How is the output voltage of an alternator regulated?

• A. By varying mechanical speed
• B. By varying the field current
• C. By changing the number of poles
• D. By adjusting the load

Answer: (B)

Voltage regulation in an aircraft alternator is achieved by controlling the excitation of the rotor windings. The amount of voltage produced in the stator is proportional to the magnetic flux created by the field (rotor) and the speed at which it’s driven. In flight, engine speed is kept fairly steady, so the Automatic Voltage Regulator tunes the field current to keep the output voltage within a required range. If the sensed output is too high, the regulator lowers the field current, weakening the magnetic field and reducing voltage; if the output is too low, it increases the field current to raise the voltage.

Changing mechanical speed would alter frequency as well as voltage, which isn’t how regulation is done in practice. The number of poles is fixed by the machine’s design and isn’t adjusted during operation. Adjusting load might affect voltage temporarily, but it doesn’t regulate to a set value by itself. The field-current control is the correct method for maintaining a stable output voltage.