Explain the operation principle of a DC motor and the role of the commutator.

• A. It converts electrical energy to mechanical energy.
• B. Lorentz force on current-carrying conductors in a magnetic field; commutator reverses current direction each half-cycle to keep torque unidirectional.
• C. It provides cooling to windings.
• D. It controls speed by varying the supply voltage.

Answer: (B)

A DC motor converts electrical energy into mechanical energy by the forces produced on current-carrying windings in a magnetic field. When current flows through a winding in the rotor, the magnetic field from the stator interacting with the current exerts a Lorentz force that creates torque on the rotor, causing it to turn. As the rotor reaches the point where the direction of torque would try to flip, the commutator switches the connection of the winding to the supply every half turn. This reversal is achieved through the brush-commutator arrangement, so the current in the winding changes direction just as the rotor passes through the midpoint of its rotation. Because the torque remains in the same rotational direction, the rotor keeps spinning rather than stalling or reversing. In short, energy moves from electricity to motion, and the commutator is the mechanism that keeps the rotation smooth by reversing current at the right moments to maintain unidirectional torque.