An alternator consists of 3-phase winding on the stator and a d.c. the field winding on the rotor.
1. Stator
It is the stationary part of the alternator and it is made up of laminated sheet-steel having slots on its inner periphery. A 3-phase winding is placed in these slots and serves as the armature winding of the alternator which is placed in a stationary part. Generally in Alternator armature is connected in star and neutral connected to the ground.
2. Rotor
The rotor carries a field winding which is supplied with direct current through two slip rings by a separate d.c. source. This d.c. the source which is also known as exciter is generally small dc generator or dc power source which mount on the shaft of the alternator.
Rotor construction is of two types, namely;
(i) Salient pole type
(ii) Non-salient pole type
(i) Salient pole type
In this type of rotor construction, the salient poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator as shown below.
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| Salient Pole Type Rotor |
The individual field pole windings are connected in series in such a way that when the field winding is energized by the exciter, adjacent poles have opposite polarities. The speed of this type of rotor is about 120-400 RPM which is driven by water turbine or diesel engine due to following reasons-
- The salient poles rotor cause excessive windage loss if it is driven at high speed and tends to produced noise.
- Salient-pole rotor construction cannot be made strong enough to withstand the mechanical stresses to which they may be subjected at higher speeds. In India, 50 Hz frequency is required for generating AC so it has a large number of pole and short axial length.
(ii) Non-salient(Cylindrical) pole type
In this type of rotor construction made up of smooth solid forged steel radial cylindrical having number of slots along the outer periphery.
The field windings are placed in these slots and are connected in series to the slip rings through which they are also energized by the exciter. The fig is shown the non-salient pole type rotor.
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| Cylindrical Pole Type Rotor |