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INTRODUCTION
The alternator are of salient pole, brushless revolving field type and self excited by an overhug AC exciter(rotating armature type)through a rotating rectifier assembly. The machine is itself regulated through a generator controlled unit (GCU) which controlled excitation to the exciter field.
CONSTRUCTION
STATOR
The stator winding consist of coils wound either with synthetic enamel copper wires or with bare copper bars and with suitable slot insulation
ROTOR
The rotor consists of shaft winding and core. Rotor field windings are wound with high conductivity copper strips insulated suitably.
BEARINGS
Bearings are of ball bearing type for single bearing machine. For double bearing machines the non drive end bearing is ball type and the driving end side is either roller or ball bearing.
ROTATING RECTIFIER ASSEMBLY (RRA) AND THE EXCITER
The exciter rotor and stator are overhung on the non drive end side of the generator. The stator carries field coils and the rotor carries the armature coils. The exciter rotor are connected to an RRA which is full wave bridge rectifier. The RRA consist of 6 power diodes,3 numbers of cathode stud and 3 numbers of anode stud to convert AC to DC.The DC voltage from RRA is fed to the main field of generator.
Rotating Rectifier Assembly
TERMINAL BOX
The AC outputs are brought out to top side of the machine and are terminated in terminal box. The terminals box also accommodates the generator control unit(GCU).The GCU comprises of an automatic voltage regulator and filter unit.
OPERATING PRINCIPLE
In the separately excited system (PMG) there is separate source of exciter field power from a small permanent magnet field AC generator mounted on same shaft as the main machine. Hence the machine title is separately excited.
The permanent magnet produces and output voltage that is only dependent on speed and is independent of load condition. This constant output voltage is fed to the exciter field winding through the AVR.By comparing the main output sensed voltage with the set reference voltage the AVR decides on the proportion of permanent magnet machine output to rectify and fed to the exciter field.The exciter rotor output would then increase, establishing a strong main field and there for a marked increase in main output field.
The AVR senses and compares voltages and adjusts exciter field. Excitation continues until desired output voltage is developed.