Answer / vikas

Synchronous machine can run on both AC and DC
excitaion...The speed at which the machine will rotate will
depend on the relative frequency of stator and rotor
supply...i.e.if machine is connected to grid of 50 Hz
frequency and rotor is also provided with 50 Hz frequency
source then relative frequency will be 0 and machine will
not rotate because average torque produced will be 0 as
someone has already mentioned, but if rotor frequency is
say 20 Hz then the relative frequency will be 50-20=30 Hz.
so the machine speed will be 120f/p..substitute f= 30
Hz..when the rotor supply is DC then the relative frequency
is 50-0= 50 Hz. so the machine speed is given by
substituting f= 50 Hz in this case...

Answer / ghanshyam singh

because only dc current can produce the fixed pole i.e. the
poles produced by the dc current does not change with time
as in the case of the poles produced by ac current.
now when the stator of the machine is supplied the ac
current the rotating magnetic current is produced.
the dc excitation is necessary so that the poles produced
by the dc current and ac current both attrect each other
which will be responsible for the production of the torque.

Answer / b. b. shukla

In a synchronous motor, a rotating magnetic field is produced in stator by giving 3 phase AC supply.Now as per working principle of syn. motor,rotor must have a magnetic locking with stator field. For this purpose we need permanent magnetic poles in rotor so that rotor pole will form magnetic locking with nearest opposite pole of rotating stator field and rotates with stator field at synchronous speed.
Now to form permanent magnetic poles in rotor, DC excitation is fed to rotor winding of synchronous motor.

Answer / sanjeet swarnakar

if AC excitation is given then alternate north and south
pole is created on rotor field
the rotor move half cycle clockwise and half cycle anti
clockwise and finaly stop to rotate
due to this dc current is given to the rotor of the
syncroneous motor

Answer / kunal

TO PRODUCE MAGNETIC FIELD

Answer / pranav patel

it's all about on ac when we applied 3-p supply to the syn
motor it's indused rotating magnetic field and this
magnetic field rotate in synchronous speed so if u r stator
winding is two pole so ur rotor it's also two pole so when
we given supply same pole creat attrection so rotor rotate
in half round and when polarity change differnt pole ripple
so second time its rotate half round in another direction
so we need

Answer / rahul

Excitation is provided to rotor. Now dc field which is made
rotating in space produces a rotating magnetic field. AC
exciation can be thought as two separate magnetic fields
rotating in opp. directions. Also both the fields are of
same strength and thus net torque is zero. So we must
provide DC and not ac excitaion to synchr m/c.

Answer / shyamal mandirwala

my question is why do we provide DC excitation????
can we provide AC excitation to the notor???

Answer / ashok kumar

In AC synchronous motor its stator coil is not a
permanment magentising property so it is connected with dc
exciter for initial operation purpose.

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