The Principle Of Operation Of The PMG To The Exciter To Then The Stator

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The PMG generates a vollage that is sent to a Voltage regulator.

This voltage regulator is the normally labelled AVR.

The AVR controls the Exciter generator by controlling the amount of current it allows to flow into it's field. The Current comes from the PMG. the PMG acts like a current generator and not like a voltage generator, since its own field is constant (PMG).

The exciter generates the required current to flow into the main field of the alternator. The Exciter is normally a 3 phase mini generator whic output is rectified into a DC voltage. The voltage is varied by the AVR, and this results in a varied current (DC) that will flow in the rotating field of the main alternator.

In my previous setup, I was working on synchronous generator 2.0MW, 2250KVA 11KV, 750 rpm, 50Hz, with brushless excitation system.

An alternator with brushless exciter is a combined unit of two alternators. One is brushless excitation system unit and other is main alternator. Complete Burshless excitation system is directly mounted on the common shaft of the alternator.

The brushless exciter mainly consist of

Three phase rotating Rectifier,

Three phase main exciter,

Three phase pilot exciter (PMG)

metering & supervisory equipments

Three phase main exciter is an AC exciter with rotating armature and stationary field. Armature is connected to 3-phase rotating rectifiers to rectify the AC voltage to DC voltage fed to the armature.

The Pilot exciter is actually a permanent magnet generator (or PMG). PMG has stationary armature and rotating field. When common shaft rotates at the rated speed due to the attached prime mover, the PMG exciter generates electricity which is fed to the automatic voltage regulator (or AVR).

AVR measures and compares the voltage and current generated by the PMG or pilot exciter to predetermined values and then sends the current to the 3-phase main exciter via rotating rectifers as per requirement.

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