Electrical system

If you are really interested you can read about fine details at other places like in automotive engineering or physics books. This is just a quick general overview of the system.

The electrical system contains two major electrical power sources. There is a battery that supplies energy to start the car and run automotive electronics while the alternator is putting out too little power or no power, and an alternator that charges the battery and supplies energy to run electrical devices while the alternator pulley is spinning fast enough. The minimum charged battery voltage in use today is around 12.6 volts. That voltage may come down a little under heavy loads because of resistance in the battery and in the battery wiring, but 12.6 volts is a reasonable estimate for modest loads when a battery is supplying all energy.

To charge the battery even at a very slow rate, the alternator has to produce 13.8 volts. The ideal voltage to charge a battery is over 14 volts, but less than 14.7 volts. At 14.7 volts the battery will start to "boil" or gas excessively. This releases acid and harmful vapors. You can read how to test your charging system at this link.

Ever hear this claim? "Slowing the alternator speed by changing pulleys frees up horsepower." This idea is totally wrong! Slowing the alternator too much can actually cost horsepower.

An alternator has a field winding in the rotating part. The field winding produces a magnetic field. The strength of this magnetic field depends on the electrical current flowing through the field winding and the number of turns in that field winding. Current gets to the spinning field winding through very hard carbon brushes. The field current comes through a voltage regulator that controls the current in the winding based on the alternator output voltage.

As the rotor spins magnetic force lines cut across a stationary winding called the stator winding. The stator consists of several windings, but they are usually grouped so they act like three windings. Each stator winding has a coil of heavy wire. These are very heavy high current windings.

As the magnetic flux in the stator varies with the passing flux from the rotating armature, each stator winding produces voltage. That voltage is alternating current, very much like the electricity in your house. There are several rectifier diodes that act like a "gate". They convert the alternating polarity to a single polarity that pulses. Since there are at least three windings spaced strategically around the stator, there are several pulses that are very slightly time-delayed from each other. When they are added together the output is an almost steady direct current, very similar to a battery. (If you have ever heard a faint whining tone in the middle pitch range of musical tones in a car radio or stereo, this is the very small ripple or imperfection caused by summing all the alternator's positive voltage pulses together with the diodes.)

The stationary windings, when the alternator drives a load, has current. This net current is the same as the load current. If you draw 100 amperes from the alternator, the averaged sum of currents in all the stator windings must be 100 amperes! This current just like the current in the rotor generates a magnetic field. As a matter of fact anytime we have current flowing a magnetic field is created.

This magnetic field "bucks" or pushes back against the magnetic field of the rotating armature. This "bucking" or dragging from the rotor and stator's mixing fields is how energy gets transferred from the pulley to the load on the alternator. The mechanical load on the rotating shaft is directly proportional to the load power on the alternator. The more electrical stuff you run from the alternator, the more you load the alternator shaft. Since the energy conversion is not 100% efficient a portion of the horsepower supplied at the pulley is wasted as heat.

The voltage regulator looks at the alternator output voltage and adjusts the field current supplied to the rotating field windings on the rotor. The regulator attempts to hold the alternator output voltage at a predetermined value that is suitable for charging the battery, generally around 14.5 volts. The regulator might supply 3 or more amperes under heavy electrical loads or when the alternator is not spinning fast enough to keep up with the load. More current from the regulator increases the magnetic flux in the rotor, and that increases the drag on the alternator shaft. This is how the alternator draws the right amount of horsepower to make the correct amount of electricity!

Think a little bit about how this works. Nothing is free. If we load the alternator with a great big electric fan and/or an electric water pump the alternator has to draw MORE mechanical energy than it would take to drive the fan blades or pump directly. We will always lose significant energy through conversions from mechanical to electrical and back to mechanical. The alternator, at best, is about 60% or so efficient. The electric motors are, at best 85% efficient and are more likely around 70% efficient. This means the overall efficiency to convert crankshaft load to electrical energy and back to mechanical energy is 40-50%. If a water pump and fan took 1 horsepower to drive directly, it would take about 2 horsepower to drive it fully from an alternator.

