Construction:

• Stator (Armature): The stationary part where the AC voltage is generated.
• Stator Frame: Provides mechanical support and houses the core.
• Stator Core: Made of laminated silicon steel (to minimize eddy current and hysteresis losses), with slots on its inner periphery.
• Armature Windings: Three-phase insulated copper windings placed in the stator slots. These windings are typically connected in a star (Y) configuration. This is where the output AC voltage appears.
• Rotor (Field System): The rotating part that produces the main magnetic field. It carries the DC field winding, which is excited by an external DC source (exciter).
• Types of Rotors:
  • 1. Salient Pole Rotor:
  • Description: Has distinct, projecting poles (like a star shape) that are bolted to the rotor shaft. The field windings are wound around these pole pieces. The pole faces are usually laminated.
  • Characteristics: Large diameter, short axial length. Provides good ventilation. Suitable for large number of poles.
  • Speed: Used for low-speed alternators (e.g., in hydroelectric power plants, driven by slow-speed water turbines).
  • 2. Cylindrical (Non-Salient Pole) Rotor:
  • Description: Has a smooth, cylindrical forged steel rotor with slots milled out of its periphery. The field windings are placed in these slots, forming a distributed winding that approximates a sinusoidal flux distribution.
  • Characteristics: Small diameter, long axial length. Provides a uniform air gap. Mechanically robust and quiet at high speeds.
  • Speed: Used for high-speed alternators (e.g., in thermal, nuclear, and gas turbine power plants, driven by high-speed steam/gas turbines). Typically 2 or 4 poles.
• DC Exciter and Slip Rings/Brushes: A separate DC power source (exciter) provides the DC current to the rotor field winding. For conventional alternators, this DC current is supplied to the rotating field winding through stationary carbon brushes making contact with rotating slip rings mounted on the rotor shaft. Modern large alternators often use brushless excitation systems, which eliminate the need for brushes and slip rings.

Working Principle (EMF Generation):

1. Field Excitation: A DC current is supplied to the rotor's field winding, creating a steady magnetic field (north and south poles) on the rotor. The strength of this magnetic field (flux Φ) can be controlled by varying the DC field current.
2. Prime Mover and Rotation: The rotor is mechanically driven by a prime mover (e.g., a turbine for large power plants, or a diesel engine for backup generators) at a very precise and constant speed, known as the synchronous speed.
3. Flux Cutting and Induced EMF: As the rotor (with its magnetic field) rotates, its magnetic flux lines cut the stationary conductors of the stator (armature) windings. According to Faraday's Law, this relative motion induces an electromotive force (EMF) in the stator conductors.
4. Three-Phase Output: Since the stator has three-phase windings spatially displaced by 120∘ electrical degrees, the rotating rotor flux induces three sinusoidal EMFs in these windings. These induced EMFs are equal in magnitude and displaced by 120∘ electrically from each other, thus generating a balanced three-phase AC voltage output.

EMF Equation of a Synchronous Generator (Alternator):

The magnitude of the RMS phase EMF induced in the stator windings is determined by the machine's design and operating parameters.
- Formula (RMS Phase EMF): Eph = 4.44Kw fΦTph (Volts) Where:
- Eph : RMS value of the induced EMF per phase (Volts).
- 4.44: A constant derived from the sinusoidal nature of the flux and the relationship between average and RMS values for a sine wave.
- Kw : Winding factor (or winding distribution factor and pitch factor combined). It accounts for how the windings are distributed in the slots and how the coil sides are pitched. It's typically less than 1 (e.g., 0.9 to 0.98), reducing the effective turns and thus the induced EMF.
- f: Frequency of the generated AC voltage (Hz).
- Φ: Magnetic flux per pole (Webers). This is directly controlled by the DC field current.
- Tph : Number of turns per phase in the stator winding.
- Significance: This equation highlights the factors that determine the generated voltage. For grid-connected operation, frequency (f) must be constant. Therefore, the generated voltage (Eph) is primarily controlled by varying the field flux (Φ) through adjustment of the DC field current.

Concept of Synchronous Speed (Ns):

• Definition: For an alternator, the rotor's mechanical speed must be precisely equal to the synchronous speed to generate AC power at the desired output frequency (e.g., 50 Hz or 60 Hz for power grids). The name "synchronous" comes from the fact that the rotor's mechanical speed is synchronized with the speed of the rotating magnetic field it produces (or the frequency of the generated voltage).
• Formula: Ns =(120f)/P (in RPM)
• This is the same formula as for the RMF of an induction motor. Here, f is the desired output frequency, and P is the number of poles on the rotor.
• Example 5.1: A 2-pole synchronous generator needs to produce power at 50 Hz. Its rotor must spin at Ns =(120×50)/2=3000 RPM.
• Example 5.2: A hydroelectric generator has 12 poles and is designed for 60 Hz. Its rotor must spin at Ns =(120×60)/12=600 RPM.

Applications:

• Utility Power Generation: Large synchronous generators are the backbone of central power stations (thermal, nuclear, hydroelectric, gas turbine plants), supplying electricity to national and regional grids.
• Standby/Emergency Power: Smaller alternators, often coupled with diesel engines (diesel gensets), provide backup power for critical facilities like hospitals, data centers, and industrial plants during grid outages.
• Marine and Aviation: Used as the primary source of AC power on ships and aircraft.
• Synchronous Condensers: Synchronous machines operated without a prime mover, purely to supply or absorb reactive power to improve power factor in the grid.