Construction of an Alternators

CONSTRUCTION OF AN ALTERNATORS

An Alternator has 3‒phase winding on the Stator and a DC 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. In DC machines, the outer frame (or yoke) serves to carry the magnetic flux but in alternators, it is not meant for that purpose. Here it is used for holding the armature stampings and windings in position. The armature core is supported by the stator frame and is built up of laminations of special magnetic iron or steel alloy. The core is laminated to minimise loss due to eddy currents. The laminations are insulated from each other and have spaces between them for allowing is placed in armature slots and serve as the armature winding on the alternator. The armature 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 DC source. This DC source (called exciter) is generally a small DC shunt or compound generator mounted on the shaft of the alternator. Two types of rotors are used in alternators. (i) Salient pole type and (ii) Smooth cylindrical type.

1. Salient (or projecting) pole type

In this type, salient or projecting poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator as shown in Fig. 4.1. The individual field pole windings connected in series in such a way that when the field winding is energised by the DC exciter, adjacent poles have opposite polarities.

It is used in low‒medium speed (120‒400 rpm) alternators. This type of alternators are driven by diesel engines or water turbines. Low‒speed rotors always posses a large diameter to provide the necessary space for the poles.

2. Non salient pole (smooth cylindrical) type

In this type, the rotor is made of smooth solid forged‒ steel radial cylinder having a number of slots along the outer periphery. Such rotors are designed mostly for 2‒pole (or 4‒pole) turbo‒generators running at 3600 rpm (or 1800 rpm). The field windings are embedded in these slots and are connected in series to slip ring through which they are energised by the DC exciter. Two (or four) regions corresponding to the central polar areas are left unslotted as shown in Fig. 4.2.

High speed alternators (1500 or 3000 rpm) are driven by steam turbines. Since steam turbines run at high speed and a frequency of 50 Hz is required, we need a small number of poles on the rotor of high‒speed alternators (also called turbo‒alternators). We can use number less than 2 poles and this fixes the highest possible speed. Turbo alternators posses 2 or 4 poles and have small diameters and very long axial lengths.