How does a generator convert mechanical energy into electrical energy?

A generator is a device which converts mechanical energy into electrical energy.

It is based on the principle of electromagnetic induction.

The armature coil of the generator rotates by using the mechanical energy. As the armature of the generator rotates, the magnetic flux linked with the coil changes. When the magnetic flux linked with the coil changes, an induced emf is developed.

the magnitude of the induced emf is equal to the rate of change of magnetic flux.