Energy for the operation of most electrical equipment in an aeroplane depends upon the electrical energy supplied by a generator. A generator is any machine which converts mechanical energy into electrical energy by electromagnetic induction. It is designed to produce alternating-current energy is called an AC generator, or alternator, a generator which produces direct-current energy is called a DC generator. Both types operate by inducing an AC voltage in coils by varying the amount and direction of the magnetic flux cutting through the coil.
For aeroplanes equipped with direct-current electrical systems, the DC generator is the regular source of electrical energy. One or more DC generators, driven by the engine, supply electrical energy for the operation of all units in the electrical system, as well as energy for charging the battery. The number of generators used is determined by the power requirement of a particular aeroplane. In most cases, only one generator is driven by each engine. But in some large aeroplanes, two generators are driven by a single engine. Aircraft equipped with alternating-current systems use electrical energy supplied by AC generators also called alternators.