An automatic voltage regulator will stabilise the voltage output on a generator as the load changes on start up and running. The first units used were electromechanical systems but modern units are solid state devices. The AVR is a feed back control system that measures the output voltage of the generator and compares it with a set point. When this varies it generates an error signal that is used to adjust the excitation of the generator. As the excitation current in the field winding of the generator increases the terminal voltage also increases. This allows for a high level of control in the output voltage.