1. Construction of alternator
Stator- it is built up of sheet steel laminations having slots on its inner periphery .a 3 phase winding is
placed in these slots & serves as the armature wdg of d alternator . see fig in j.b.gupta …
Rotor-The rotor carries a field wdg which is supplied with D.C. through two slip rings by a separate D.C.
source .This D.C source (called exciter ) is generally a small D.C. shunt or compound generator mounted
on the shaft of the alternator.based on the construction rotor are of two types
(a) Salient pole type- In this type ,salient or projecting pole are mounted on a large circular steel
frame which is fixed to the shaft of alternator .The individual field pole wdgs are connected in
series such a way that when the field wdg is energized by a D.C. exciter ,adjacent poles have
opposite polarities.low and medium speed alternator (120-400 rpm) those driven by diesel
turbine & oil engines have salient pole type rotors due to following reasons-
(1)The salient field poles would cause an excessive winding loss if driven at high speed & would
tend to produce noise .
(2)salient pole construction can’t be made strong enough to withstand the mechanical stresses
to which they may be subjected at higher speeds.
For low speed alternator ,50 Hz frequency we need more no of poles.For accommodating large
no of pol;es ,rotor diameter must be comparating increased & from mechanical consideration
,salient pole construction has been found to suit best for low speed primover
(b) Non slient pole type - In this type ,the rotor is made of smooth solid forged steel radial cylinder
having a no of slots along the outer periphery .The field wdg are embedded in these slots & are
connected in series to the slip rings through which they are energized by D.c. exciter .such rotors
are design mostly for 2 pole or 4 pole turbogenerator running at 1500 or 3600 rpm.
The regins forming the poles are usually left unslotted .The poles areas are
surrounded by the field wdg placed in slots.The field coils are so arranged aroud there polar
areas that flux density is maximum on the polar central line .If we use salient pole construction
for high speed alternator the rotor structure may not be able to withstand the enormous

2. centrifugal forces developed by the salient poles.so cylindrical rotor construction is best suited
from mechanical consideration for high speed alternator.
Reason for non salient rotor for high speed alternator---
(i)This type of construction has mechanical robustness & give noiseless operation at high
speeds .
(ii)The flux distribution around the periphery is nearly a sine wave & hence a better e.m.f
waveform is obtained than in case of salient pole type.
Working principle of synchronous motor .
Consider a 3 phase synchronous motor having two rotor poles Nr & Sr . then the stator will also
be wound for two poles Ns & Ss .The motor has direct voltage applied to the rotor wdg &
3phase voltage applied to the stator wdg .The stator wdg produces a rotating fieldwhich
revolves round the stator at Ns. the D.C. sets up a two pole field which is stationary.
Suppose at any instant the stator poles are at position A & B.It is clear that poles Ns & Nr repel
each other & so do the poles Ss & Sr ,Therefore the rotor tends to move in the anticlockwise
direction .after half cycles 1/(2f ) =0.01 sec . the polarities of stator poles are reversed but the
polarities of rotor poles remain same .now Ss & Nr will attract each other & so do Ns & Sr
therefore rotor tends to move in clockwise direction .since the stator poles change their
polarities rapidly,they tend to pull the rotor first in one direction & then after a half cycle in the
other .Due to high inertia of the rotor motor fails to start. Hence synchronous motor
has no self starting torque.
Steady state electromagnetic torque is developed in a synchronous motor only when magnetic
locking between stator & rotor fields takes place.This can happen only when relative speed
between stator & rotor fields is zero.
If stator and rotor fields rotate in same direction & at same speed ,then rotor S pole is
magnetically locked with stator N pole & rotor N pole with stator S pole & in this manner steady
state torque is developed.this means that rotor must be brought up to a speed equal to the
rotating stator field speed for production of torque.

3. This can be done by two methods .(1) auxiliary motor starting (2) damper winding starting
For these two topics plz refer P.S.BHIMBHRA …PAGE NO 676-678
In the stator wdg ,two effects are to b considered the effect of stator field on the stator wdg &
the effect of the rotor field cutting stator conductors at synchronous speed ..
(1)the effect of stator field on the stator conductors is accounted for by including an inductive
reactance in the armature wdg .This is called synchronous reactances Xs. Ra is consider for the
copper loss in the stator wdg.
(2)The second effect is that a voltage is generated in the stator wdg by the synchronously
revolving field of rotor .This generated emf Eb is known as Back emf & opposes V (stator
voltage) .Eb depends on rotor speed & rotor flux phi per pole or field current.
for two topics (1) effect on motor at load (2) effect of change of field variation (v & inverted v
curves)… go with j.b.gupta …..v.k.mehta is good for numerical…