Thread: Pls help to explain the generator excitation of older system

Originally Posted by By Hadi Saadat

The generator excitation system maintains generator voltage and controls the reactive power flow. The generator excitation of older systems may be provided through slip rings and brushes by means of dc generators mounted on the same shaft as the rotor of the synchronous machine. However, modern excitation systems usually use ac generators with rotating rectifiers, and are known as brushless excitation.

I don't understand this: "...by means of dc generators mounted on the same shaft as the rotor of the synchronous machine". Could someone pls elaborate more the differences between older system and new brushless excitation system? Thanks.

Hi

The are 2 circuits: One is the main circuit or Generator, and the second is the exciter circuit. For the main circuit the field is in the rotor and the output is at the armature. But in the case of the second circuit the field is at other armature (lets call it B) and the induced current is obtained at the rotor B which is mounted in the same shaft as field A (which is in the rotor). As the induced current in rotor B is alternating current, rectifying diodes are employed in order to deliver DC to the field A, there is no mechnical contacts, as you may appreciate.

I hope it helps

Sal

hi
as you know a DC current is required in the main rotor to generate voltage in the main stator
this DC current was supplied (before) in the same principle of injecting DC current to a DC generator by using brushes,and that DC current was controlled before enjection by AVR
New generators are using the electromagnetic feild insted of brushes to supply power to the rotor by using a small synchronous generator mounted on the same shaft of the main gnerator called exciter (but in reverse direction) that means the DC power in the stator (instead of the rotor )and teken from the output of the main genrator and rectified and used to obtain magnetic feild which will induce an AC power in the stator (by transferring power by the magnetic feild) ,it is exactly a synchronous generator working in reverse manner
then the AC power in the rotor is rectified by the rotating diodes and used to feed the winding of the generator stator .
note that the excitor is smaller than the manin generator and its function is only to facilitate power transfer from generator stator to its rotor by electromagnetic field.
hopping its clear
thanks