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This type of rotor is used for slow speed machies which have large diameters and small axial lengths.
The pole are made of thick steel laminations rivetted together and attached to a rotor by a devetail joint. The overhang of the pole gives mechanical support to the field coil.
In most of the alternators, where the oscillation or the hunting effect is very high, the damper winding in the pole faces is provided. The copper bars short circuited at both ends are placed in the specially 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 there will be relative motion between the damper winding and the main field. This will induce current 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 is so shaped that the radial gap length increases from the pole centre to pole tips. This makes the flux distribution over the armature uniform uniform to generate sinusiodal waveform of e.m.f.
The salient pole field structure has the following special feature
1. They have large diameter and short axial length.
2. The pole shoe covers about 2/3rd of pole pitch.
3. Poles ar e laminated to reduce eddy current losses.
4. These employed with hydraulic turbines or diesel engines. The speed is 100 to 375 r.p.m.