What does a commutator do in a direct-current generator?

In a direct-current (DC) generator, the commutator acts as a mechanical rotating switch that converts the inherently alternating voltage induced in the rotor coils into a unidirectional output current.1

As the generator's rotor turns, the voltage induced in the coils naturally alternates. The commutator, which consists of a series of bars or segments connected to the armature coils, reverses the connections to the stationary output terminals at the specific moments in each rotation when the rate of change of magnetic flux is zero.1

In practical machines with multiple coils, the commutator ensures that the output voltage is the sum of the voltages induced in the coils, resulting in a unidirectional and relatively constant voltage with only a small ripple. By working in conjunction with fixed brushes, the commutator allows the machine to supply a steady DC output.12