A Quick Look

The alternator is designed to keep the car battery fully charged, it is not designed to charge flat  batteries. They are on the whole very reliable and will spin away for years without failing. They work by producing alternating current (hence alternator). The current is produced through a series of windings, which sit in a steel (normally) housing around the one moving part of an alternator. This winding is called the stator, and the part that spins Ė the rotor.


Alternating current is fine but it has a problem Ė you canít store it. All batteries hold DC voltage and you therefore need to convert the output. The device for this is called the rectifier.

The rectifier is a series of diodes. Diodes are electrical one way valves. Each winding from the starter (on average 3) has its own pair of diodes one only allows positive (+) through and the other negative (-) The schematic below shows how these operate the current.

                                                                   Positive ++++++++ To Battery Positive

+-+-+-+-+-+-+ Rectifier

                                                           Negative - - - - - -To Battery negative

Voltage Regulation

Left alone an alternator has no method of controlling its output, so it will over-charge the  battery. To prevent this a voltage regulator is introduced. All regulators work in the same way this is by cutting off the supply to the rotor.

As  said previously, the rotor is spinning around inside the stator. The rotor is an electro-magnet, the power to energise this, being transferred onto a component known as the slip-ring via a pair of brushes. All the time the magnet is switched on the alternator produces current, if it is interrupted it stops. The voltage regulator is responsible for switching this off and on. Typically a regulator switches the magnetic field off at about 14.5 volts and turns it back on at 13.5 this can happen numerous times each second giving an illusion of a steady voltage of around14v.


Although that is basically how  every alternator works,  there are two distinct variations, one called self excited and the other battery excited. The most common type is the self excited alternator. These have an extra diode added to the rectifier (known as the excitation diode). When you turn the ignition on the car the battery light comes on this is because it is connected to the rotor. With the light on there is a small flow of electricity around the rotor and at the same time a small electro- magnetic field is generated. When the alternator turns it generates a small output some of this is filtered through the excitation diodes which are in turn connected to the rotor, so as the magnetic field increases the alternator generates more power until in a few milliseconds it is charging.

The battery excited alternator on the other hand has a supply direct to the rotor and produces a full magnetic field immediately the ignition is turned on.

Advantages ?

The advantage of the self-excited alternator is that the wiring is a bit simpler and the alternator is unlikely to be damaged should the ignition be left on for an extended period of time. The disadvantage is that if the bulb on the dash blows it wonít charge.

The drive belt is not a musical instrument !!!

The alternator is normally driven by a belt. One of the main causes of bearing failure is over tightening.  Work on the maxim that if you can twang it Ė itís too tight. Itís a drive belt, not a guitar. Some of the latest vehicles are now fitted with elasticated drive belts these are fitted using special equipment and are non adjustable.

A Note Of Caution

Once again technology continues to advance, and a number of vehicles are now manufactured with smart charging systems fitted. If this is the case the alternator is controlled by the vehicles computer and is part of a Control Area network (CAN). If this is the case although the rectification will be the same, the regulation will not. For more information look in "Teccie Talk".


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