The academic study of electric machines is the universal study of electric motors and electric generators. By the classic definition, electric machine is synonymous with electric motor or electric generator, all of which are electromechanical energy converters: converting electricity to mechanical power or mechanical power to electricity. The movement involved in the mechanical power can be rotating or linear. The apparatus that converts energy in three categories: generators which convert mechanical energy to electrical energy, motors which convert electrical energy to mechanical energy, and transformers which change the voltage level of an alternating current.