Which circuit provides the excitation current to the alternator's rotor?

• A. AC circuit
• B. Battery charging circuit
• C. DC circuit
• D. Ignition circuit

Answer: (C)

The rotor in an alternator must have a steady magnetic field to induce a stable AC output in the stator. That field is created by direct current flowing through the rotor windings. So the excitation current is supplied by a DC circuit, typically from the aircraft’s DC power source and controlled by the automatic voltage regulator to adjust the field strength. An AC circuit would continuously reverse the rotor’s magnetization and can’t provide the steady field needed for proper operation. The battery charging circuit is used to recharge the battery from the alternator’s output, not to feed the rotor’s field, and the ignition circuit governs spark plugs, not the alternator.