Rotor Construction of 3-Phase Synchronous Machine
Rotors of an electrical machine are classified as:
• Salient pole rotors and
• Non-salient pole rotors.
The term salient means ‘projecting’. A salient pole consists of poles that are projected out from the surface of the rotor core. These are used for the rotors for four or more poles.
A non-uniform air gap accompanies a salient-pole synchronous machine.
The air gap is minimum under the pole centers, and it is maximum in between the poles. The pole faces are so shaped that the radical air gap length increases from the pole center to the pole tips so that the flux distribution in the air gap is sinusoidal. This will help the machine to generate sinusoidal e.m.f.
Cylindrical Rotor or Non-salient pole rotors
Cylindrical-rotor machines are also known as non-salient pole rotor machines. The construction of the rotor is such that it forms a smooth cylinder. It has no physical poles as in the salient-pole construction. These rotors are made up of solid forgings of high-grade nickel-chrome-molybdenum steel.
3-phase synchronous machine rotor construction:
• Salient pole rotor: larger diameter, shorter axial length
• Cylindrical pole rotor: smaller diameter, larger axial length
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