

1. It will remain the same

2. It will increase

3. It will decrease

4. It will fluctuate

The correct answer is: It will decrease

In a three-phase, full-wave rectifier, the output DC voltage is derived from the input AC voltages converted through diodes. These rectifiers are designed to maintain a relatively stable DC output voltage under normal operating conditions. When a fuse blows, it typically indicates a failure in the electrical circuit, such as an overload or short circuit condition. This interruption in the circuit will now prevent one of the phases from contributing to the rectifier's operation. Since DC voltage in a three-phase rectifier is dependent on the effective contribution of all three phases, losing one phase will lead to a decrease in the overall input voltage available for rectification. With less voltage available from the input phases, the rectifier can only convert the remaining phases into a lower DC output voltage. This scenario results in a diminished ability to maintain the original output voltage, thus effectively causing a decrease in the DC voltage observed at the output terminals of the rectifier. This concept aligns closely with the nature of three-phase systems—losing one phase doesn't just stop the output but reduces it, as the system is designed to leverage the strength of multiple phases working in tandem.