What is the primary function of the Automatic Voltage Regulator (AVR) in a synchronous generator?

• A. Control speed via governor to keep frequency constant.
• B. Limit field current to protect the rotor from overheating.
• C. Maintain nominal terminal voltage by sensing voltage and adjusting field excitation accordingly.
• D. Detect and indicate generator faults.

Answer: (C)

Automatic voltage regulation works by sensing the generator’s terminal voltage and adjusting the field excitation to keep the output voltage near its rated value. The internal EMF of a synchronous generator depends on the field current, so the AVR increases or decreases excitation in response to voltage deviations caused by load changes. If the load pulls voltage down, the AVR increases field current to raise the voltage; if the voltage tends to rise, the AVR reduces field current to lower it. This keeps the voltage steady without changing the engine speed, which is governed by the governor to maintain frequency.

The other functions mentioned are separate: the governor controls speed to hold frequency, a protective limiter deals with preventing damage from excessive field current, and protection circuits detect faults rather than continuously regulating voltage.