The main purpose of a split-ring commutator in a motor is to:

The main purpose of a split-ring commutator in a motor is to switch the current direction. In a motor, the coil of wire (armature) rotates within a magnetic field. As the coil turns, the direction of the induced electromotive force (emf) changes, which would cause the coil to experience a reversal in torque if the current direction were not altered. The split-ring commutator performs this vital function by periodically reversing the connection of the coil to the external circuit, ensuring that the current flowing through the coil matches the required direction to maintain continuous rotation of the motor.

This mechanism allows the motor to produce a steady rotational motion rather than alternating forward and backward movement. The other options are not the primary functions of a commutator; for example, while controlling speed or supplying constant voltage is essential in motor operation, these functions are usually achieved through other components or control methods rather than the split-ring commutator itself.