Car Parts and Accessories

The generator has two weak permanent magnets inside its round casing. When current flows through the coils of wire around the magnets, they are transformed into electro magnets. An armature— mounted in a front end plate and supported in a rear plate—generally containing about 28 loops of wire, is suspended and rotates between these field coils. The rear end plate of the generator contains two brushes which, under spring loading, make contact with the commutator of the armature.

When the generator is turned by the engine, the armature revolves in the weak magnetic field caused by the permanent magnets, and a weak current is generated in its windings.

This current leaves each winding through the commutator and the carbon brushes in contact with it, one brush receiving negative current and the other positive, so the output is DC.

Most recent generators are shunt wound—that is, one of the brushes is `tapped’ so that some of the current generated in the armature windings is fed into the field windings connected in series around each pole piece, with the result that the permanent magnets are transformed into electro magnets as the generator starts charging. This strengthening of the field value increases the generator’s output.

The amount of current generated depends not only on the speed at which the armature is turning, but on the strength of the field magnets. This will vary, according to the amount of current they receive.

When the generator is charging a low battery, or when the lights and other high-current consuming components are n use, it can take about half a kW to turn he armature at the necessary speed. A Jack fan belt will slip and stop the venerator from producing sufficient current, and the battery will gradually discharge.

Wear on the commutator causes the raised copper segments to become flush with the insulating material separating hem, or the insulating segments become logged with particles of the carbon brush s it wears away. This, combined with worn brushes, reduces the amount of Current which can pass and so reduces output.

It is possible to re-polarise a generator n older cars with a positive-earth electrical system so that modern negative-earth adios can be used.

Auto Control Unit Regulates Output

Because the generator’s output increases with engine speed, it has to be regulated by a control unit. The unit limits the output to about 15.5 volts, so that the battery is not over-charged or electrical components damaged.

The control unit also limits the current, to prevent damage to the generator itself; and, by means of a cut-out switch, it stops the battery discharging itself back through the generator. An alternator limits its own current output.

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