What happens to a DC motor's speed under light load conditions?

In light load conditions, a DC motor's behavior can indeed be influenced by the characteristics of its field winding and the overall design of the motor. When the field winding of a DC motor is short-circuited, the field flux decreases significantly, which reduces the electromagnetic torque opposing the mechanical side of the motor. As a result, the speed of the motor actually increases. This phenomenon occurs because the speed of a DC motor is inversely proportional to the field flux—less flux allows the motor to spin faster for the same level of armature current.

It's also important to recognize that in typical applications, maintaining a certain level of torque becomes more essential as loads vary. This behavior under light loading conditions illustrates why short-circuiting the field winding could lead to an increase in speed, as the limiting factor of torque diminishes. On the other hand, the other options focus on aspects like constant speed or efficiency, which do not directly capture the specific behavior of a DC motor under the discussed circumstances.