The difference between salient pole generators and non-salient pole generators

Differences Between Salient-Pole Generators and Nonsalient-Pole Generators

Salient-pole generators and nonsalient-pole generators are two common types of synchronous generators, differing significantly in structure, performance, and application. Below is a detailed comparison of the two:

1. Rotor Structure

• Salient-Pole Generator:

• Rotor Shape: In a salient-pole generator, the rotor has distinct magnetic poles that protrude from its surface, forming visible pole shoes. Each pole typically consists of an iron core and an excitation winding.

• Number of Poles: Salient-pole generators usually have fewer poles (such as 2, 4, 6, 8), with noticeable gaps between the poles ( interpolar regions).

• Application: Salient-pole generators are primarily used in low-speed, high-capacity applications, such as hydroelectric generators and steam turbine-driven generators.

• Nonsalient-Pole Generator:

• Rotor Shape: The rotor of a nonsalient-pole generator has a smooth, cylindrical surface without any visible protruding poles. The excitation windings are embedded in slots within the rotor.

• Number of Poles: Nonsalient-pole generators typically have more poles (such as 12, 16, 24), distributed evenly around the rotor, with minimal interpolar regions.

• Application: Nonsalient-pole generators are mainly used in high-speed, medium to small capacity applications, such as steam turbine generators and gas turbine-driven generators.

2. Air Gap Distribution

• Salient-Pole Generator:

• Non-uniform Air Gap: Due to the protruding poles, the air gap in a salient-pole generator is smaller at the poles and larger in the interpolar regions. This non-uniform air gap leads to a non-sinusoidal magnetic field distribution, affecting the quality of the output voltage waveform.

• Harmonic Content: The non-uniform air gap can result in higher harmonic content in the output voltage, particularly third harmonics.

• Nonsalient-Pole Generator:

• Uniform Air Gap: The air gap in a nonsalient-pole generator is nearly uniform around the entire circumference, resulting in a more sinusoidal magnetic field distribution and better output voltage waveform quality.

• Harmonic Content: The uniform air gap minimizes harmonic content, producing a cleaner voltage waveform.

3. Electromagnetic Characteristics

• Salient-Pole Generator:

• Direct Axis and Quadrature Axis Reactance: In a salient-pole generator, the direct axis reactance (Xd) and quadrature axis reactance (Xq) differ. Xd is larger because the magnetic flux through the poles encounters less reluctance, while Xq is smaller due to the higher reluctance in the interpolar regions.

• Short-Circuit Ratio (SCR): Salient-pole generators have a lower short-circuit ratio, typically ranging from 1.0 to 2.0. This results in higher short-circuit currents but slower voltage recovery during faults.

• Nonsalient-Pole Generator:

• Direct Axis and Quadrature Axis Reactance: In a nonsalient-pole generator, the direct axis reactance and quadrature axis reactance are almost equal due to the uniform air gap and symmetrical flux path.

• Short-Circuit Ratio (SCR): Nonsalient-pole generators have a higher short-circuit ratio, typically ranging from 2.0 to 3.0. This results in lower short-circuit currents and faster voltage recovery during faults.

4. Mechanical Characteristics

• Salient-Pole Generator:

• Large Rotor Inertia: The larger poles in a salient-pole generator contribute to a higher rotor inertia, making it suitable for low-speed, high-inertia systems, such as hydroelectric turbines.

• Ventilation and Cooling: The gaps between the poles facilitate the design of cooling ducts, providing better ventilation and cooling performance.

• Nonsalient-Pole Generator:

• Small Rotor Inertia: The compact rotor structure of a nonsalient-pole generator results in lower inertia, making it suitable for high-speed, low-inertia systems, such as steam turbines.

• Ventilation and Cooling: The smooth rotor surface of a nonsalient-pole generator makes ventilation and cooling more complex, often requiring specialized cooling systems.

5. Starting Characteristics

• Salient-Pole Generator:

• High Starting Torque: Due to the larger poles, salient-pole generators provide higher electromagnetic torque during startup, making them suitable for applications requiring significant starting torque.

• Nonsalient-Pole Generator:

• Lower Starting Torque: Nonsalient-pole generators have relatively lower starting torque but exhibit better dynamic response during high-speed operation.

6. Applications

• Salient-Pole Generator:

• Primarily used in low-speed, high-capacity power generation systems, such as hydroelectric power plants and nuclear power plants. The low-speed characteristics of salient-pole generators make them ideal for use with hydro turbines or low-speed steam turbines.

• Nonsalient-Pole Generator:

• Primarily used in high-speed, medium to small capacity power generation systems, such as thermal power plants and gas turbine power plants. The high-speed characteristics of nonsalient-pole generators make them ideal for use with steam turbines or gas turbines.

Summary

• Salient-Pole Generator: Features distinct magnetic poles, non-uniform air gap, and is suitable for low-speed, high-capacity applications like hydroelectric generators. Its advantages include higher starting torque and better cooling performance, but it may have more harmonic content in the output voltage.

• Nonsalient-Pole Generator: Has a smooth rotor surface, uniform air gap, and is suitable for high-speed, medium to small capacity applications like steam turbine generators. Its advantages include better output voltage waveform quality and faster short-circuit recovery, but it has lower starting torque.

The choice between a salient-pole generator and a nonsalient-pole generator depends on the specific application requirements, including speed, capacity, starting characteristics, and the mechanical and electrical needs of the system.