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Salient pole rotor vs. non-salient pole rotor

Rotors of an electrical machine are classified as: (i) Salient pole rotors and (ii) Non-salient pole rotors. Both types are explained below.

Salient pole rotor

salient pole rotor

In salient pole type of rotor consist of large number of projected poles (salient poles) mounted on a magnetic wheel. Construction of a salient pole rotor is as shown in the figure at left. The projected poles are made up from laminations of steel. The rotor winding is provided on these poles and it is supported by pole shoes.
  • Salient pole rotors have large diameter and shorter axial length.
  • They are generally used in lower speed electrical machines, say 100 RPM to 1500 RPM.
  • As the rotor speed is lower, more number of poles are required to attain the required frequency. (Ns = 120f / P   therefore, f = Ns*p/120   i.e. frequency is proportional to number of poles). Typically number of salient poles is between 4 to 60.
  • Flux distribution is relatively poor than non-salient pole rotor, hence the generated emf waveform is not as good as cylindrical rotor.
  • Salient pole rotors generally need damper windings to prevent rotor oscillations during operation.
  • Salient pole synchronous generators are mostly used in hydro power plants.

Non-salient pole (cylindrical) rotor

non-salient pole (cylindrical) rotor

Non-salient pole rotors are cylindrical in shape having parallel slots on it to place rotor windings. It is made up of solid steel. The construction of non-salient pole rotor (cylindrical rotor) is as shown in figure above. Sometimes, they are also called as drum rotor.
  • They are smaller in diameter but having longer axial length.
  • Cylindrical rotors are used in high speed electrical machines, usually 1500 RPM to 3000 RPM.
  • Windage loss as well as noise is less as compared to salient pole rotors.
  • Their construction is robust as compared to salient pole rotors.
  • Number of poles is usually 2 or 4.
  • Damper windings are not needed in non-salient pole rotors.
  • Flux distribution is sinusoidal and hence gives better emf waveform.
  • Non-salient pole rotors are used in  nuclear, gas and thermal power plants.