Car Parts and Accessories

Producing Current for the Auto Electrical System

All cars have a device for generating electricity. Without this, the electrical demands could drain a fully charged battery in a matter of hours.

The device may be either a dynamo (generator) which produces direct current (DC), or an alternator, which produces alternating current (AC).

An alternator can produce more current than a dynamo of the same size, and so charge the battery when the engine is idling. This gives it an advantage in heavy traffic and permits the use of extra accessories. But a battery- is charged by DC current only, so an alternator needs a rectifier to convert AC into DC current.

Both the dynamo and the alternator generate current through the movement of an armature. The generator windings in a dynamo are inside the armature and move with it; those of the alternator are outside the armature, and are stationary.

Because of the difficulty in passing current from the armature, with its complicated windings, and of cooling the moving armature, a dynamo’s maximum output is limited to about 30 amp. and its maximum speed to about 6 000 rpm.

For engines whose maximum speed is 6 000 rpm, the dynamo is made to turn at the same speed as the engine.

But a generator rotating at below 1 200 rpm will not produce sufficient current to charge the battery; so when an engine is idling, at say 600 rpm, the battery will not be charged by a dynamo.

An alternator presents no major cooling problem because its generating windings are stationary. It can be designed to produce more than 45 amp and, since the armature is less complicated, it can turn at over 12 000 rpm. When an engine is idling at 600 rpm, the alternator, driven by a pulley that is smaller than the engine’s pulley, turns at 1 200 rpm fast enough to charge the battery.

How the Auto Alternator Works

The alternator has its generating windings on the inside of a stationary soft-iron ring—the stator. The armature, more properly called a rotor, is mounted on bearings inside the stator, and is driven by the fan belt.

The armature contains only one winding coil, with each end connected to a separate slip ring. Current is fed to the slip rings through two small stationary carbon brushes; and when current flows through the armature coil it turns into a magnet-one end becoming the north pole, the other the south pole.

Current is generated in the stator winding when a magnet passes each stator coil ; and the more magnets passing each coil in a given time, the higher the current generated. The armature, though a single magnet, behaves like a set of magnets; for its ends are formed into metal fingers, each of which becomes a magnet. These fingers interlock but do not touch.

Unlike the dynamo, an alternator has no commutator to ensure that its output is DC. As a succession of north and south poles passes each stator winding, they generate alternately positive and negative current in the winding.

This AC current is changed into the DC current needed to charge a battery by one- way electronic valves known as diodes, or rectifiers, which are built into the alternator. Some pass only negative and others pass only positive current, so that the final output to the terminals is DC.

Before the advent of the semi-conductor diode, or transistor, rectifiers were very large and difficult to cool. This meant that alternators could be used only on large commercial vehicles.

An alternator limits its own current output. The rectifiers, which will not pass current in reverse, do the job of the cutout. For this reason the alternator needs only voltage regulation and the control box can be fully transistorised and is sometimes fitted into the alternator casing.

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