ALTERNATOR WARNING LIGHT
"What does that little red light that says ALT mean when it comes on?" Very basicallyit means that either the alternator output voltage is lower than the battery voltageor the battery voltage is lower than the alternator output voltage. If the light gets dimmer as you rev up the enginethen you most likely have a problem with the alternator. If it gets brighterthen the battery is most likely bad.
That's all well and goodbut just exactly what does all that mean? To get a good ideait is first necessary to understand how an alternator works. You don't need an engineering degreejust a basic understanding of the general principles. Figure 1belowis a block diagramor a "functional" diagramof an alternatorand its connections to the remainder of the automobile electrical system. Following the figure is a description of the various components that make up an alternatorand a description of how each operates to keep the battery charged in your car.
ALTERNATOR ROTOR
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/Cor direct current. In other wordsthe current flows in one direction onlyand is supplied to the wire coil by a set of brushes and slip rings. The magnetic field produced hasas any magneta north and a south pole. The rotor is driven by the alternator pulleyrotating as the engine runshence the name "rotor."
STATOR
Surrounding the rotor is another set of coilsthree in numbercalled the stator. The stator is fixed to the shell of the alternatorand does not turn. As the rotor turns within the stator windingsthe magnetic field of the rotor sweeps through the stator windingsproducing an electrical current in the windings. Because of the rotation of the rotoran alternating current is produced. Asfor examplethe north pole of the magnetic field approaches one of the stator windingsthere is little coupling taking placeand a weak current is producedAs the rotation continuesthe magnetic field moves to the center of the windingwhere maximum coupling takes placeand the induced current is at its peak. As the rotation continues to the point that the magnetic field is leaving the stator windingthe induced current is small. By this timethe south pole is approaching the windingproducing a weak current in the opposite direction. As this continuesthe current produced in each winding plotted against the angle of rotation of the rotor has the form shown in figure 2. The three stator windings are spaced inside the alternator 120 degrees apartproducing three separate setsor "phases," of output voltagesspaced 120 degrees apartas shown in figure 3.
OUTPUT DIODES (BRIDGE RECTIFIER)
A/C voltage is of little use in a D/C systemsuch as used in an automobileso 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 directionwhile blocking current flow in the other direction. The BRIDGE RECTIFIER consist of six diodesone pair for each winding. One of the pair is for the negative half cycleand the other for the positive half cycle. As a result of this diode rectificationthe output of the alternator looks as shown in figure 4.
Surprisingly enoughthe output of the alternator is not a pure D/C as one might expectbut a pulsating D/C. Because there are three windingseach with a positive and a negative halfby the time the voltage is passed through the diodesthere are six pulsations for each rotation of the rotor. This is close enough to D/C for most automotive components. Critical componentssuch as radioshave their own internal filtering circuits to further smooth out the waveform to a purer D/C.
DIODE TRIO
The diode trio consistsas the name suggestsof three diodesone per phasewhich provides field current to the alternator regulator. This output will be discussed in more detail later in the "field current supply" section.
VOLTAGE REGULATOR
The regulator has two inputs and one output. The inputs are the field current supply and the control voltage inputand 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 dropsthe regulator senses thisby means of the connection to the batteryand allows more of the field current input to reach the rotorwhich increases the magnetic field strengthwhich ultimately increases the voltage output of the alternator. Converselyif the battery voltage goes upless field current goes through the rotor windingsand the output voltage is reduced.
FIELD CURRENT SUPPLY
Field current supply is provided from two different sources - from the alternator itselfvia the diode trioand from the batteryvia the alternator warning lamp. When you first get in the car and turn the key onthe engine is not running and the alternator is not spinning. At this timethe voltage/current source for the field current is from the batterythrough the ignition switchand through the warning lamp. After the engine is startedand the alternator is up to speedthe output of the diode trio is fed back to the regulatorand serves as a source of current for the field current. At this timethe alternator is self sustainingand the battery is no longer needed to power the automobiles electrical system WARNING!!! This is theoretical only - in actual practicethe voltage surges resulting from disconnecting the battery can seriously damage the regulator circuitry. All alternator manufacturers strongly advise NOT doing this! This test will not prove the functionality of the alternator anywayas the engine may still run with a weak alternator output.
WARNING LAMP
This brings us back full circle to the starting point - the alternator warning lamp. As can be seen from figure 5a schematic for an actual alternatorthere is a path to ground from the field current supply input [1] to the regulator. As a resultwhen the key is turned oncurrent flows through the warning lampthrough the resisterstransistorsand field coiland then to groundcausing the lamp to illuminate. Once the alternator is at full outputvoltage from the diode trioalso applied to [1]equals the battery voltage. At this timewith 12 volts on both sidesthe lamp is out.
If the alternator should failvoltage from the diode trio would dropand once again the lamp would light from the battery voltage. If the alternator output is only a little lowthe lamp will be dimly lit. If the alternator fails completelyand the output voltage goes to zerothe lamp will be lit at full brilliance. Converselyif the battery should failand the battery voltage dropswith the output voltage of the alternator on one side and the low battery voltage on the otherthe lamp will also light.
As stated earlierif the light grows dimmer as the engine is revved upit is because the alternator voltage is rising with the RPMproducing more voltage on the alternator side of the lamp. The closer the output voltage gets to the battery voltagethe dimmer the bulb becomes. By the same wayif the light gets brighter with increasing RPMit is because as the alternator voltage increasesit is getting higher than the battery voltage. The higher the voltage with respect to the battery voltagethe greater the voltage difference across the lampand the brighter it gets.
SUMMATION
In summarythenwe can say that field current through the rotor coils produces a magnetic fieldwhich is coupled over to the stator coilsproducing an AC voltage. This AC voltage is converted by the output diodes into pulsating DC voltagewhich charges the battery.
The field current is supplied from either the batteryvia the warning lampor from the diode trio. The amount of field current allowed to pass through the regulator to the rotoror field coilis controlled by the voltage feedback from the battery.
And there you have it - the complete operation of an alternator in a nutshell. The next time you see the little red lightyou will know exactly what it is trying to tell you.