The problems of designing the brushless a. c. generator are mainly concentrated to the design of the main rotating excitation system. This system contains the semiconductor rectifier circuit which is relatively weak in electrical and mechanical loading.
The rectifier elements have many problems those are as less allowance of such as the heat capacity, the amount of instantaneous excess currents, the amount of reverse voltages, and the instant break-down under the surge voltages. Therefore, the careful considerations for the design of the ratings or protection of the rectifier elements are expected, and for further inspections the exact simulation of the excitation system will be required.
In this paper, as the analytical method, the state variable approach is developed, with which the analyses of the general switching circuits can also easily be performed. Moreover, as the first step for the exact inspections for the excitation system, the faults of one of rectifier elements are presented.
There are cases of short circuit or open circuit about the faults of the rectifier elements, where the excess currents and excess voltages occur in the system respectively. The results of the annlyses are the following.
(1) The field votage under the fault shows the wave form same as that of steady state after the instance of the fault on both cases of the short circuit and the open circuit.
(2) The excess voltages at the open circuit fault are not observed at some initial conditions of computation results, of which the tendency should be analyzed with the considerations of the model of the open circuit and with the contributions of the floating capacities.
(3) On the sound rectifier element the excess currents of the short circuit fault should have the harmful influences.
(4) Since the amount of the short circuit fault currents do not nearly decrease and contain the large amount of d. c, component, it should have some intence influences on the armature windings of the a. c. exciter and the control systems combined with the exciter field winding.

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When short circuit failures occur in a marine electric power generating system, the current of main a. c. generator should be maintained at not less than three times of it's rated value for two seconds. To satisfy this condition, the main generator must be excited by the current transformer (CT) at these short circuit faults.
The short circuit current decays with transient time constant of the main generator, but the current heats the feeder line and the voltage induced by the inrush current from CT may give a harmful influence upon the exciting system. Therefore, it is especially successful to make the short current rapidly reduce and to keep a certain short circuit current constant, from the view point of the generator protection and the circuit breaker behavior.
We have used the current limit cotrol apparatus annexed the bypass thyristor type AVR for above pourpose and we have analyzed the sudden short circuit current of the self-excited ac generator. The analytical results obtained are very much in agreement with the experimental results.

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