Motor Vehicle Electricity

Motor Vehicle electricity

The Alternator. The alternator is an AC (alternating current) generator. It is driven by the crankshaft via a rubber vee or poly-vee belt. It produces electricity to charge the battery and to supply all the electrical loads. The alternator will have an output of 14-15V. This is slightly higher than battery voltage so that the alternator can drive current through the battery in order to charge it. An average alternator would have a current output of 60A but this can be higher depending on the electrical requirements eg; heated seats, electrically adjustable seats,etc.

The main components in the alternator are; the rotor, the stator, the rectifier and the voltage regulator. The rotor is driven by the drive belt. As it rotates within the stator its magnetic field sweeps across the stator windings inducing a voltage within them. This output must be rectified (converted from AC to DC) and its voltage regulated.

The rotor is an electromagnet. Its poles are folded back and interlocked so as to provide alternating north and south poles as it rotates. Current for the coil (which produces the electromagnet) is provided by slip-rings and brushes. These are used in order to supply current to a shaft as it rotates. The coil uses up to 4A. The rotor is supported by bearings at both ends of the shaft. A pulley bolted to the front of the shaft provides the drive. A fan is usually attached to the rotor to draw air across the diodes in the rectifier to cool them. The Stator is a collection of three windings wound around but not in contact with the rotor. The windings are formed around a laminated soft iron yoke. They are usually wired in star formation where one end of each winding is joined and the other three ends are connected separately to the rectifier. It is in the stator that the electricity is induced. The size of the stator windings determines the maximum current output of the alternator.

The Rectifier converts the AC output from the stator to DC. It is made up of six diodes which redirect the flow of the current as it leaves the stator so that rather than constantly changing direction (AC) it all flows in one direction (DC). This is necessary in order to charge the battery and for the ignition system.

The Voltage Regulator determines the voltage output of the alternator. It does this by controlling the strength of the rotor magnet by controlling the flow of current to the rotor coil. If the output voltages above 15V bulbs may blow and the battery will be overcharged (the electrolyte will be boiled off). Controlling the output this way allows the rotor to keep spinning without producing an output. It also means that the alternator output is not determined by engine speed which constantly changes.