Car Engine Alternator And Its Working Principles
The alternator generates current in the same way as the dynamo. The field winding, however, is incorporated in the armature known as the rotor, and current is generated in stationary windings mounted around the outside cover and called the stator.
Current is fed to the rotor winding through two stationary graphite brushes, which make contact with two smooth slip-rings formed on one end of the rotor.
Each end of the rotor is formed into fingers which mesh together but do not touch. When current is passed through the rotor windings, one end of the rotor and its fingers become the north pole of a magnet, and the other end and its fingers, the South Pole.
As the rotor turns, each stator winding is subject to a change in the magnetic field as each finger moves past it. The succession of magnetic north and south poles passing the stator winding generates alternatively positive and negative current in the stator.
As the stator windings in which current is generated are stationary, the alternator cannot use a commutator and brushes to sort out the polarity.
The positive and negative output from each stator winding, there are normally three of them is alternating current (AC), which varies according to the speed of the rotor.
Because a number of components such as a radio, will be damaged if they are fed with alternating current, and also because alternating current cannot be used to charge the car battery, the polarity must be rectified to produce direct current (DC).
The rectification is achieved by using small electrical ‘one-way valves’ known as diodes. These will allow only positive current to pass through them in one direction and negative current in the other direction.
By connecting a number of diodes into a circuit known as a rectifying ‘bridge’ the alternating current can be rearranged so that positive current is fed to the battery positive terminal and negative current to its negative terminal, changing the alternator output into direct current.
Most car alternators have nine diodes in a rectifier pack. They act as a one-way valve. Six of them correct the polarity and three act as control diodes, supplying current to operate the control box and rotor winding.
The three stator windings in the alternator are each connected to two main diodes and one control diode.
Alternator control box
The control box is usually fitted inside the same body of the alternator. It works in the same way as a dynamo regulator to limit output voltage.
But often, instead of having contact points, it uses a transistor as a solid-state switch. It receives current from the control diodes which also supply the rotor winding, and are at the same voltage as the output terminal.
Warning light
In most cars, there is a charge warning light known as the ignition warning light. This is a bulb connected between the ignition switch and the generator.
In a wiring diagram, it can illustrate that if the engine is stopped and the ignition is switched on, current from the battery passes through the bulb to earth in the generator.
This causes the bulb to light because one side of it is connected to earth and the other to a 12-volt supply. When the engine is started the bulb goes out.
This is because it now has 12 volts from the battery on one side of it and 12 volts from the generator on the other, so nothing passes through it.
At engine idling speed it may glow very dimly, indicating that some current is passing from the battery through the bulb because the generator output is not as high as that of the battery.
