Introduction:
In a brushless excitation system of modern alternators, the main exciter is an AC generator. This system is widely used in modern alternators due to its advantages over the traditional brush-type excitation system. The AC generator, also known as the main exciter, plays a crucial role in providing the necessary field current to the main generator.

Explanation:
The brushless excitation system eliminates the need for carbon brushes and slip rings, which are commonly used in the traditional brush-type excitation system. The main exciter in a brushless excitation system is an AC generator, and it performs the function of producing the field current required for the main generator.

Advantages of AC Generator as Main Exciter:
- Higher Efficiency: AC generators are more efficient compared to DC generators, as they do not have the energy losses associated with commutation and brush contact.
- Lower Maintenance: AC generators have fewer moving parts compared to DC generators, resulting in lower maintenance requirements and longer service intervals.
- Enhanced Reliability: The absence of brushes and slip rings reduces the chances of wear and tear, ensuring a more reliable operation.
- Improved Voltage Regulation: AC generators provide better voltage regulation, as they have inherent characteristics that help maintain a stable output voltage.
- Simplified Control System: The control system for the brushless excitation system is relatively simple compared to the traditional system, resulting in easier operation and troubleshooting.

Working Principle:
The main exciter, an AC generator, is usually a three-phase synchronous machine. It is driven by the prime mover, such as a steam turbine or a gas turbine, through a mechanical coupling. The rotor of the main exciter is connected in parallel with the rotor of the main generator.

When the prime mover starts rotating, it drives the main exciter, which generates three-phase AC voltage. This AC voltage is rectified by a three-phase rectifier bridge, converting it into DC voltage. The DC voltage is then supplied to the field winding of the main generator through a rotating diode assembly.

The field winding of the main generator creates a magnetic field, which induces the generation of electricity in the stator windings. Thus, the AC power generated by the main generator is used for various applications.

Conclusion:
In the brushless excitation system of modern alternators, the main exciter is an AC generator. This system offers several advantages over the traditional brush-type excitation system, including higher efficiency, lower maintenance, enhanced reliability, improved voltage regulation, and simplified control system. The AC generator, also known as the main exciter, plays a crucial role in providing the necessary field current to the main generator.