Construction of alternatorStator- it is built up of sheet steel laminations having slots on its inner periphery .a 3 phase winding isplaced 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 mountedon 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
centrifugal forces developed by the salient poles.so cylindrical rotor construction is best suitedfrom 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 highspeeds .(ii)The flux distribution around the periphery is nearly a sine wave & hence a better e.m.fwaveform 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 alsobe 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 fieldwhichrevolves 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 repeleach other & so do the poles Ss & Sr ,Therefore the rotor tends to move in the anticlockwisedirection .after half cycles 1/(2f ) =0.01 sec . the polarities of stator poles are reversed but thepolarities of rotor poles remain same .now Ss & Nr will attract each other & so do Ns & Srtherefore rotor tends to move in clockwise direction .since the stator poles change theirpolarities rapidly,they tend to pull the rotor first in one direction & then after a half cycle in theother .Due to high inertia of the rotor motor fails to start. Hence synchronous motorhas no self starting torque.Steady state electromagnetic torque is developed in a synchronous motor only when magneticlocking between stator & rotor fields takes place.This can happen only when relative speedbetween stator & rotor fields is zero. If stator and rotor fields rotate in same direction & at same speed ,then rotor S pole ismagnetically locked with stator N pole & rotor N pole with stator S pole & in this manner steadystate torque is developed.this means that rotor must be brought up to a speed equal to therotating stator field speed for production of torque.
This can be done by two methods .(1) auxiliary motor starting (2) damper winding startingFor these two topics plz refer P.S.BHIMBHRA …PAGE NO 676-678In 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 inductivereactance in the armature wdg .This is called synchronous reactances Xs. Ra is consider for thecopper loss in the stator wdg.(2)The second effect is that a voltage is generated in the stator wdg by the synchronouslyrevolving field of rotor .This generated emf Eb is known as Back emf & opposes V (statorvoltage) .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 vcurves)… go with j.b.gupta …..v.k.mehta is good for numerical…