What is the relationship between generators and motors?

Answer 1

Both generator and motor are energy converters. Subject to losses, both convert one form of energy into another.

All motors and generators use a form of motion on one side of their energy conversion process (typically, but not necessarily, rotary motion).

For example, a petrol engine is a motor that converts combustible fuel into rotary motion, an electric generator converts (typically rotary) motion into electric energy.

Complimentary motor/generator pairs are only possible among certain forms of energy. For example, an electric motor can be connected to a generator, thus converting electric energy into rotary motion, and converting rotary motion back into electric energy.

Other complimentary motor/generator pairs are only possible by indirection. For example, a wind turbine converts wind energy into rotation, which can be converted into electric energy with an (electric) generator. This generator's output can then supply an electric motor, which converts it into rotary motion, which in turn might drive a fan blade, thus converting rotary motion into wind energy.

Not all energy forms support complimentary motor/generator pairs. Combustible fuels such as petrol, for example, can be converted into rotary motion which a combustion engine. However, a generator capable of converting rotary motion into combustible fuel has yet to be invented.