Two sources of power, one AC and one DC, are required for running a(n):

• A. Induction Motor.
• B. Squirrel-Cage Motor.
• C. Synchronous Motor.
• D. Wound-Rotor Motor.

Answer: (C)

Two sources of power are used to run this type of motor: AC power for the stator to create a rotating magnetic field, and DC power for the rotor to establish a steady magnetic field. The DC excitation on the rotor allows the rotor’s magnetic field to be fixed in strength while the stator’s rotating field drives the rotor to turn at the same speed as the rotating field. That locking in step is what makes it a synchronous machine. Other common motors run on AC in the stator and rely on induced currents in the rotor, so they don’t require a DC rotor excitation. Hence, the motor described is a synchronous motor.