Thanks to Steve Reynolds for this explanation of Alternators (especially of interest to those of us with UDPs).
Here's an overview of how an alternator works.
The alternator consists of the following parts:
ROTOR = a coil called the field winding that spins in the center of the alternator. It can also be called armature or wrongly called commutator winding.
The rotor consists of:
Slip Rings = 2 Copper rings about 1/4" wide which the brushes press on to. One slip ring is connected to one end of the rotor winding and the other slip ring is attached to the end of the winding. Brushes = Strips of usually carbon about 1/4" square by about 1" long which couple current from the stationary contacts to the moving slip rings/rotor inside.
BEARINGS = 1 bearing is at each end of the rotor shaft (front and rear of the alternator). The rotor winding is wrapped around a laminated iron core which is attached to the shaft which the pulley is connected to. STATOR= 3 stationary windings attached to the outside case which surrounds the Rotor. These coils produce the voltage/current to charge the battery.
DIODES (or rectifiers)= Typically 6 diodes which rectify the AC coming from the stator winding to convert it to DC.
REGULATOR= On our SHOs an electronic module which samples the vehicles voltage to adjust the alternator output to provide the correct voltage.
How it works.
To create electricity we need to move a magnetic field across a wire. The wire breaking the lines of magnetic flux induces a voltage in that wire. The stronger the magnet or the faster we break these magnetic flux lines the more voltage is created.
The Regulator supplies the Rotor (field winding) with a current up to about 5 amps. This creates a magnetic field around the rotor. (The current flows from the battery through the regulator through 1 brush and slip ring through the field winding through the second slip ring/brush and back to the battery usually through the chassis ground).
As the fan belt moves it spins the rotor which causes the lines of flux to be broken by the Stator inducing a AC voltage in the windings. The AC flows through the diodes which only let 1/2 of the AC through causing the AC to become pulsating DC. (This pulsating DC is what causes alternator whine in some radios.) The shape of this waveform is what the mechanic is looking at when he states the diode's waveform is good or bad. The negative side of this output is usually connected to the alternator case. The positive side connects to the battery.
If there was no regulator and the field was connected directly to the battery, the faster the engine spins the higher the voltage output of the alternator would be.
The regulator senses the battery voltage and adjusts the rotor field current up or down to control how much output the alternator has. If the engine runs too slowly the regulator will turn the field current full on but there may not be enough output to keep up with the SHOs electrical needs. When this happens you will see the headlights start to dim. (This is what can happen with under drive pulleys.)
Now if the motor spins the alternator too fast usually there is no electrical problem, the regulator can shut down the rotor current to zero to keep the alternator from putting out to much voltage. The problem is the rotor is spinning so fast that the centrifugal force on the rotor windings and laminated core cannot take the force and they begin to fly apart. Once that happens it usually takes out the stator winding and the whole alternator is ready for the trash.
One over simplified point to note is the higher the alternator output voltage is the higher the output current of the alternator can be.
The basic functional difference between a generator and an alternator is a generator has no output at low RPMs while an alternator has some output. At a given field current the output of an alternator is somewhat linear with the RPM speed of the alternator. Double alternator speed double current output. This holds true until either the magnetics are saturated or the regulator reduces the output to either protect the alternator (diodes/overheating from over current) or to keep from overcharging/overvoltage.
At idle most alternators will not supply enough output to run all the accessories in the car.
For example if the alternator puts out 20 AMPS at idle and you have just the engine running you might only need 10 Amps for the coil/engine electronics (10 Amps is my guess). This would mean you could have 10 Amps to charge the battery with. Now suppose you turn on your headlights and put your heater fan on high you now need an additional 25 Amps (plus the 10 Amps for the engine). Your have a load of 35 Amps but the alternator is still only capable of delivering 20 Amps. Your battery is now supplying 15 Amps to run the car. You are in effect discharging the battery. If you keep your car running like this for hours you will drain your battery. But this is not a real problem because as soon as you drive away the engine speeds up and the output of the alternator climbs. If you double your engine speed your output goes to 40 Amps and now you have the 35 Amps for the load and 5 additional amps that can charge your battery. As the car goes faster more current is available for charging the battery.
When under drive pulleys are installed the alternator speed goes down and so does it's output. Now at idle you need more of your battery's current to run your car. The engine speed has to run faster before you hit the break even point where the alternator supplies all the current. And you need to have the engine speed run even faster to recharge the battery.
There isn't a problem until you have more time spent discharging the battery than charging the battery. In this situation you will kill (discharge) the battery. This still isn't usually a big problem if you monitor what's going on. You are in control of most electrical use in your car. If you slow down the heater fan or only run the rear window defroster until the window is clear you'll be reducing the amount of current you need. That way you will be lowering the break even speed for your alternator.
In the winter I always get a kick out of people that say "I was in a traffic jam in a snow storm and I got a dead battery. I couldn't believe the bad luck I had." Of course they don't tell you they were idling with the headlights on, blower on full speed, rear window defogger running constantly and wipers going full tilt. There's no way the alternator and battery is going to keep up with that kind of load at low engine speeds.
The dimming of the headlight is an indication that your alternator is not keeping up. The charging voltage has dropped off so the lights are dimmer. But this doesn't mean anything is necessarily wrong.
I don't even consider most stereos to be a significant load on any alternator. If you are not playing the stereo at high volumes and do not have the bass cranked up excessively your probably drawing less than 2 Amps. As the bass picks up the so does the current. Even then the high current pulse are in short duration followed by longer delays between beats. This means the average current may still not be excessively high. I think running around with the headlights always on is potentially more of a concern. The true audio guys like to add additional alternators but I feel this is more to keep the voltage from dropping at anytime so maximum audio output is always available.
So the point is enjoy your UDPs and if you think your alternator isn't keeping up shut off a few electrical items in your SHO.
Time to wake up! Class Dismissed
SHO Specific Alternator Info
'89-93 3.0L 5-spd 90 amp unit
'94-95 3.0L 5-spd 130 amp unit
'93-95 3.2L auto 120 amp unit
Note that all the units used in the SHO's are NOT interchangeable with any of the other units used in other 1989-95 Taurus models. Also, all of these units use a six groove serpentine pulley, although they may not be exactly the same.