1. DC Exciter
Earlier, excitation systems comprised of a small DC generator coiled to the same shaft to which rotor was coiled. And thus, the rotating rotor, the power for the electromagnet is produced by the rotor. Also, by the variation in the field current of the exciter, the exciter output can be controlled. Then the magnetic field of the rotor is controlled by the exciter. In result, a constant output voltage is produced by the generator.
2. Static Exciter
Nowadays, the exciters in the generators are static. The main generator itself provides the DC power for the electromagnet. The rotor is worked through the ring slips and for this a number of thyristors are used. These thyristors convert AC into DC. This way the maintenance and operating problem of another rotating machine is eliminated. The static exciters give a better output control as compared to the electromagnetic control. At the time of the starting, generator offers no output therefore, a large battery bank provides the required power for the excitation.
3. Brushless Exciter
In this system, the exciter’s armature is on the shaft of the rotor itself. After the rectification by the solid-state devices, the DC output of the armature goes to the coils of the rotor. In this system, there is no need of the slip rings, as the armature and the rotor are on the same shaft. Therefore, the operational and maintenance requirements are reduced, improving the reliability.