An alternating voltage is generated in a single conductor or alternating coil rotating in a uniform magnetic field with stationary poles an alternating voltage will also be generated in stationary armature conductor when the field poles rotate past the conductors. thus , we see that as long as there is a relative motion between the armature conductors and field flux there will be a voltage generated in the armature conductors. in both cases the wave shape of voltage is a sine curve.

A.C. generator are usually called alternator the are also called synchronous generators. rotating machines that rotate at a fixed speed by the supply frequency and the number of poles are called synchronous machines

A synchronous generator is a machine for converting mechanical power from a prime mover to a.c. electrical power at a specific voltage and frequency. a synchronous machine rotates at constant speed is called synchronous speed. synchronous generators are usually 3-phase type because of the several advantages of 3-phase generation, transmission and distribution. Large synchronous generators are used to generate bulk power at thermal, hydro and nuclear power stations. 

Synchronous generator with power rating of several hundred MVA are very commonly used in generating stations. the biggest size used in india has a rating of 500 MVA used in super power thermal power stations. synchronous generator are primary sources of the world's electric power system today.For bulk power generation , stator winding of synchronous generators are designed for voltage ranging from 6.6 kV to 33 kV.

Construction of three -phase synchronous machines
An alternator consists of two main parts namely, the stator and the rotor. the stator is the stationary part of the machine. It carries the armature winding in which the voltage is generated. the output of the machine is taken from the stator. the rotor is the rotating part of machine. the rotor produces the main field flux.

Stator: the various parts of the stator include the frame, stator core , stator windings and cooling arrangement.

Frame  may be cast iron for small-size machines and welded steel type for large size machines. 

In order to reduce hysteresis and eddy-current losses, the stator core is assembled with high grade silicon content steel lamination. 

A 3-phase winding is put in the slots cut on the inner periphery of the stator as show in fig the winding of each phase is distributed over a several slots. when current flow in a distributed winding it produces an essentially sinusoidal apace distribution of emf.

Rotor: There are two types of rotor construction namely, the salient pole and cylindrical rotor type.

Salient Pole Rotor: The term Salient means protruding or projecting thus a salient- pole rotor consists of poles projecting out from the surface of the rotor core.

since the rotor is subjected to changing magnetic fields, it is made of thin steel laminations to reduce eddy current losses . the salient poles rotor have concentrated winding on the poles .

Damper bars are usually inserted in the pole faces to damp out the rotor oscillations during the sudden change in load condition.

A salient pole synchronous machines has nonuniform air gap. the air gap is minimum under the pole centres and it is maximum in between the poles. the pole faces are so shaped that the radial air gap length increases from pole centre to pole tips so that the flux distribution in the air gap is sinusoidal. this will help the machine generate sinusoidal emf.  

The individual field pole windings are connected in series to give alternate north and south polarities. the end of field winding are connected to a d. c. source through the brushes on the slip rings. the slip rings are metal rings mounted on the shaft  and insulated from it.  they are used to carry current to  or from the rotating part the machine via carbon brushes. 

Salient pole generator have a large number of poles, and operate at lower speeds. A salient pole generator has comparatively a large diameter and short axial length. the large diameter accommodates  a large number of poles. 

Salient pole generators driven by water turbines are called hydro-alternators or hydrogenerators. 

Cylindrical Rotor: A cylindrical rotor machine is also called a non-salient pole rotor has its rotor so constructed that it forms a smooth cylinder. the construction is such that there are no  physical poles to be seen as in salient pole construction.  

Cylinder rotor are made from solid forgings of high grade nickel chrome-molybdenum steel. In about two third of the rotor periphery, slots are cut in regular intervals and parallel to the shaft. The d.c. field wings are accommodated in these slots. the winding of distributed type. the unslotted portion of the rotor forms pole faces.

The smooth rotor of the machine makes less winding losses and  the operation is less noisy because of uniform air gap.

fig Show end views of 2 pole and 4 pole cylindrical rotors. cylindrical-rotor machines are driven by steam or gas turbines.   cylindrical-rotor synchronous generators are called turboalternator or turbogenerator.such machine are always horizontal configuration installation.the machine are built in a number of ratings from 10MVA to 1500MVA.  


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