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Alternator Rotor Construction

Rotors used in alternators are of two types. They are,

  • Salient pole type rotor
  • Smooth cylindrical type rotor

Salient pole type rotor

Salient pole type rotor is also called projected pole type as all the poles are projected out from the surface of the rotor. Salient pole type rotor is mainly used for slow-speed alternators which have larger diameter and small axial lengths.

The poles are made of thick steel lamination rivetted together and attached to a rotor by a devetail joint. The overhang of the pole gives mechanical support to the field coil. The oscillation or the hunting effect is very high in most of the alternators, thus the damper winding in the pole faces in provided. The copper bars are short circuited at both ends are places in the specially provided holes. The relative velocity of the damping winding with respect to main field will be zero when the speed is steady but as soon as it departs from the synchronous speed, hence there will be a relative motion between the damper winding and the main field. As a result the current will induce in them. This induced current will exert a torque in such a way as to bring the alternator to operate at synchronous speed. The pole face of salient pole type rotor is so shaped that the radial air gap length increases from the pole centre to pole tips. This construction makes the flux distribution over the armature uniform to generate sinusoidal waveform of e.m.f.

Smooth cylindrical type rotor

Smooth cylindrical type rotor is also non salient type or non-projected pole type of rotor. Smooth cylindrical rotor is used for alternators which are coupled to steam turbine which run as very high speeds. The diameter of the rotor is reduced and axial length is increased to reduce the peripheral speed of the alternator. The number of rotor poles are 2 or 4.

Smooth cylindrical type rotors are made from solid forgings of alloy steel. The outer periphery of a smooth cylindrical rotor has slots in which the field winding in placed. Whereas unslotted portions of the cylinder itself act as the poles. The poles of the smooth cylindrical type rotor are not projecting out and the surface of the rotor is smooth which maintains uniform air gap between stator and the rotor.

Salient pole type rotor vs Cylindrical type rotor

Salient pole type rotor Cylindrical type rotor
Air gap is no uniform. Air gap is Uniform.
Mechanically weak. Mechanically strong and robust.
Poles are projecting out from its surface. Poles are non projected from its surface.
Diameter is high and axial length is small. Diameter is small and axial length is high.
Preferred only for low speed alternators of ranging from 125 r.p.m to 500 r.p.m. Preferred only for high speed alternators of ranging from 1500 r.p.m to 3000 r.p.m.
Seperate damper winding is necessary. Seperate damper winding is not necessary.
For same size of smooth cylindrical type rotor, the rating is smaller. For same size of salient pole type rotor, the rating is higher.
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