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As the
electromagnetic induction principle states, “when a coil of copper wire is rotated in a magnetic field in such a way as to
cut across the lines of magnetic force, an electric charge is created or
induced in the wires”; the generator exciting system also follows this
principle as when the magnetic circuit of the generator rotor rotates, the
generator field winding excites the rotating magnetic circuits to
establish field flux linkage (FFL) and consequently due to this FFL an internal
voltage which have a proportion amplitude to the magnitude of  FFL is generated on the rotor field winding. This
generated voltage will be delivered to the whole system connected to the
generator unit. But the stator field winding finds its starting power from the
excitation system connected with it. This excitation system is part of generating units in which it produces flux by
passing current in the field winding of the exciter to supply its output to the
field winding of the generator through brushes and slip rings to run or excite
the synchronous machine.


However, if the
excitation system fails to deliver power to the generator by any reason, the
generator field winding will directly stop exciting the rotating magnetic
circuits and as a result the FFL will start to decrease gradually until the time
constant limit it. Thus, the generator internal voltage which its magnitude
highly depends on the value of FFL, will also starts to decay with decrease of
the field flux linkage. The reactive power of the generator is highly dependent
on the internal voltage, thus when decay in internal voltage also decreases the
reactive power on the generator terminal. At this case the generator starts
pursuits a way to stay in synchronism to generate power and starts to consume
reactive power from the system connected with it rather than supplying it which
consequently will decrease the reactive power and voltage on the system. This
situation is called excitation loss and the generator losses its synchronism
and is said to be asynchronous machine.

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The synchronism of
the generator also can be affected by faults on the system connected to it as
the FFL of the generator increases when a short circuit fault happen on the
transmission line with effect of automatic voltage regulator operation during
this fault condition as can be seen on Fig. 1 for synchronous machine shown in
Fig. 2. This FFL variation leads to increase the amount of power injected by
the generator to the grid to control the terminal voltage variation by AVR.
High power deliver on the transmission side will increase the field current and
field flux linkage which in result again will increase both the generator
internal and terminal voltage.

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