Construction of alternator and rotor

Construction of an Alternator

~ An alternator is a synchronous type machine.
~ In this type of machines, field windings are placed on the rotor and the armature winding is housed in suitable shaped slots in the stator.
~ The filed windings (rotor) is connected to an external dc source called exciter, through a pair of sliprings as shown in Fig. 9.3.1.


~ The armature winding (stator) is a 3 phase winding and thel induced voltage in this winding is applied to the load. This is
a 3 phase ac voltage.
~ A prime mover is mechanically coupled to the shaft of the machine. The armature is rotated at a speed called synchronous speed (Ne).
~ The rotating field, fixed armature arrangement is preferred due to certain advantages.
~ Fig. 9.3.2 shows the detail construction of a three phase aiternator.
~ The stator frame made from cast iron supports the stator core.
~ The stator core has slots on its inner periphery in which the stator conductors are placed.

~ The rotor has alternate N and S poles placed on its outer rim.
~ These magnetic poles are excited from an external dc source (typically 125 to 600 V dc).
~ When the excited rotor rotates, its magnetic field cuts the tationary stator (armature) conductors as a result of which a 3-phase ac voltage gets induced into the armature winding.
~ The frequency of this induced stator emf depends on the umber of N and S poles moving past a conductor in one econd and the direction of induced emf is given by Fleming's right hand rule.

Construction of a rotor

Fig. 9.3.3 shows the construction of rotor of an alternator.


Types of rotors :
There are two types of rotor based on its constructions as follows:
1. Salient pole type rotor
2. Smooth cylindrical type rotor.

1. Salient pole type rotor :

~ Fig. 9.3.4 shows the salient pole rotor. They are also called as projected pole type rotor.
In this type of construction the poles attached to the cylindrical rotor are of protruding or projecting type.
~ As the poles are projecting, the air gap between stator and rotor is non uniform. The poles are made up of thick steel laminations and they are bolted out to the rotor as shown in Fig. 9.3.4.
~ The field winding is provided 0n the pole shoe.
~ For such a construction it is possible to have a large number of poles (16, 32 etc.)
~ The only disadvantage is that the size of rotor is big.
~ The mechanical strength of salient pole rotor is less due to the projecting poles. Hence, it is preferred for low speed alternators.
~ The prime movers that dnive such rotors are water turbines and IC engines and steam engines as listed in Table 9.2.1
~ Smooth cylindrical type rotor (Non salient pole type rotor) :
~ They are also called as nonsalient type rotor or non-projected pole type rotor. Fig. 9.3.5 shows smooth cylindrical rotor.
~ In such a type of construction, the rotor is kept unslotted. The slots carry the field winding and the unslotted parts become
poles. 'The air gap between the stator and the rotor is uniform.
~ The rotor is in the form of a smooth solid steel cylinder which has a large number of slots to accommodate the field winding.
~ The slots are covered at top, with steel or manganese wedges.
~ The poles in this type of rotor are not projecting out and surface of rotor is smooth.
~ These rotors have small diameters and large axial lengths, inorder to keep the peripheral speed within limits. These
rotors are mechanically strong.

~ They are used for high speed alternators, which range from 750-3000 rpm. The prime overs that drive such rotors are steam turbines, electric motors.