The voltage regulator maintains the output voltage of the generator within the specified limits. It has developed from a variety of ingenious electro-mechanical devices to the versatile electronic devices of today. The accurate and skilled assembly that the electro-mechanical regulators demanded is not compatible with modern manufacturing techniques, and friction and the inertia of moving parts set a limit to the accuracy and speed of response.

All regulators are now electronic, the final control being by transistors instead of variable resistors, which has led to improved accuracy and speed of response. Typical performance figures appear in ISO 8528 and are included in the section titled Starting Mechanisms.

It is important to note that the voltage regulator on its own does not determine the recovery time of the generator output voltage, there are other and longer time constants in the system. The regulator can respond very quickly to a voltage dip by increasing the voltage across the exciter field circuit, but the exciter field circuit is inductive and introduces a time constant. The exciter in turn increases the voltage across the main field which is highly inductive and introduces an additional and longer time constant. The overall response time is improved by field forcing, or applying say twice the normal voltage to the exciter field thereby increasing the rate of rise of current in the main field.

For voltage rises the regulator can do no more than reduce the exciter field voltage to zero, leaving the main field current...