Alternators produce the current needed to meet vehicle electrical demands while the engine is running. They also replenish the energy that the battery used to start the vehicle. An alternator has the ability to produce more current at lower speeds than the DC generators that were once used by most vehicles.
Alternator  ALTERNATOR  & Starter alternator

An alternator consists of an internal rotor mounted supported by bearings at the front and rear. The rear section of the alternator is called the slip frame. It is where the rear bearing is mounted as well as the brushes and diodes needed. The front half of the alternator is called the drive end. The drive end contains the front bearing which supports the front shaft of the rotor. The center section of the alternator contains the stator. The stator consist of three separate coiled wire sections assembled in the shape of a ring. The rotor used in the alternator has two slip rings at the rear of the rotor that contact the alternator brushes. The rotor contains wire wrapped in coils around a soft metal core, usually cast iron. The wire coils in the rotor are called the field coils. When power is applied through the rotor it becomes a magnet. When the drive belt rotates the magnetized rotor inside the stator, an electrical current is produced. The current that is produced is alternating current due to the rise and collapse of the magnetic field as the rotor is spun. The diodes are used to convert the alternating current into a direct current. Because of the design of the alternator it is able to produce electricity more efficiently than a generator at low RPM. Battery life is extended due to less frequent discharge at low speeds.

Alternators can fail in many different ways. Rotors can develop an internal short or open causing no output since the rotor loses its ability to produce a magnetic field. Stator windings can short or become open, reducing the amount of current produced from the alternator. Diodes can short or become open, causing low current output. Alternator diodes can be damaged by improper testing. Removing the battery cable from the battery while the engine is running can cause catastrophic failure of the alternator diodes. The vehicles battery helps to control the voltage output of the alternator. When the battery cable is removed when the engine is running with a working alternator, the voltage produced by the alternator can reach above eighty volts. The high current flow can cause damage the diodes as well as other electrical components of the vehicle.