NEIEP Mechanics Practice Exam

If the DC output of a three-phase rectifier is lower than specified, what would be the most likely cause?

1. One input phase is missing

2. The rectifier is damaged

3. The load is too high

4. The output capacitor is failing

The correct answer is: One input phase is missing

A lower than specified DC output from a three-phase rectifier can often be attributed to one input phase being missing. In a three-phase system, each phase contributes equally to the total output voltage. If one phase is lost or not functioning correctly, the overall voltage output will be reduced because the rectifier is not able to harness the full potential of the three-phase input. This scenario leads to an imbalance, causing the rectifier to output a lower DC voltage than expected. In contrast, while a rectifier being damaged could lead to reduced output, it is often associated with the entire system or components failing rather than a lower output under normal operating conditions. A load that is too high would typically cause output voltage to sag under load conditions but not necessarily indicate a persistent lower output. Similarly, a failing output capacitor might cause ripple or instability in voltage rather than a systematically lowered output voltage. Therefore, the absence of one phase stands out as the most direct cause of an insufficient DC output in this scenario.