Excitation Systems

Excitation Systems

Giving d.c. supply to the field winding, for the production of the necessary flux is called excitation.

For the small machines, the required d.c. supply is obtained from d.c. generator called exciter. It is mounted on the main shaft of the alternator. The d.c output of the exciter is given to the field winding of the alternator through slip ring and brush assembly as generally the field winding is on the rotor. In some machines, a current is supplied to the exciter by another smaller d.c. generator called pilot exciter. But this arrangement is not very sensitive, quick and effective if it is required to change the excitation of the alternator.

For the medium size machines instead of d.c. generators, a.c. generators are used called a.c. exciters. The output of such a.c. exciters is rectified and then given to the rotor of the main alternator using slip ring and brush assembly. The excitation can be varied and controlled as per requirement in such method.

Still these two methods are not very much suitable for the large alternators hence a brushless excitation system is used for the large alternators.

1. Brushless Excitation System

With the increase in rating of an alternator, the supply of necessary magnetic field becomes difficult as the current values may reach upto 4000 A. If we use conventional excitation systems such as a d.c. generator whose output is supplied to the alternator field through brushes and slip rings then problems are invariably associated with slip rings commutators and brushes regarding cooling and maintenance. Thus modem excitation systems are developed which minimizes these problems by avoiding the use of brushes. Such excitation system is called brushless excitation system which is shown in the Fig. 1.7.1. 

It consists of silicon diode rectifiers which are mounted on the same shaft of alternator and will directly provide necessary excitation to the field. The power required for rectifiers is provided by an a.c. which is having stationary field but rotating armature.

The field of an excitor is supplied through a magnetic amplifier which will control and regulate the output voltage of the alternator since the feedback of output voltage of alternator is taken and given to the magnetic amplifier. The system can be made self contained if the excitation power for the magnetic amplifier is obtained from a small permanent magnet alternator having stationary armature which is driven from the main shaft. The performance and design of the overall system can be optimized by selecting proper frequency and voltage for a.c. excitor. The additional advantage that can be obtained with this system is that it is not necessary to make arrangement for spare excitors, generator-field circuit breakers and field rheostats.

Review Question

1. Explain the brushless excitation system used in an alternator. Methods of Ventilation