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.
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Effect of Change in Excitation
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