Figure below shows one old and classic generator exciting system arrangement consisting of a main exciter with manual or automatic control of the field can be classified in a general way as “slow response” systems. The “regulator” in this case detects the voltage level and includes a mechanical device to change the control rheostat resistance. One such directacting rheostatic device (the “Silverstat” regulator) consists of a regulating coil that operates a plunger, which in turn acts on a row of spaced silver buttons to systematically short out sections of the rheostat.
In application, the device is installed as shown in Figure below. In operation, an increase in generator output voltage will cause an increase in dc voltage from the rectifier. This will cause an increase in current through the regulator coil that mechanically operates a solenoid to insert exciter field resistance elements. This reduces excitation field flux and voltage, thereby lowering the field current in the generator field, hence lowering the generator voltage. Two additional features of the system in Figure are the damping transformer and current compensator. The damping transformer is an electrical “dashpot” or antihunting device to damp out excessive action of the moving plunger. The current compensator feature is used to control the division of reactive power among parallel generators operating under this type of control. The current transformer and compensator resistance introduce a voltage drop in the potential circuit proportional to the line current. The phase relationship is such that for lagging current (positive generated reactive power) the voltage drop across the compensating resistance adds to the voltage from the potential transformer. This causes the regulator to lower the excitation voltage for an increase in lagging current (increase in reactive power output) and provides a drooping characteristic to assure that the load reactive power is equally divided among the parallel machines.