THE WORKINGS OF AN ALTERNATOR
ALTERNATOR WARNING LIGHT
What does the little red light that says ALT, GEN or sometimes CHARGING mean when it comes on? Without being scientific, 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 the engine up, then you most likely have a problem with the alternator. If the light gets brighter, then the battery is most likely the problem.
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.
STATOR
Surrounding the rotor is another set of coils, three in total, called the stator. The stator is fixed to the shell of the alternator, and does not turn. As the rotor turns within the stator windings, the magnetic field of the rotor sweeps through the stator windings, producing an electrical current in the windings. Because of the rotation of the rotor, an alternating current is produced.
OUTPUT DIODES
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 “output diodes” and in the “diode trio.” Diodes have the property of allowing current to flow in only one direction, while blocking current flow in the other direction. The output diodes 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.
DIODE TRIO
The diode trio consists 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.
FIELD CURRENT SUPPLY
Field current supply is provided from two different sources – from the alternator itself, via the diode trio, and from the battery, via the alternator warning lamp. When you turn the key to “on”, the engine is not running and the alternator is not spinning. At this time, the voltage/current source for the field current is from the battery, through the ignition switch, and through the warning lamp. After the engine is started, and the alternator is up to speed, the output of the diode trio is fed back to the regulator, and serves as a source of current for the field current. At this time, the alternator is self sustaining, and the battery is no longer needed to power the automobiles electrical system.
REGULATOR
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.
WARNING LIGHT
The alternator warning lamp travels a path to ground from the field current supply input to the voltage regulator. As a result, when the key is turned on, current flows through the warning lamp, through the resisters, transistors, and field coil in the alternator, and then to ground, causing the lamp to illuminate. Once the alternator is at full output its voltage will equal 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. As stated earlier, if the light grows dimmer as the engine is revved up, it is because the alternator voltage is rising with the RPM, producing more voltage on the alternator side of the lamp. The closer the output voltage gets to the battery voltage, the dimmer the bulb becomes. By the same way, if the light gets brighter with increasing RPM, it is because as the alternator voltage increases, it is getting higher than the battery voltage. The higher the voltage with respect to the battery voltage, the greater the voltage difference across the lamp, and the brighter it gets.