The reversibility principle of electrical machines states that an electrical machine can be used to operate either as a motor, converting electrical to mechanical energy, or as a generator doing the reverse, i.e. converting mechanical to electrical energy. The module will therefore also be concerned with electrical generation. In addition to drives and actuators, the module will include a study of transformers.These also rely on electromagnetic principles, but are not intended to produce mechanical motion. They are widely used in electromechanical systems for 'stepping up or down' the magnitude of an AC voltage or current.To understand how motors and generators work, it is first necessary to review the basic electromagnetic principles upon which they depend.