Principle Of Operation

• Operate on principle of electromagnetic induction• Stationary armature and rotating field• Rotor is rotated using a prime mover• When the rotor rotates, the stationary conductors

are cut by the magnetic flux • Hence emf is induced• In three phase alternators, the rotor flux will

induce 3 voltages displaced in time by 120 degree• Emf induced is AC in nature• AC voltage will give rise to AC current when a

load is connected to stator

Advantages of stationary armature

• Current can be delivered directly from fixed terminals on the stator. This reduces voltage drop due to brushes

• Easy to insulate stationary armature windings• Bracing of armature windings is easier.

Deformation of windings can be avoided.

Construction of Alternators• Stator frame• Armature core

Cylindrical rotor

Salient pole rotor

Damper windings

•Also called as amortissuer windings• Copper bars located in slots in pole shoe•Bars are SC at both ends by heavy copper rings•Alternators are not self starters so they provide starting torque

Ratings of an Alternator

• Line to line voltage• Full load KVA• Power factor• Field voltage• Maximum field current• Resistance of stator winding

• When the alternator is loaded, the terminal voltage drops. This depends on the load and load power factor