In this article, we shall discuss the differences between Salient Pole & Non-Salient Pole Rotor. In electrical machines, the rotor is a moving or rotating part of an electromechanical system like an electric motor or generator, or alternator.
The rotor is used to house the rotor winding and magnetic poles. It is attached to a shaft with help of a key. Thus, the rotor is considered a rotating part of a machine that rotates inside a magnetic field when torque is developed. There are two types of rotor constructions used namely, salient pole rotor and non-salient pole rotor.
What is a Salient Pole Rotor?
Salient Pole Rotors are large in diameter and shorter in length. They are typically used in slow-speed (usually 100 RPM to 1500 RPM) electrical machines. The poles are made of steel laminations and are fixed to the rotor by a dovetail joint.
Salient Pole rotors normally require damper windings to avoid oscillations of rotors during operation. The damper conductor bars are short-circuited at both ends with the help of copper rings. The rotor winding is provided on these salient poles and supported by the pole shoes.
Features of Salient Pole Rotor
The salient pole rotor has the following special features:
- Salient pole rotors are used in low-speed electrical machines.
- Salient pole cores are laminated to reduce eddy current losses.
- The pole shoes cover around 2/3 of the pole pitch.
- Since the salient pole rotor speed is slower, thus a larger number of poles are required to attain the required frequency because the frequency is proportional to the number of poles. Typically, the number of salient poles ranges from 4 to 60.
- Salient pole rotors generally use damper winding to prevent rotor oscillations while operating.
- The flux distribution is relatively poor as compared to the non-salient pole rotor, hence the induced emf waveform is not as uniform as the non-salient pole rotor.
- These are usually used with hydraulic turbines or diesel engines.
What is a Non-Salient Pole Rotor?
Non-Salient Pole Rotors are cylindrical-shaped rotors having parallel slots on their outer periphery to place rotor windings. They have a relatively smaller diameter but a longer axial length. They are typically used in high-speed alternators usually 1500 RPM to 3000 RPM. The non-salient pole rotor is made up of solid steel.
They are also called drum rotors or cylindrical rotors. The surface of the non-salient pole rotor is smooth because of the absence of the outside projection of poles, therefore, they maintain a uniform air gap between the stator and rotor.
Features of Non-Salient Pole Rotor
The non-salient pole rotor has the following special features:
- Non-salient pole rotors are smaller in diameter but have longer axial lengths.
- The windage losses and noise produced is less than that of the salient pole rotors.
- Non-salient pole rotors are generally designed for 2-pole or 4-pole machines.
- Non-salient pole rotor construction is mechanically very strong as compared to salient pole rotor construction.
- The damper windings are not required in the case of non-salient pole rotors.
- Non-salient pole rotors are mostly used in high-speed electrical machines.
- In the non-salient pole rotor, the magnetic flux distribution is sinusoidal and thus they give better emf waveform.
Difference between Salient Pole Rotor and Non-Salient Pole Type Rotor
The important differences between the salient pole rotor and the Non-salient pole rotor are listed in the following table:
| Parameter | Salient Pole Rotor | Non-Salient Pole Rotor |
| Definition | A salient-pole rotor is a type of rotor that consists of projected poles keyed on a cylindrical core. | A non-salient pole rotor is a cylindrical rotor with slots on its outer periphery to house rotor windings. |
| Windage loss | The salient pole rotor has more windage losses. | Non-salient pole rotor has fewer windage losses due to their smooth surface. |
| Number of poles | A salient pole rotor has more poles. | In a non-salient pole rotor, usually, 2 or 4 poles are present on the rotor. |
| Flux distribution | In the salient pole rotor, the magnetic flux is not uniform due to the air gap between poles. | In the non-salient pole rotors, the flux is uniformly distributed in the air gap. |
| Speed | The salient pole machine will have a typical speed ranging from 100 RPM to 600 RPM. | Non-salient pole rotor will have higher speeds around 1500 RPM to 3000 RPM. |
| Pole construction | In the Salient pole rotor, the poles are projected. | In a non-salient pole rotor, the rotor poles are not projected, i.e. it has an outer smooth surface. |
| Suitable prime mover | With a salient pole rotor, the water turbines are generally used to drive the machine. | Non-salient pole rotor is generally used with steam turbines. |
| Balanced operation | Salient pole rotors are not balanced. | Non-salient pole rotor provides perfectly balanced operation. |
| Construction | The construction of a salient pole rotor is complex. | Non-salient pole rotor has a simple construction. |
| Need of damper windings | Salient pole rotors require damper windings to prevent rotor oscillations during operation. | The damper windings are not required in the non-salient pole rotors. |
| Diameter and axial length | Salient pole rotors have larger diameters and smaller axial lengths. | Non-salient pole rotors have a smaller diameter and larger axial length. |
| Applications | Salient pole synchronous generators are mostly used in hydropower plants. | Non-salient pole rotors are often used in thermal power plants, nuclear power plants, gas power plants, etc. |
Conclusion
Thus, all these are the key differences between the salient pole rotor and the non-salient pole rotor. The most significant difference between the two rotor constructions is that the salient pole rotor has projected poles whereas the non-salient pole rotor has cylindrical poles.