Answer / tareq harb

There are 3 types of excitation systems:
AREP, PMG & Self excitation.

in self excitation the AVR (Automatic Voltage Regulator)
takes its power directly from the out put terminals of the
generator, the main disadvantage of this method is that
when the generator output volatage drops the feeding to AVR
will be insufficient. suppose SHORT CKT fault takes place
on generator terminals as a result the output voltage will
drop too much & AVR shuts down directly with out feeding
this short CKT, if running on island mode (load sharing
mode) the hole system will trip.

But in PMG excitation system, PMG will feed AVR with the
same power nevertheless out put volatge, in case of short
ckt PMG alternator can supply 3 times its rated current for
10 seconds giving enough time for generator breaker to trip
eliminating the short ckt with out affecting the system.

as we see the main advange of PMG ALTERNATORS is its
enhanced immunity against short ckts.

Self excited systems is preferable in single mode operation
as it is cheaper & shorter prime movers.

PMG is strongly recommended in island mode operation.

Answer / j.c.das

PMG or Permanent Magnet Generator are used in brushless
excitation system. It is advantageous from the maintenance
point of view. It doesn't employ carbon brushes as it does
not have a commutator and so no regular wear and tear as in
carbon brushes.

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