The synchronous machines
whether alternator or motor are necessarily separately excited machines.
Such machines always require d.c. excitation for their operation. The
field systems are provided with direct current which is supplied by a
d.c. source at 125 to 600 V. In many cases the exciting current is
obtained from a d.c. generator which is mounted on the same shaft of
that of alternator. Thus excitation systems are of prime importance.
Many of the conventional system involves slip rings, brushes and
commutators.
1.1 Brushless Excitation System
With the increase in rating of 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 invariable associated with slip
rings commutators and brushes regarding cooling and maintenance. Thus
modern excitation systems are developed which minimizes thees problems
by avoiding the use of brushes. Such excitation system is called
brushless excitation system which is shown in the Fig. 1.
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| Fig. 1 |
It consists of silicon diode rectifier 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.
excitor 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 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 space excitors, generators-field
circuit breakers and field rheostats.
