• AC excitation systems


    In AC excitation system, the DC generator is replaced by an alternator of sufficient rating, so that it can supply the required field current to the field of the main synchronous generator. That is AC excitation system utilize ac machines as source of excitation power. Usually, the exciter is on the same shaft as the turbine-generator. In this scheme, three phase alternator voltage is rectified (which may be stationary or rotating) by either controlled or uncontrolled rectifiers  and the necessary DC supply is obtained. Generally, two sets of slip rings, one to feed the rotating field of the alternator and the other to supply the rotating field of the synchronous generator, are required.  It is suitable for Medium size hydro alternators. This system eliminates the commutator and brush. Frequency: 50-250 Hz (High frequency).


    AC Excitation with stationary rectifier

    DC output to the main generator field is supplied through slip rings and exciter is driven from main shaft. Voltage regulator derive its power from exciter output voltage. AC regulator maintain main generator terminal voltage while DC regulator is used when ac regulator get faulty or needed to disable. Auxiliary inputs to ac regulator provide additional control & protective functions.
    When uncontrolled rectifiers are used, the regulator controls the field of the ac exciter which in turn control exciter output voltage and exciter field is derived through thyristor rectifiers.
    When controlled rectifiers are used, the regulator directly controls the dc output voltage of the exciter. Voltage regulator controls firing angle of thyristors and exciter uses an independent static voltage regulator to maintain its output voltage.



    AC Excitation with rotating rectifier


    The dc output of rotating rectifiers is directly fed to main generator field so the need for slip rings and brushes is eliminated. Such systems are called brushless excitation systems. They were developed to avoid problems with the use of brushes when supplying the high field currents of large generators. They do not allow direct measurement of generator field current or voltage.  Field of ac pilot exciter, armature of ac exciter, diode rectifiers are rotating. Rectified output of pilot exciter energizes the field of ac exciter and voltage regulator controls the field of ac exciter.