Which of the following is not a part of Brushless Excitation scheme used for synchronous machines?

Brushless Excitation System:

The schematic arrangement of Brushless DC Excitation is shown below

• The brushless excitation system consists of an alternator, rectifier, main exciter, pilot exciter (a permanent magnet generator alternator).
• The main and the pilot exciter are driven by the main shaft.
• The main exciter has a stationary field and a rotating armature which connected through the silicon rectifiers to the field of the main alternators.
• The pilot exciter is a shaft driven permanent magnetic generator having rotating permanent magnets attached to the shaft and a 3-phase stationary armature, which feeds the main’s exciter through a 3-phase full-wave phase-controlled thyristor bridge converter.
• This excitation system has the advantage that there is no use of commutator, collector, and brushes.
• It has a short time constant and a response time of fewer than 0.1 seconds.
• It is used in large turbo-generators having a capacity above 500 MW.