Final answer:
d. Armature reaction is the loss of flux in the air gap between the stator and rotor due to increased air gap reluctance, which is different from eddy currents causing energy losses and magnetic damping.
Step-by-step explanation:
The loss of flux in the air gap between the stator and rotor due to the increased reluctance compared to the cores is known as armature reaction. This effect occurs because the air gap presents a higher reluctance path for the magnetic flux, thereby reducing the overall magnetic coupling between the stator and rotor. This is different from eddy currents, which are currents induced within conductors by a changing magnetic field and can lead to energy losses and magnetic damping. Unlike armature reaction, eddy currents are minimized by constructing the cores out of thin, insulated sheets of iron to reduce resistive heating.
Magnetic damping is a form of drag caused by these eddy currents as seen in physics demonstrations where a metal pendulum swinging through a magnetic field experiences drag due to induced currents. Examples include the demonstration of the pendulum bob described in the provided text, where motion is quickly damped when the bob is metal and undamped when the bob is nonconducting, due to the absence of significant eddy currents.