Which statement best describes the rotor design attribute of AC induction motors?

• A. The rotor uses carbon brushes
• B. The rotor includes a commutator
• C. The rotor is described as not using brushes or armature
• D. The rotor must be powered by direct current

Answer: (C)

AC induction motors rely on a rotating magnetic field produced by AC in the stator, and the rotor remains a passive conductor with currents induced in it rather than being fed directly from the supply. Because there is no external electrical connection to the rotor, there are no brushes or a commutator for typical designs. The common rotor type is the squirrel-cage rotor, which consists of conducting bars shorted at the ends, allowing induction to generate torque without a direct armature circuit. In other words, the rotor does not require brushes or an armature to be powered.

Some wound-rotor designs do use slip rings and brushes to feed the rotor winding, but those are less common in basic AC induction motor concepts. The rotor in a standard induction motor is not powered by direct current; the rotor currents are induced by the stator’s AC field.