The alternator keeps the battery fully charged when the engine is running and provides power to the vehicle electrical systems. The alternator provides this electrical power by converting mechanical energy into electrical energy. Michael Faraday performed an experiment in 1831 that demonstrated that when a magnet is passed through a coil of wire an electric current is produced in the circuit of the coil. This is called electro-magnetic induction and it is around this principal that alternators are based, a magnet rota is turned inside the windings of a fixed stator producing alternating current (AC) due to the fact that the magnetic pole changes as the rotor turns. AC current is not suitable for maintaining the battery charge as the polarity rapidly changes from positive to negative so alternators are fitted with a rectifier, that through the use of semi conductors called diodes convert the AC to direct current (DC). The voltage produced by an alternator can be very high and need to be controlled so that output does t not to exceed 14.2 volts. A voltage regulator that is fitted inside the alternator casing that controls the current supplied to the rotor field does this. If the current is reduced then the strength of the magnetic field is reduced and the power generated is reduced.
Alternators are generally very reliable and will provide many years and thousands of miles of service. However on occasion a fault will arise. The most obvious sign that something is wrong is when the battery warning light is illuminated as this light indicates that the battery is not being charged. When this warning light come on it is essential to have this check immediately. A qualified technician must carry out the test procedure as follows: test the battery for serviceable life, check the drive belts condition and tension, check the alternator for security, check cables and connections, check output current with an ammeter, test for a voltage drop and finally test the regulator. Another indication that an alternator has a fault and will need replacing, is excessive audible noise, which indicates internal bearing failure. A common cause of alternator failure is oil contamination, which has resulted from an engine oil leak coating the alternator rotor and stator which effects electro magnetic induction.
The energy used to rotate the alternator comes from the engine itself and a drive belt is used to connect the crankshaft pulley to the alternator pulley. It is important that routine checks are made on the condition of the belt, because when a belt is unserviceable, worn and starts to fray, some of the belt material can get caught up in the crankshaft pulley. This drive belt debris will then typically work its way onto the lower cam belt gear and cause the cam belt to jump a few teeth, which will result in catastrophic engine failure.