dynamo

A dynamo is an electrical generator that creates direct current using a commutator, and uses self-powering electromagnets for the stator field rather than permanent magnets. Dynamos are self-starting by using residual magnetic field left in the iron cores of the electromagnets. If a dynamo were never run before, it was usual to use a separate battery to excite or flash the field of the electromagnets to enable self-starting. Dynamos were the first practical electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter.