Answer / musa abdul razak

We uses dc to get a steady torque and to synchronise the rotor.

Answer / ashish bhagat

alternator works on the base of electromagnetic induction.
according to electromagnetic induction, EMF will induced in
the conductor(armature) when it cuts magnetic flux.
therefore to produced EMF it neccesery produced constant
magnetic flux. therefore magnetic poles are excited by DC
source only beacuse flux produced by AC supply will having
changing frequency.

Answer / saroj kanta nayak

DC Exciation is mostly used, Because We need a constand
magnetic field for ouy power generation. So DC Voltage or a
Natural Magnet can Solve this problem. As using a natural
magnet will be bulky,So insted we use DC windings with
inverter power which draws power from the same Generator
during generation.

Answer / shyamalmandirwala

Actually the answer is like this:
When we are giving DC excitation to the AC machine,we are
imposing DC on AC.
In this case the Resulting Frequency (Fr) will be the
difference of the two.
Hence in this case,Fr = 50 - 0 = 50Hz
and hence we get the excitation at desired frequency,where
as if we provide AC excitation, then Fr will become 0Hz &
so no excitating magnetic field will be created...

Answer / raj

NOTHING IS THERE WE KNOW WHENEVER CONDUCTOR CUTS THE
MAGNETIC FIELD THE EMF INDUSED. DC CREATES ONLY MAGNETIC
FEIELD. AC CREATES ONLY MAGNETIC FLUX.

Answer / r.thirumoorthy

THE FIELD STRENGTH IS CONSTANT FOR DC SUPPLY, BUT AC
SUPPLYS FIELD STRENGTH IS VARIABLE.

Answer / glenn20sal

AC SUPPLY IS GIVEN TO THE STATOR WHICH PRODUCES ROTATING
MAGNETIC FIELD ROTATING AT CONSTANT RPM
INORDER FOR THE ROTOR TO CATCH UP WITH THE ROTATING FIELD
THE ROTOE SHOULD HAVE ONE DIRECTION MAGNETIC FIELD ONLY
THEN IT CAN ROTATE.
THEREFORE THE EXCITATION IS DC

Answer / ashish

as we know that alternator work on the fraday's law ie when
a conductor cut the magnetic flux then a emf is induced. If
we give a.c supply than flux will also alternate and we will
get proper cuts of flux. hence we use dc supply.

Answer / vknt

if we are using DC supply for field it will create the
rotating magnetic field as the rotor rotates. so there will
be a relative speed between field and stator coil to produce
voltage.

if we are using AC supply for field this will create
alternating field in rotor. as the rotor and field both are
rotating the net speed of the field will be zero i.e there
will not be a relative speed between filed and stator. so
voltage will not be produced in stator.

Answer / saeid

In theory, there are 3 kinds of generator or alternator
that are synchronus, induction and DC.

So the question is why DC excitaion is used for
alternators. The answer is becuase this a synchronus
generator. And that is philosophy of this kind. For example
if you use AC Excitaion it will not be synchrounus but it
will be induction generator

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