Synchronous generators – brushless excitation concept In the brushless excitation system used on modern alternators, what is the type of the main exciter that feeds the rotating rectifier assembly to supply direct current to the rotor field?

Introduction / Context:
Brushless excitation eliminates slip rings and carbon brushes by generating field current entirely on the rotor side. Understanding which machine serves as the main exciter is fundamental to alternator excitation systems used in power plants and large gensets.

Given Data / Assumptions:

• Main exciter mechanically coupled to the alternator shaft.
• Output must be rectified on the rotor by a rotating diode wheel to feed the field winding with direct current.
• No brushes or slip rings are employed in brushless systems.

Concept / Approach:

The main exciter is typically a rotating armature alternator (AC generator). Its three-phase AC output is immediately rectified by the rotating rectifier (diode bridge) mounted on the same shaft, producing DC for the main machine field. A static or pilot PMG (permanent magnet generator) and AVR regulate exciter field to control alternator voltage.

Step-by-Step Solution:

Verification / Alternative check:

Standard OEM brushless diagrams show PMG → AVR → exciter field → exciter armature (AC) → rotating diodes → main field (DC). This chain confirms AC main exciter.

Why Other Options Are Wrong:

• dc generator: would not need a rotating rectifier; contradicts typical brushless practice.
• either dc or ac generator: brushless scheme specifically uses AC plus rotating diodes.
• induction generator: requires external reactive support and is not used as the main exciter here.

Common Pitfalls:

Confusing the pilot PMG (a small alternator) with the main exciter; both are AC machines but serve different roles.

Final Answer:

ac generator