Operation of the Circuit
The current generated from the regulator arrangement produces a magnetic field. When rotating, the rotor causes a magnetic flux to be produced, and this flux is cut by the stator windings. This results in the generation of a voltage and current output in the stator windings.
A small part of the current produced in the stator windings is passed to the shunt field so as to provide excitation for the DC exciter itself. The amount of current flow through the exciter shunt field is controlled by a resistance, which is made up of carbon discs or a carbon pile packed into a ceramic tube.
The resistance of the carbon disc is varied by pressure change. This pressure is controlled by a magnetic field produced by an electromagnet coil. The current for this electromagnetic coil is supplied through the transformer and rectifier circuit from alternator output to the terminal board. This means that as the load changes the alternator voltage also varies. The strength of the electromagnet also increases or decreases due to this load change, resulting in the change in the compression on springs and thus the resistance of the carbon pile.
The resistance of the carbon is least when the pressure on springs is least and on the armature greatest. This occurs only when there is low output voltage which causes solenoid to be weak. Due to this effect, low resistance and more current flows to the shunt and high excitation is produced. This high excitation when fed to rotor produces more voltage.
In the same manner, the pressure will be low when strong solenoid field is present in the arrangement, i.e. when alternator voltage is high. Due to this a small amount of current is conveyed to the shunt, mainly because of high resistance, resulting in less excitation and reduced output voltage.