Which of the following is a motor that can be run on both AC and DC?

• A. Universal motor
• B. Induction motor
• C. Synchronous motor
• D. DC motor with separate windings

Answer: (A)

The universal motor is designed so the field and armature windings are in series, so the same current flows through both. With AC, the current direction alternates, but because the windings are in series and the device uses a brushed commutator, the torque produced stays in the same rotational direction. This lets it run smoothly on alternating current. With DC, the current is unidirectional, and the series windings still produce torque in a consistent direction, so it runs just as a DC motor would.

Induction motors need a rotating magnetic field created by AC; if you feed them DC, there’s no rotating field to pull the rotor along, so they won’t operate properly. Synchronous motors also rely on an AC rotating field to maintain speed in step with the supply frequency, so DC won’t do. A DC motor with separate windings is essentially a DC machine; feeding AC isn’t typical or practical for normal operation because the commutation and windings aren’t arranged for AC torque development.

So the motor that can run on both AC and DC is the universal motor. It’s favored for small appliances requiring high speed and compact size, despite being noisier and wearing brushes faster.