If the commutator fails to reverse current direction each half-cycle, what happens to motor torque?

• A. Torque would remain forward constant.
• B. Torque would reverse every half cycle, causing oscillation.
• C. Torque would be zero.
• D. Torque would double.

Answer: (B)

In a brushed DC motor, the commutator reverses the rotor winding current every half turn so the magnetic torque stays in the same rotational direction, keeping the motor moving forward. If that reversal fails, the current keeps its original direction, and as the rotor reaches the next pole alignment the magnetic forces flip sign. The result is torque that would push the rotor one way for a half turn, then push it the opposite way for the next half turn. The rotor can’t make steady forward progress, so it oscillates back and forth, with almost no net rotation. The average torque over a full cycle is near zero, so the motor stalls or vibrates rather than spinning continuously. This is why the correct description is that the torque would reverse every half cycle, causing oscillation.