This is a good logical question; and it has a logical answer. If we watch the fan operate we see it often does not run at all. Unlike a mechanical fan, the manufacturer can turn the electric fan completely off when not needed. Over a long period of time, even with greatly reduced efficiency, the much shorter operating time consumes less fuel. This is a major advantage.

A second much less important factor is the electric fan can be operated at the optimum speed for the blade and motor design. The mechanical fan is not always operating at a speed where it optimized, the electric fan can be operated at peak efficiency all the time, and this offsets a little bit of the efficiency deficit.

A third factor is ease of design. Can you imagine mounting a belt driving a mechanical fan with a sideways-mounted engine? Even with a rear-wheel drive, it is physically easier and cleaner to use an electric fan.

Finally, even if we doubled the power loss, a fan only requires about 2 horsepower maximum.

For the racecar driver concerned with every last horsepower, we probably don't want to use an electric fan driven from an alternator! Fortunately, the power used to turn a clutch or flex fan is so low that doubling the power loss makes very little difference. The cleanliness of the installation might mean more than the one horsepower loss we can expect from driving an electric fan from an alternator. If the fans run from the battery, and if we do not recharge the battery from the engine, an electric fan makes excellent sense because we can get that energy from an external charger. We can cool the car in the pits while the engine is off. That's an advantage!

Now let's do a sniff test for advertising "bullshit". I've seen claims where an electric fan can free up 10-20 horsepower. Let's see if that makes sense. One horsepower equals 746 watts. 746 watts is 746/13 = 57 amperes of current at 13 volts. If our electric fan is 80% efficient we would use 71 amperes of current draw to run a 1 HP electric fan. Normally I don't use words like "bullshit" in a technical discussion, but there is no other accurate way to describe what advertising people are feeding us. If the fan needed 10 horsepower's worth of mechanical power to run, we would need 12.5 horsepower's worth of electrical energy with an 80% efficient motor and a 100% efficient alternator. That would be 746*12.5 = 9,325 watts or 720 amperes at 13 volts from any alternator we use.

Do you have a 720 ampere fan? Do you really think the fan blade requires 10 horsepower? If you do, you must be as technically incompetent as advertising people. 10 HP average power is enough to run many homes.

The fan spinning around on the front of the water pump actually draws about 2 horsepower if we spin it at very high RPM. It draws much less if it is a clutch or flex fan, probably in the order of 1/2 horsepower or so. If we got rid of a good mechanical clutch or flex fan and ran an electric fan totally off a battery charged in the pits, we would gain about 1 horsepower. If we ran it off the alternator we would LOSE about 1/2 horsepower more, or about 1 horsepower total.

There are two good reasons to use an electric fan. They are:

Mechanical. A cleaner engine compartment with more room, or easier fan mounting
Operational. Cooling the radiator down while the engine is off

Freeing up Power from the Alternator

Here's another good myth. If we slow the alternator down we free up power. That's just as silly as the 15-horsepower electric fan myth!

When we slow an alternator down the output voltage drops. When the voltage drops, the regulator turns the field current up. This loads the pulley more and drags on the belt and crank more. By the time the closed loop of regulator and alternator stabilizes, we are often loading the pulley with very slightly more horsepower load than if the alternator was spinning faster! While this may seem contradictory, it occurs because the alternator system is more efficient operating at higher pulley speeds.

In truth though these changes are all so small that we might not do noticeably worse, but the laws of physics dictate we can't do any better! For a given load in watts the alternator has to consume a certain amount of horsepower. Because the alternator is around 60% efficient, it consumes about 1 horsepower for every 450 watts of load. In a 13.8-volt charging system this means our alternators draw one horsepower for every 33 amperes of load current. If we've slowed the alternator down to make more power, we really have wasted money. Worse yet we probably created idle charging problems, especially if we added an electric fan.

If we really are hell-bent on reducing alternator drag, we have to turn alternator field current off. That's the only way to do it. We can't possibly run lights, charge the battery, and/ or run fans off the alternator without loading the engine down. If we draw 20 amps at 13.8 volts, we have to supply at least the same amount of power to the alternator if we slow it 50% or speed it 100%. It is the load power it delivers that determines higher RPM horsepower the alternator uses, not the RPM of the pulley.

The alternator is a closed loop system. If the output terminal voltage is below a certain voltage, the alternator