Details of Excitation system of Alternator

Excitation system of Alternator is one of the most important topics of Electrical Engineering. DC voltage is responsible to generate Magnetic field in alternator. This DC voltage can be supplied to the alternator separately or may be produced from the AC current produced by it thorough conversion. This mechanism is called excitation system. We all know that on the basis of excitation system, Alternators mainly are of two types-(1)Separately Excited system,(2) Self Excited system.

Separately and self excited system of Alternator

The basic difference is that separately excited system have Permanent Magnet Generator (PMG) for powering the Automatic Voltage Regulator(AVR)and there is one Isolation Transformer for sensing the voltage for AVR and in the other hand for self-excited Alternator, there is residual magnetism in the exciter field to initialize the voltage ramp-up process and after voltage build-up, Isolation Transformer which is responsible for voltage sensing for AVR , start powering the AVR as well.

Which means in self-excited Alternator, Isolation Transformer is responsible for two things-(1) Sensing Voltage (2) Powering AVR after initial voltage build-up due to residual magnetism.

1. Separately excited system of Alternator

operation of separately excited machine
Operation of separately excited machine

2. Self excited system of Alternator

operation of self excited machine
Operation of self excited machine

Self excited alternator problem- lack of Residual Magnetism

Sometimes you may face one problem that there is no voltage build-up in the Self-excited Alternator. The reason is lack of Residual Magnetism. Then what you have to do, remove the AVR, flash the exciter field by connecting a 12V DC battery and a blocking diode to the exciter field and after that start the engine, if you see now voltage is building up, then it is confirm that Exciter field has been flashed successfully. Now stop the engine and remove the battery & blocking diode, connect the AVR and again start the engine to make sure that the Alternator is back in operation.

loss of residual magnitism in self excited alternator

Advantages of PMG Excitation Support Systems

01. A PMG with its appropriate regulator can enhance a generator’s transient performance because it will provide a constant Ac voltage input to the automatic voltage regulator(AVR) regard less of the generator’s terminal voltage.

02. When the load on the generator is nonlinear due to thyrister (SCR) power supplies such as UPS Systems, variable speed drives etc. the load may produce notches on the voltage severe enough to cause misfiring of the power rectifiers in a shunt excited generator’s AVR. When this occurs , the generator’s terminal voltage will become unstable.

If the generator is fitted with a PMG, the input power to the AVR is isolated from these disturbances, and no voltage un-stability will occur.

03. A PMG is constructed with a permanent magnet rotor which has a very strong magnetic field. This eliminates the need for field flashing which is sometimes necessary with shunt type synchronous generators.

04. Strong and very fast voltage build-up.

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