What is a universal motor?

• A. A motor that can run on alternating current only.
• B. A motor that can run on direct current only.
• C. A motor that cannot start under load.
• D. A motor that can run on both AC and DC supply.

Answer: (D)

A universal motor is a motor designed to run on either alternating current or direct current because its field and armature windings are connected in series. The same current flows through both windings, so they create magnetic fields that reinforce each other. With direct current, that’s straightforward—the fields build and rotate. With alternating current, the current and the fields continuously reverse, but since both windings reverse together, the torque direction remains the same and the motor keeps rotating. This is why a single motor can operate from either AC or DC supplies.

Universal motors are typically small, high-speed machines with good starting torque, found in many handheld appliances like vacuum cleaners and power tools. They use brushes and a commutator, which is part of how they maintain torque under both types of current. They’re not as energy-efficient as some other motor types and can be noisy, but their ability to run on either current and deliver strong starting torque is what makes them useful in those applications.