The circuit that supplies the field winding of an aircraft alternator is best described as which type?

• A. AC circuit
• B. DC circuit
• C. Automatic control circuit
• D. Signal circuit

Answer: (B)

The field winding is excited with direct current to create a steady magnetic field in the rotor. Although the alternator’s output is alternating current, the excitation circuit that feeds the field winding uses DC, delivered through brushes and slip rings and controlled by a voltage regulator. This DC excitation allows the regulator to increase or decrease the field strength and thus regulate the generator’s output voltage. Using AC for the field would produce a changing magnetic field and unpredictable output, so the field circuit is classified as a DC circuit. The other choices don’t fit because this path’s purpose is to supply power (excitation) to the field, not to carry AC power to the load, be an automatic control loop, or serve as a signaling pathway.