The principle of electric generator is Electromagnetic Induction. The schematic is shown below:

An electric generator is a device that converts mechanical energy obtained from an external source into electrical energy as the output. The flow of electric charges is induced by moving an electrical conductor, such as a wire that contains electric charges, in a magnetic field. This movement creates a voltage difference between the two ends of the wire or electrical conductor, which in turn causes the electric charges to flow, thus generating electric current.

The function of brushes is to make proper contact with the rotating rings because through them the current is supplied to the coil