Which statement accurately describes a main difference between AC and DC motors?

Which statement accurately describes a main difference between AC and DC motors?

• A. A motor that can run on both AC and DC supply.
• B. A DC motor uses brushes and commutators to switch current direction.
• C. DC motors require no maintenance.
• D. AC motors are always more efficient than DC motors for all tasks.

Answer: (B)

Key concept: how the motor’s current is controlled to produce torque. In a DC motor, the rotor current is reversed by a commutator and brushes as the rotor turns, keeping torque in the same direction. This mechanical switching is the defining difference from most AC motors, which rely on the alternating supply to create a rotating magnetic field without brushing the rotor. So the statement that a DC motor uses brushes and a commutator to switch current direction directly describes this essential distinction. The other statements aren’t general truths: while there are motors that can run on both AC and DC (universal motors), that isn’t the standard difference between AC and DC motors; DC motors do require maintenance due to brush wear; and AC motors aren’t universally more efficient than DC motors in all tasks—their efficiency depends on design and application.

Key concept: how the motor’s current is controlled to produce torque. In a DC motor, the rotor current is reversed by a commutator and brushes as the rotor turns, keeping torque in the same direction. This mechanical switching is the defining difference from most AC motors, which rely on the alternating supply to create a rotating magnetic field without brushing the rotor. So the statement that a DC motor uses brushes and a commutator to switch current direction directly describes this essential distinction. The other statements aren’t general truths: while there are motors that can run on both AC and DC (universal motors), that isn’t the standard difference between AC and DC motors; DC motors do require maintenance due to brush wear; and AC motors aren’t universally more efficient than DC motors in all tasks—their efficiency depends on design and application.