Refer to figure.
The purpose of the voltage regulator is to control the output voltage of the generator at varying loads and speeds. For instance, if the electrical load of an AC generator providing a constant output voltage increases, the voltage regulator will increase the excitation current of the field coil, thus increasing the output of the generator and keeping the voltage constant for the increased load. The voltage regulator is wired in series with the field coil because it acts on it to reduce the voltage. Therefore, a failure of the regulator would cause the output voltage to increase.