Q18Basic Electrical Engineering

A Synchronous Generator, universally known as an Alternator, is the primary source of all electrical energy consumed globally. It is an electromechanical energy conversion device that transforms mechanical energy (from a turbine) into three-phase AC electrical energy. It is termed 'synchronous' because the electrical frequency of the generated voltage is strictly locked and proportional to the mechanical speed of the rotor.

Constructional Features: Unlike standard DC generators, alternators have a stationary armature winding and a rotating magnetic field. This design is crucial for handling massive high voltages (e.g., 11kV to 33kV) without dangerous sparking at slip rings.

• 1. Stator (Armature): The stationary outer frame built from stacked, laminated silicon steel stampings to drastically reduce eddy current losses. The inner periphery contains deep slots where the heavy 3-phase armature windings (usually star-connected) are securely placed. Because it is stationary, high-voltage output can be drawn directly through robust bolted connections.
• 2. Rotor (Magnetic Field): The rotating core coupled to the prime mover. It carries the field windings, which are supplied with low-voltage DC power via two smooth slip rings and brushes to create distinct North and South magnetic poles. Rotors are of two types: - Salient Pole Rotor: Large diameter, short axial length, featuring protruding (salient) poles. Used in low-speed hydraulic turbines (hydroelectric plants). - Cylindrical (Non-Salient) Rotor: Small diameter, long axial length, perfectly smooth cylinder. Used in high-speed steam or gas turbines (thermal plants) to reduce windage loss.

Working Principle: The fundamental operation is based on Michael Faraday's Law of Electromagnetic Induction. The prime mover (e.g., steam turbine) forcibly rotates the rotor at a constant synchronous speed ($N_s$). The DC supply on the rotor creates a constant, powerful magnetic flux. As the rotor spins, this magnetic flux sweeps across the stationary stator windings. The physical cutting of magnetic flux lines by the stationary conductors induces a dynamic alternating Electromotive Force (EMF) in the 3-phase stator windings. Because the windings are physically offset by $120^{\circ }$, a perfectly balanced 3-phase voltage is produced.

The frequency of the generated voltage strictly follows the equation: $f=\frac{N_s\times P}{120}$, ensuring stable grid synchronization.