When the engine is running and voltage output is low, the regulator switches the rotor circuit to ground and maximum current flows through the rotor field winding. The high intensity magnetic field created raises the value of the induced voltage in the stator windings and alternator output rises. The output voltage is also impressed on the exciter diode circuit and the output voltage is sensed by the regulator control circuits via the regulator “L” terminal.
When the maximum allowable voltage has been reached, the control circuits switch the rotor field circuit off and the magnetic field at the pole shoes reduces in size, or “decays”. The decaying magnetic field reduces the magnitude of the voltage induced in the stator windings and lowers the alternator output voltage. This again is sensed by the voltage regulator control circuits and the rotor circuit is switched on once more.
The regulator switches rapidly between the ‘ON’ and ‘OFF’ conditions, within the pre-set maximum and minimum voltages, to allow the alternator to maintain an output voltage of approximately 14 volts and at the same time deliver the current needed for electrical system operation.