For which of the following motors are brushes NOT employed?

The correct answer is option 2.

Explanation

• Three-phase squirrel cage induction motor: A squirrel cage induction motor is a type of three-phase induction motor that does not use brushes. Instead, it has a squirrel cage rotor, which consists of short-circuited conductive bars embedded in a laminated iron core.
• Three-phase slip ring induction motor: Uses brushes and slip rings to connect external resistance to the rotor for speed control.
• DC series motor: Uses brushes and a commutator to supply current to the rotating armature.
• Three-phase synchronous motor: Usually has a wound rotor with brushes (unless it's a brushless type) to supply DC excitation for synchronous operation.