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Question
Synchrounous motor is three phase motor,but its not self 
starting .why?
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
 
Answer
# 1
synchronous motor stator has 3 ph winding n rotor s suplied
by a dc suply to form a north n south pole..supose two
magnets are aligned axialy. now rotate one of them slowly.
the other one also starts rotating with dat. thatz how the
working of syn motor can be xplained in a very simple way.
if we rotate the magnet fastly the other one cant move with
that much speed becoz of its inertia. when 3 phase suply s
given to the stator winding, a rotating magnetic field is
created with syn speed. but the rotor cant rotate. so at
first the rotor s rotated by some external means to the syn
speed n then give suply to the stator.so that the rotor
continues to rotate with syn speed
 
Is This Answer Correct ?    58 Yes 6 No
Ashli
 
Answer
# 2
let assume the stator field as Ns and Ss..the rotor field as
Nr and Sr..when the given to supply to stator windings,which
cause rmf may occur..assume Ns is repel Nr..as the direction
of rotor is clock wise..(in first half cycle)..in next cycle
Ss is attract Nr(anti clk wise direction)..so the rotor cant
able to rotate in same direction.... so having only low
torque ...so that provides resudual magnetism at
innitialy..(ie dc excitation like alternator)
 
Is This Answer Correct ?    27 Yes 5 No
Saravana Babu
 
 
 
Answer
# 3
the essential requirement for starting a synchronous machine
is that in the initial stages the rotor( which houses d.c
excited field windings) must be rotated at the synchronous
speed(determined using(120*f)/p where'f'is the supply
frequency and 'p' is the no. of poles) so that the rotor
poles may lock on to the stator rotating poles( created by
three phase balanced excitation given to the stator armature
windings) following which the rotor will rotate
synchronously with stator field at that speed. hence without
the use of an external prime mover in the beginning, the
syn. motor cannot self start....
 
Is This Answer Correct ?    11 Yes 1 No
Arnab Choudhury
 
Answer
# 4
In order to understand the principle of operation of a
synchronous motor, let us examine
what happens if we connect the armature winding (laid out in
the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source
and the field winding to a D.C
source of appropriate voltage. The current flowing through
the field coils will set up sta-
tionary magnetic poles of alternate North and South.On the
other hand, the 3-phase currents flowing in
the armature winding produce a rotating magnetic field
rotating at synchronous speed. In
other words there will be moving North and South poles
established in the stator due to the
3-phase currents i.e at any location in the stator there
will be a North pole at some instant of
time and it will become a South pole after a time period
corresponding to half a cycle. (after
a time = 1/2f
where f = frequency of the supply). Let us assume that the
stationary South
pole in the rotor is aligned with the North pole in the
stator moving in clockwise direction
at a particular instant of time.These two poles get
attracted and try to maintain this alignment ( as per lenzís
law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the
production of torque in the clockwise
direction. However the rotor cannot move instantaneously due
to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator
pole would quickly (a time
duration corresponding to half a cycle) change its polarity
and becomes a South pole. So
the force of attraction will no longer be present and
instead the like poles experience a force of repulsion.In
other words, the conditions are now suitable for the
production of torque in the anticlockwise direction. Even
this condition will not last longer
as the stator pole would again change to North pole after a
time of 1/2f. Thus the rotor will
experience an alternating force which tries to move it
clockwise and anticlockwise at twice
the frequency of the supply, i.e. at intervals corresponding
to 1/2f seconds. As this duration is
quite small compared to the mechanical time constant of the
rotor, the rotor cannot respond
and move in any direction. The rotor continues to be
stationary only. so need some prime mover
 
Is This Answer Correct ?    6 Yes 1 No
Abhishek Kumar
 
Answer
# 5
synchronous motors are not self-starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field, but instead acts as a synchronous generator (alternator) and creates a very large counter-current that opposes the rotation 
Is This Answer Correct ?    2 Yes 0 No
Santosh Kumar Sharma
 
Answer
# 6
As per the operating principle of the Synchronous motor,
due to continuous & rapid rotation of stator poles,the
rotor is subjected to a torque which is rapidly reversing
i.e. in quick succession,the rotor is subjected to torque
which tends to move it first in one direction & then in the
opposite direction.Owing to its large inertia,the rotor
cannot instataneously respond to such quickly-reversing
torque,with the result that it remains stationary or in
other words it is not self starting.
 
Is This Answer Correct ?    2 Yes 1 No
Sumesh Sapkota
 
Answer
# 7
Since the rotor is supplied from a DC source the rotor have
a permanent polarity . But applying 3 phase supply to
stator, a 3 phase magnetic field is produced , but polarity
changing according to the supply frequency . Actually at
some regular time intrvals the stator poles and rotor poles
comes into exat 120 degree phase shift (this will exist
only for some micro seconds)the motor try to rotate a
clockwise direction . Another time intervels motor getting
torque to counter clockwise direction .
Thus the rotor poles oscillating to clockwise and
anticlockwise direction, due to the inertia of the moving
system it cannot move into any direction
 
Is This Answer Correct ?    1 Yes 0 No
Leo Thomas
 
Answer
# 8
when a 3-phase supply is provided to the stator winding of
the synch motor, a rotating field is produced n adjacent N-
S poles r formed on the stator windings.....due to rotor
inertia the rotor is not able to follow the rotating
magnetic field rotating at synch speed......therefore some
means has to be provided to make it self
starting........the solution lies in this
principle............the rotor is generally of squirrel
cage construction which speeds up as a induction motor and
when the motor picks up speed close to synch speed, the
rotor windings r excited by d.c. thus forming adjacent
poles on the rotor.....and in this way the stator poles
locks in with the rotor poles with opposite polarity to
rotate the rotor at synch speed...........that why synch
motor is DOUBLY EXCITED.
 
Is This Answer Correct ?    1 Yes 0 No
Raul
 
Answer
# 9
in synchronous motor due to continuous and rapid rotation of
stator poles, the rotor is subjected to a torque which is
rapidly reversing and tends to move it first in one
direction and then in the opposite direction .owing to its
large inertia the rotor cannot instantaneously respond to
such quickly reversing torgue with the result that it
remains stationary i.e it is not self starting
 
Is This Answer Correct ?    1 Yes 0 No
Pooja Vanan
 
Answer
# 10
"A synchronous motor is not a self starting motor because
torque is developed when running at synchronous
speed,therefore,the motor needs some type of device to
bring the rotor to synchronous speed."

EXPLAINATION:
As per operating principle of synchronous
motor,due to continuous and rapid rotation of stator
poles,the rotor is subjected to a torque which tends to
move it first in one direction and then in the opposite
direction.Owing to its large inertia,the rotor cannot
instantaneously respond to such quickly reversing
torque,with the result that it remains stationary or in
other words it is not self starting.
 
Is This Answer Correct ?    1 Yes 0 No
Prasanth
 

 
 
 
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