How does a brushless excitation system supply DC to the main field without brushes?

• A. It uses external DC supply to supply main field with no rotating parts.
• B. It uses a small rotating exciter with rectifiers feeding the main field via slip rings, eliminating the need for brushes on the main generator.
• C. It uses static exciters mounted on the stator with no rotor components.
• D. It uses carbon brushes to feed the main field as a compromise.

Answer: (B)

In a brushless excitation system, the main field current is supplied without using brushes on the main generator. The idea is to power the rotor’s field from a separate, small rotating machine—the exciter—that sits on the same shaft as the main generator. The exciter generates AC, which is converted to DC by a rectifier. That DC is then fed to the main field winding through slip rings arranged in the rotating assembly, so the main generator itself has no carbon brushes for exciting its field. This arrangement eliminates brushgear on the main machine while still delivering a steady DC to the field.

Other approaches would either rely on a separate external DC source without a rotating exciter, use a static (stator-mounted) exciter without involving the rotating excitation path, or simply employ carbon brushes to feed the main field, which defeats the purpose of a brushless system.