During operation of a DC rheostat motor starter, the resistance is increased in the field winding and reduced in the armature until the motor reaches full speed. Which option describes this change?

• A. increased / reduced
• B. reduced / increased
• C. increased / increased
• D. reduced / reduced

Answer: (A)

In a DC rheostat starter, speed is controlled by two ideas: how much current the armature can draw and how strong the field flux is. The motor runs faster as the field weakens (less flux) and as the armature current is allowed to increase to develop the needed back-EMF toward the supply voltage.

The described sequence fits this: you increase the resistance in the field winding, which reduces the field current and weakens the magnetic field, allowing the speed to rise. At the same time, you reduce the resistance in the armature, removing the current-limiting effect and letting more armature current flow as speed builds, helping the motor approach full speed. This combination—field resistance up (field weakening) and armature resistance down (less starting current limit)—drives the motor toward its rated speed.

If the armature resistance were increased, or if the field were strengthened (lower field resistance), the motor would slow down rather than accelerate toward full speed.