What device is used to reverse the direction of current in DC motors?

The device that reverses the direction of current in DC motors is the split ring commutator. In a DC motor, a split ring commutator serves a crucial role in ensuring that the rotational motion can be maintained by changing the direction of the current flowing through the armature windings.

As the armature rotates, the split ring commutator, which is attached to it, periodically switches the connections of the coil to the power supply. This switch happens at the optimal time during the rotation, allowing the current to reverse just as the rotation reaches the point where the magnetic forces would otherwise start to push it back. By reversing the current in the coil, the motor maintains its rotational direction, ensuring smooth and continuous operation.

The other options provided serve different functions: the shield wire pertains to protecting signals from interference, the stator is the stationary part of the motor that provides a magnetic field, and a step-up transformer is used in AC systems to increase voltage, but does not have any direct relevance to the operation of DC motors. Therefore, the split ring commutator is fundamentally important for the functioning of DC motors through its ability to reverse current direction.