What is the source of excitation current for most aircraft alternators?

• A. DC circuit
• B. AC circuit
• C. Pneumatic circuit
• D. Battery management circuit

Answer: (B)

The excitation current for most aircraft alternators is DC. The rotor field must be fed with a steady direct current to establish and maintain the magnetic field that produces electricity. In practice, the DC for the field comes from the aircraft’s battery (through the voltage regulator). In many brushless designs, there is a small exciter that generates AC, which is then rectified to DC to feed the rotor; the DC input ultimately originates from the aircraft’s DC supply. The regulator meters this excitation to keep the system output around the proper voltage. If the excitation were AC, it would not create a stable magnetic field and the alternator would not regulate properly.