Charging Systems- Alternators

Alternators

An alternator is needed on a vehicle because they are equipped with power consuming accessories such as radios, heaters, horns, lights etc. Because of these components there needs to be a generating system that can produce a considerable output of electrical energy. The construction of a alternator is made up of the following component parts

·         A stationary winding assembly, the stator

·         A rotating electromagnet, the rotor

·         A slip ring and brush assembly

·         A rectifier assembly

·         Two end frames

·         A cooling fan

The stator is composed of a cylindrical, laminated iron core with three sets of windings inserted in slots on the inside. These windings are arranged in such a way that a separate alternating waveform is included in each winding as the rotating magnetic field cuts it. The rotor is an electromagnet consisting of a coil wound on an iron core, which is pressed on a shaft. When current is passed through the winding, magnetic poles are established at the ends of the iron core and shaft. An iron end-piece is fastened on each side of the coil assembly so that projections on the end-pieces interlace. These projections take on the same polarity as the ends of the shaft on which they are mounted, forming pairs of north and south poles around the periphery of the rotor. The rotor rotates within the stator with a very small air gap between the two parts so that as strong a magnetic field as possible cuts the stator windings and the maximum possible current is induced in the windings. The ends of the rotor coil are connected to insulated slip rings mounted on the shaft. A current supplied from the battery passes through the brushes and slip rings to energise the rotor windings and produce the magnetic field.

Although the alternating current could be used for lighting or heating purposes, direct current is needed to charge the battery, this is why the rectifier is required. The operating temperature of the rectifier must be kept within reasonable limits. To prevent overheating, the rectifier assembly is mounted in a piece of heat-conducting metal known as the “heat sink”. A fan is also mounted on the front end of the shaft which provides a flow of air for cooling the assembly. Alternators are constructed with a three-phase stator winding and six diodes are required to provide full-wave rectification of the current produced.

Alternators are designed to prevent production of a current great enough to cause damage to the alternator parts. The charging current will not rise above the maximum safe output of the alternator irrespective of speed or electrical load, so that no external current regulator is needed.