Exam Question

The Shaft Generator is a synchronous motor, which may be fitted directly in line with the propulsion shaft, with a frequency that is variable. To keep a constant frequency must be separately regulated using a "frequency converter"
The SG output feeds a three-phase diode bridge rectifier; this output voltage is converted in DC voltage. The controlled inverter converts that DC voltage to AC voltage with a constant frequency by sequenced Thyristor switching. The excitation system is designed such that it is able to maintain its full output voltage down to 60% of engine speed.
The thyristor is only turned-off when its current is reduced (approximately) zero. This is a problem for the inverter thyristors when driving into the ship's inductive load (typically about 0.8 power factor lagging). In this case the current continues to flow in a thyristor after its voltage has gone through a zero point causing disruption of the inverter switching sequence.
It is necessary to have the thyristor current in-phase-with its voltage. To overcome this problem it is necessary to supply a leading reactive power (KVar) to the bus-bar, such that voltage and current are in phase and automatically turning off the thyristor at the end of each AC half-cycle.