How does a direct-current generator produce voltage that flows in one direction?

A direct-current (DC) generator produces unidirectional voltage through the use of a mechanical rotating switch called a commutator.1

While the voltage induced in the rotor coils is inherently alternating—as the magnetic flux through the coils increases and decreases during rotation—the commutator functions to rectify this output. It achieves this by reversing the connections between the rotor coils and the stationary output terminals at the precise moments when the rate of change of flux in the coil is zero.1

In a simple machine with one rotor coil, this process results in a pulsating unidirectional voltage. In practical machines, multiple coils are symmetrically arranged around the rotor and connected to a multi-bar commutator. By summing the induced voltages from these coils, the generator produces an output voltage that is approximately constant, containing only a small ripple. Stationary carbon brushes bear on the rotating commutator to collect this current and deliver it to the external circuit.12