Why is the voltage induced in a rotating DC generator coil naturally alternating at first?

In a rotating DC generator, the voltage induced in the armature coils is inherently alternating because it is generated by the changing magnetic flux through the coils as the machine rotates. As the rotor turns, the magnetic flux through a coil increases and then decreases, causing the induced voltage to oscillate with a zero average value.1

This alternating nature is a fundamental result of electromagnetic induction, where the electromotive force is proportional to the rate of change of the magnetic field linked with the circuit. To convert this alternating voltage into the unidirectional output required for a DC generator, the machine utilizes a mechanical rotating switch known as a commutator. The commutator reverses the connections at the specific instants in each rotation when the rate of change of flux is zero, effectively rectifying the output to be unidirectional.12