CONSTANT  FREQUENCY SHAFT GENERATOR  SYSTEM

      The variable frequency ac output from shaft generator due to variable speed of main engine, is converted to dc by the thyristor rectifier. The thyristor inverter then converts the dc to fixed frequency 3phase ac. Changes in frequency and voltage caused by changes in propeller shaft speed or by electrical load variations are met by adjustment of the shaft generator excitation and thyristor trigger control by the control system. The rectifier/inverter unit cannot supply the reactive power (KVAr) for the ship's system. This is provided by the synchronous condenser, which is a large dedicated synchronous motor or an auxiliary generator declutched from its diesel, running as a motor. The synchronous Condenser is run on no-load at a leading power factor and operates essentially as a power factor correction capacitor. The shaft generator provides the system kilowatts at unity power factor and the synchronous condenser provides the KVAr. The reactor connected between the Thyristor inverter and the busbar improves the voltage waveform by smoothing out dips caused by commutation of Thyristors and also limits the short circuit current.

Most system can provide full power output down to 75% or 80% of rated speed and reduced power down to approximately 50% rated speed as shown is below.

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