Salient Pole Alternator Theory, Two-Reaction Theory, and Vector Diagrams
The sources provide a detailed explanation of salient pole alternators, their operation, and related concepts like the two-reaction theory.
Salient Pole (or Projected Pole) Alternators
Salient pole alternators, also known as projected pole alternators, are characterized by their prominent poles that project outward from the rotor structure. These alternators are typically used in low and medium-speed applications, ranging from 120 to 400 RPM. The sources list several reasons for their suitability in these speed ranges:
- Windage Loss and Noise: Salient poles create significant wind resistance, leading to increased losses and noise at higher speeds.
- Mechanical Strength: The construction of salient poles is not robust enough to withstand the stresses experienced at high speeds.
The design of salient pole rotors often involves a large diameter to accommodate the necessary space for the poles, resulting in a short axial length. The sources explain that this configuration is suitable for achieving the desired frequency (e.g., 50 Hz) at low speeds by using a higher number of poles.
Two-Reaction Theory
The two-reaction theory is a method for analyzing the effects of armature reaction in salient pole synchronous machines, including alternators. This theory takes into account the non-uniform air gap in salient pole machines, resulting in different reluctances along the direct axis (d-axis) and the quadrature axis (q-axis).
- Direct Axis (d-axis): The axis aligned with the rotor’s magnetic field, characterized by lower reluctance and higher flux.
- Quadrature Axis (q-axis): The axis perpendicular to the rotor’s magnetic field, characterized by higher reluctance and lower flux.
The theory states that the armature current (Ia) can be resolved into two components:
- Direct-axis component (Id): Component of armature current perpendicular to the excitation voltage (E0).
- Quadrature-axis component (Iq): Component of armature current aligned with the excitation voltage (E0).
Each of these components is associated with a corresponding synchronous reactance:
- Direct-axis synchronous reactance (Xd): Represents the combined effect of armature leakage reactance and direct-axis armature reaction.
- Quadrature-axis synchronous reactance (Xq): Represents the combined effect of armature leakage reactance and quadrature-axis armature reaction.
The two-reaction theory helps in understanding the performance characteristics of salient pole synchronous machines, taking into account the variations in magnetic reluctance along different axes.
Vector Diagram
A vector diagram illustrating the two-reaction theory for a salient pole synchronous generator operating at a lagging power factor is shown in Figure (10.34) in the sources. The diagram shows the relationships between the following phasors:
- Terminal voltage (V)
- Excitation voltage (E0)
- Armature current (Ia)
- Direct-axis component of armature current (Id)
- Quadrature-axis component of armature current (Iq)
- Direct-axis synchronous reactance drop (IdXd)
- Quadrature-axis synchronous reactance drop (IqXq)
- Power angle (δ): Angle between E0 and V.
The vector diagram aids in visualizing the effects of armature reaction and the differences in reactance along the direct and quadrature axes, providing a graphical representation of the generator’s operating conditions