The purpose of commutators in a direct current motor is to

Step 1: Analyze the question We are asked about the role of the commutator (split-ring) in a DC motor.

Step 2: Explain the concept A DC motor has a coil in a magnetic field. When current flows, the forces on the two sides of the coil (by Fleming's left-hand rule) are opposite, which produces a turning effect (couple).

If the current direction stayed fixed, then after the coil turned through 90° to the vertical position (the neutral position), the forces would begin to turn it back the other way. The coil would just oscillate and settle, not rotate continuously.

The split-ring commutator solves this. Each half of the ring is connected to one end of the coil, and the carbon brushes touch the halves. Every half rotation (at the vertical position), the brushes switch from one half of the ring to the other. This reverses the current in the coil. The force on each side therefore keeps pushing in the same rotational sense, so the coil keeps spinning in one direction.

Step 3: Check the options

• A. Reverse the direction of the current after every half rotation. This is the exact function of the commutator. ✔
• B. Increase rotations per second. Speed depends on current, field strength, number of turns, and so on, not on the commutator.
• C. Enable a uniform couple to act on the coil. The couple is not uniform. It varies with the coil's angle (maximum when horizontal, zero when vertical). The commutator keeps the couple in the same direction, but does not make it uniform.
• D. Increase efficiency. This is not its purpose.

Step 4: Answer

Tip: Remember "Commutator = Change direction of current every half turn." In a DC motor, the split ring commutates (reverses) the current. In a DC generator, the same split ring does the opposite job of converting the induced alternating current to direct current in the external circuit. Slip rings are used for AC generators.