"What does that little red light that says ALT mean when it comes on?" Very basically, it means that either the alternator output voltage is lower than the battery voltage, or the battery voltage is lower than the alternator output voltage. If the light gets dimmer as you rev up the engine Contact online >>
"What does that little red light that says ALT mean when it comes on?" Very basically, it means that either the alternator output voltage is lower than the battery voltage, or the battery voltage is lower than the alternator output voltage. If the light gets dimmer as you rev up the engine, then you most likely have a problem with the alternator. If it gets brighter, then the battery is most likely bad.
That's all well and good, but just exactly what does all that mean? To get a good idea, it is first necessary to understand how an alternator works. You don't need an engineering degree, just a basic understanding of the general principles. Figure 1, below, is a block diagram, or a "functional" diagram, of an alternator, and its connections to the remainder of the automobile electrical system. Following the figure is a description of the various components that make up an alternator, and a description of how each operates to keep the battery charged in your car.
We'll start our tour of the alternator where it all starts in the alternator itself - at the alternator rotor. The rotor consists of a coil of wire wrapped around an iron core. Current through the wire coil - called "field" current - produces a magnetic field around the core. The strength of the field current determines the strength of the magnetic field. The field current is D/C, or direct current. In other words, the current flows in one direction only, and is supplied to the wire coil by a set of brushes and slip rings. The magnetic field produced has, as any magnet, a north and a south pole. The rotor is driven by the alternator pulley, rotating as the engine runs, hence the name "rotor."
A/C voltage is of little use in a D/C system, such as used in an automobile, so it has to be converted to D/C before it can be used. This conversion to D/C takes place in the BRIDGE RECTIFIER . Diodes have the property of allowing current to flow in only one direction, while blocking current flow in the other direction. The BRIDGE RECTIFIER consist of six diodes, one pair for each winding. One of the pair is for the negative half cycle, and the other for the positive half cycle. As a result of this diode rectification, the output of the alternator looks as shown in figure 4.
Surprisingly enough, the output of the alternator is not a pure D/C as one might expect, but a pulsating D/C. Because there are three windings, each with a positive and a negative half, by the time the voltage is passed through the diodes, there are six pulsations for each rotation of the rotor. This is close enough to D/C for most automotive components. Critical components, such as radios, have their own internal filtering circuits to further smooth out the waveform to a purer D/C.
The diode trio consists, as the name suggests, of three diodes, one per phase, which provides field current to the alternator regulator. This output will be discussed in more detail later in the "field current supply" section.
The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.
This brings us back full circle to the starting point - the alternator warning lamp. As can be seen from figure 5, a schematic for an actual alternator, there is a path to ground from the field current supply input [1] to the regulator. As a result, when the key is turned on, current flows through the warning lamp, through the resisters, transistors, and field coil, and then to ground, causing the lamp to illuminate. Once the alternator is at full output, voltage from the diode trio, also applied to [1], equals the battery voltage. At this time, with 12 volts on both sides, the lamp is out.
If the alternator should fail, voltage from the diode trio would drop, and once again the lamp would light from the battery voltage. If the alternator output is only a little low, the lamp will be dimly lit. If the alternator fails completely, and the output voltage goes to zero, the lamp will be lit at full brilliance. Conversely, if the battery should fail, and the battery voltage drops, with the output voltage of the alternator on one side and the low battery voltage on the other, the lamp will also light.
In summary, then, we can say that field current through the rotor coils produces a magnetic field, which is coupled over to the stator coils, producing an AC voltage. This AC voltage is converted by the output diodes into pulsating DC voltage, which charges the battery.
The field current is supplied from either the battery, via the warning lamp, or from the diode trio. The amount of field current allowed to pass through the regulator to the rotor, or field coil, is controlled by the voltage feedback from the battery.
The shaft of the alternator is connected to the engine via a belt and pulley. When the engine is running, the shaft of the alternator is forced to rotate, this rotation generates electricity.
The alternator produces a type of electricity known as AC or alternating current, which is why it''s called an alternator. With AC electricity, the current of electrons flows forwards and backwards constantly. This is the same type of electricity you will find in your power outlets within your homes, but the voltage in your homes in much higher.
However, all the electrical components within the car use another type of electricity known as DC or direct current. With this type of electricity, the electrons flow in just one direction, this is the same as the electricity you get from a battery.
So the alternator converts the Alternating current into direct current via a rectifier. The output voltage of the alternator varies with the speed of the car, so the alternator also uses a regulator to limit this and maintain a near constant output.
Every modern vehicle needs electricity to operate, this is used to power things such as the lights, music system, electric windows, window wipers etc.
The engine combusts fuel. This is used to turn the crankshaft and propel the vehicle along. The engine only provides mechanical force, it does not produce electricity. So, we need a way to power all the electrical devices within the vehicle and that''s where the alternator comes in.
In the engine bay we also find a 12V lead acid car battery. This stores energy in the form of chemical energy, it does not store electricity.
When the engine is off, the battery powers the cars electrical components. This will drain the battery though.
Once the engine is running, the alternator is used to recharge the battery so that it has enough stored energy to start the engine again in future. The alternator will also power the cars electrical devices while the engine is running.
If the battery is left to discharge for too long, it will not be able to provide the large current necessary to start the starter motor and the car will need to be jump started.
Let''s have a look at the main parts of the alternator. At the front of the unit we find the pulley. This is a wheel which has grooves cut into it that help grip the belt that provides the rotational force from the engine.
The pulley wheel is attached to the shaft which runs through the entire length of the alternator.
The internal components are held inside the main housing. The housing consists of 2 parts, the front and rear bracket. There are some slots cut into the casing to allow air to pass through and remove the unwanted heat which is generated.
At the back of the unit we have the electrical connections. There are many different designs but this is an example of a simple 3 wire design, with an internal regulator and rectifier which has the following terminals:
B terminal. This is the output which charges the battery.S terminal. This allows the regulator to sense the voltage.F terminal. This is connected to the ignition and provides the initial power to the electromagnet at start up.
To complete the circuit, the electricity flows back through the cars frame to or from the battery''s negative terminal.
As this unit has an internal regulator and rectifier, we find these components at the rear of the unit, usually under a protective cover. We''ll see those in more detail shortly.
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