Which circuit is used to energize the alternator's magnetic field in most aircraft?

• A. AC circuit
• B. Battery charging circuit
• C. DC circuit
• D. Fuel system circuit

Answer: (C)

The magnetic field in the alternator is created by the rotor’s field winding, and that field needs a direct current to be established and maintained. In most aircraft, this excitation current comes from a DC circuit connected to the aircraft’s 28-volt DC electrical system and is controlled by the voltage regulator to keep the system voltage within limits. Once the field is energized, the alternator generates AC, which is then rectified for the aircraft’s electrical system. An AC circuit wouldn’t provide the stable excitation needed for magnetization, and the battery charging circuit or a fuel system circuit aren’t responsible for energizing the field. So, a DC circuit is used to energize the alternator’s magnetic field.