Answer / deepak rathor

in this case alternator wil work as induction generator, as
reactive power supplied by AVR wil be zero due to loss of
excitation, generator wil be in asynchronous mode.

Answer / abubakr

when excitation is lost due to fault or field breaker
tripping . Reactive power will be supplied by the system and
generator will work as induction generator and its speed
will rise a little . generator which was in over excitation
mode will work in underexcitation .
but there is under excitation limit which should not be
reached so we should detect loss of excitation and trip
generator

Answer / sunil kumar

when the excitation is lost,alternator tends to behave as
induction generator because there is no opposition to the
rotor as flux is zero, rotor speed tends to increase beyond
synchronous speed and by reactive power from the grid the
machine behaves as induction generator as slip is negative
and reverse power flows which will be taken care by
directional relay.

Answer / nishad

Generator will not produce any output voltage and it will trip the generator output breaker by the loss of excitation monitor protection relay or under voltage relay. If the breaker not trips by these relay, it will trip by reverse power relay. Due to the presents of Reverse Power relay, it will not allow the Generator to run as an induction generator and will trip the Generator breaker, thus it protect the generator from further severe fault. Again, if it not trips by these all protections, alternator will work as induction generator due to the residual magnetism in the field core when it is running in parallel and it will give the over load to another generator as a result, it will trip entire system. Also, as an alternator typically isn't designed to operate as an induction machine, the windings on these machines will heat up and damage. Therefore, need to make sure the reliability of protection relays to avoid such faults.

Answer / atul

if excitation fails let d case of hydro generator the prime
mover is turbine. since turbine ic moving and is coupled
with generator shaft and no flux to control its speed it
will go out of path or speed will increase beyond
synchronism

Answer / ramlakhan y adav (ballia)

when the excitation is removed then the alternator work as a
asynchronous motor will draw the power from reverse supply.

Answer / manjeet

duction generator as underexcitation .
but there is under excitation limit which should slip is
negative
and reverse power flows which will be taken care by

Answer / sudheer

if excitation failed , the alternator will stop making
voltage.because when exciter fails, the main winding will
not produce rotating magnetic field, so voltage will not
produce.

Answer / raj

Every one is right but one more thing will happen that is
Alternator will work like motor and will draw power from
other source ( with that it synchronised )known as reverse
power and further RPR will trip the Alternator breaker.
(Approx continuous rating for reverse/negative power is 10%)

Answer / vijin

if the excitation fails the motor will simply stops,because
it loses the magnetic locking capability by the removal of
dc supply. the motor has no field suplly to generate
necessary emf while running by virtue of the inertia.

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