Alternator Construction and Working

An alternator has 3 phase winding on the stator and D.C field winding on the rotor.

Stator

It is the stationary part of the machine and is built up of sheet-steel laminations having slots, on its inner periphery. A 3 phase winding is placed in these slots and serves. as the armature winding of the alternator. The alternator winding is always connected in star and the neutral is connected to ground.

Rotor

The rotor carries a field winding which is supplied with direct current, through two slip rings by a separate d.c source. Rotor construction is of two types, namely,

Salient (or) projecting pole type
Non-salient pole (‘or’) cylindrical type

Salient Pole Type

The rotor of this type is used almost entirely for slow and moderate speed alternators, since it is least expensive and provides ample space for the field ampere-turns. Salient poles. cannot be employed in high speed generators on account of very high peripheral speed and the difficulty of obtaining sufficient mechanical strength.

The salient poles are made of thick steel laminations riveted together and are fixed to rotor by a dove-tail joint. The pole faces are usually provided with slots for damper windings. These dampers are useful in preventing hunting. The pole faces are so shaped that the radial air gap length increases from the pole centre to the pole tips so that the flux distribution over the armature is sinusoidal and waveform of generated e.m.f is sinusoidal. The field coils are placed on the pole-pieces and connected in series. The ends of the field windings are connected to a d.c source through slip-rings carrying brushes and mounted on the shaft of the field structure.

The salient-pole field structure has the following special features.

They have large diameter and short axial length.
The pole shoes cover about 2/3 of pole pitch.
Poles are laminated in order to reduce eddy current losses.
They are employed with hydraulic turbines or diesel engines. The speed is from 120 to 400 r.p.m.

Smooth Cylindrical or Non-salient Pole Type

The rotors of this type are used in very high speed alternators driven by steam turbines. To reduce the peripheral velocity, the diameter of the rotor is reduced and axial length is increased. Such rotors have two or four poles.

It consists of a cylindrical steel forging which is suitably fabricated mechanically and treated thermally. The forging has radial slots in which the field copper, usually in strip form is placed. The coils are held in place by steel or bronze wedges and the coil ends are fastened by metal rings. The slots over certain portions of the core are omitted to form pole faces. The regions forming the poles are usually left unslotted as shown in figure.

The non-salient pole field structure has the following special features.

They are of small diameter and of very long axial length.
Less windage loss
The speed employed is from 1,000 to 3,000 rpm.
Better in dynamic balancing and quieter in operation.

Working of Alternator

Figure shows sectional view of a salient pole alternator. The field magnets are magnetised by applying 125 Volts or 250 Volts through slip rings. The field windings are connected such that, alternate N and S poles are produced. The rotor and hence the field magnets are driven by the prime mover.

As the rotor rotates, the armature conductors are cut by the magnetic flux. Hence an emf is induced in the armature conductors. As the magnetic poles are alternately N and S pole, this emf acts in one direction and then in the other direction. Hence an alternating emf is induced in the stator conductors.

The frequency of induced emf depends on the number of N and S poles moving past an armature conductor in one second. The direction of induced emf can be found by Fleming’s right hand rule and frequency is given by

f = PN/120

where N = Speed of rotor in rpm, P = Number of rotor poles.