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The Alternator is an energy converter which generates an alternating current by converting mechanical energy to electrical energy. In automobiles alternator is used to charge the battery and to power the electrical system when its engine is running.

The alternator consists of a stator which is a stationary set of wire coil windings, in which a rotor revolves which is an electromagnet supplied with a small amount of electricity through carbon or copper-carbon brushes (contacts) touching two revolving metal slip rings on its shaft. The rotation of the electromagnet inside the stator coils generates much more electricity inside these coils. The electricity is alternating current and the direction of its flow changes back and forth every time the rotor turns. It has to be rectified (direct current). A dynamo gives direct current but it is less efficient, and has particularly at low engine speeds, and weighs more than an alternator.  A warning light on the dashboard glows when the battery is not being adequately charged. There may also be an ammeter which shows how much electricity is being generated, or a battery-condition indicator is also used to show the battery's state of charge.

Howbeit, typical passenger vehicle and light truck alternators use Lundell or 'claw-pole' field construction. Modern vehicles now use the compact alternator layout, whereas larger vehicles may have salient pole alternators similar to larger machines.