Exam Question

A shaft generator (SG) is a synchronous machine directly coupled to a vessel's propulsion shaft. Its speed, and thus the frequency of the generated AC power, varies with the engine's speed. To produce a constant frequency output, regardless of engine speed, the SG utilizes a static converter.
This converter comprises two main sections:
Rectifier: This section, typically a three-phase diode bridge rectifier, converts the variable-frequency AC output of the shaft generator into direct current (DC). A reactor smooths out the DC current.
Inverter: This section converts the DC power back into AC power at a constant frequency. This is achieved using a controlled inverter, often employing thyristors switched in sequence. The switching sequence is precisely controlled by a gate signal to create the desired frequency. A crucial aspect here is that the thyristor current needs to be in phase with its voltage to ensure proper turn-off at the end of each AC half-cycle. If the load is inductive (as is typical in ships), a leading reactive power (kVAR) must be supplied to the busbar to achieve this phase alignment. This often involves a synchronous motor acting as a synchronous compensator, whose power factor is adjusted by regulating its DC field current.