An alternator is an electrical actuator that converts mechanical energy to electrical energy in the form of alternating current. It provides stable electricity for all the electric devices in vehicle. A starter motor is an electric one, pneumatic one, hydraulic one, an internal-combustion engine in case of very large engines or other device used for rotating an internal-combustion engine so as to initiate the engine's operation under its own power. It provides the engine for enough high speed to start the engine with its battery.