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Alternator or AC generator or synchronous generator •
An alternator is an electrical machine which converts mechanical energy into alternating electric energy.
•
Alternator works on the same principle of electromagnetic induction as DC generator.
There are two types of AC generators.
Rotating armature type: - armature rotates and field system is stationary (same as DC generator) . ow power rating (used in in many shops- small Diesel!kerosene Diesel!kerosene generator) Rotating Rotating field field type:type:- armature armature is stationary stationary and field field system system rotates. rotates. Rotating field type alternators are more advantages than Rotating armature type. "n general this type of alternator is used (#igh power rating) all power plants. Why Armature Winding Is Stationary In An Alternator?
At high voltages$ it easier to insulate stationary armature winding$ which may %e as high as &' k or more.
he high voltage output can %e directly taken out from the stationary armature. *hereas$ for a rotary armature$ there will %e large %rush contact drop at higher voltages$ also the sparking at the %rush surface will occur.
+ield e,citer winding is placed in rotor$ and the low dc voltage can %e transferred safely.
he armature winding can %e %raced well$ so as to prevent deformation caused %y the high centrifugal force. Construction
"n alternator$ construction consists of armature winding is placed in the stationery element called stator and filed winding on a rotating element called rotor. An alternator has three phase winding on the stator and a field winding (DC winding) on the rotor. Stator: "t is the stationary part of the machine and is %uildup of sheet steel laminations to
minimi/e the eddy the eddy current losses. losses. hey are provided with slots on its inner periphery . A three phase winding is placed in these slots and serves as the armature winding of the alternator. he armature winding is always connected to stator and the neutral is connected to ground. Rotor:- he rotor is rotating part of the alternator. he rotor having alternate 0 and 1 poles on
its outer periphery. hese poles are magneti/ed %y means of low DC voltage of 234 or 34' through two slip rings. his dc source (e,citer) is generally a small dc generator mounted on the shaft of the alternator.
here are two types of rotors: i)
1alient pole type rotor (5ro6ected 5ole) ii) 0on-salient pole type rotor (Cylindrical pole) having the following characteristics. 1alient pole type rotor (5ro6ected 5ole)
he poles are pro6ected arge diameter and small a,ial length 1uita%le for low speed alternator Air gap is not uniform 7ore num%er of poles are used sed in hydro electric power plant low 1peed (23' to 9'' rpm) 7ore 0oise 7ore windage loss
0on-salient pole type rotor (Cylindrical pole)
he poles are not pro6ected 1mall diameter and large a,ial length 1uita%le for high speed alternator Air gap is uniform 8nly 3 or 9 poles are used sed in thermal power plants #igh speed (24'' or &''' rpm) ess noise ess
Operation:
he rotor winding is energi/ed from the DC e,citer and an alternate 0 and 1 poles are developed on the rotor. *hen the rotor is rotated in anticlockwise direction %y prime mover$ the stator or armature conductors are cut %y the magnetic flu, of rotor poles. Conse;uently$ emf is induced in the stator due to electromagnetic induction. he direction of induced emf can %e
found %y +lemings right hand rule and fre;uency is given %y rotor in rpm= 5 < num%er of rotor poles.
Salient pole type rotor (Projected Pole
f =
NP 120
where 0< speed of
1alient means sticking out or pro6ected out. A salient pole is a magnetic pole that is pro6ected out of the rotor surface. he magnetic wheel is made of cast iron or steel with good magnetic ;uality. 5oles are mounted on a large circular steel frame which is fi,ed to the shaft of the alternator. he individual fi,ed pole windings are connected in series in such a way that when the field winding is energi/ed %y the dc e,citer$ ad6acent poles have opposite polarities. he salient pole alternators are slow-speed machines$ speed varying from 24' to >'' rp!. hese alternators are driven %y hydraulic tur%ines.
1alient type rotor has large dia!eter" s!all length and low speed . Diameter is usually %etween &-24 m. 1alient type rotor has non-uniform air-gap and more num%er o f poles. ow and medium speed alternators (#$% & '%% rp! driven %y diesel engines or water tur%ines have salient pole type rotors due to following reasons: he salient field poles would cause an e,cessive windage loss if driven at high speed and would tend to produce noise. 1alient-pole construction cannot %e made strong enough to withstand the mechanical stress at higher speeds. on-salient pole:
he rotor is made of smooth solid forged steel and cylindrical in shape. Rotor has a num%er of slots along outer periphery. he field windings are em%edded in these slots and are connected in series to the slip ring. he field windings are wound such that 0 and 1 poles occur alternately and are energi/ed %y the DC e,citer. he num%er of filed poles may %e two or four. 0on-salient pole is non-pro6ecting surface type. Rotor has small diameter$ large a,ial length and uniform air-gap. 0on-salient type rotor is used for high speed (24'' or &''' rpm) are driven %y steam tur%ines.
