Functions of an Electrical Alternator

Look through the vent ports on this alternator case and you can see the wire coils beneath.

An alternator is one of the more common transducers you're likely to encounter, but it's far from the only one. A transducer is a device that converts one type of energy into another, in this case kinetic (movement) energy from the spinning crankshaft into electrical energy. Alternators may look tremendously complex to the untrained eye, but the principles underlying their operation are grade-school simple.

Inductive Power Generation

Magnetism and electricity are two forces that go together like peas and carrots, or electrons and copper. Electricity is energy made of electrons moving from one location to another; kinetic energy on a subatomic scale. Electrons are like tiny, negatively charged magnets themselves. That means that applying a magnetic field to the electrons in any substance will cause them to move either toward the magnet or away from it, depending upon the magnet pole. Passing a magnet through a coil of wire -- or a coil of wire over a magnet -- causes the electrons in the wire to move, which creates electrical flow in a reaction known as "inductive generation."

AC vs DC Power

Think about what would happen if you were to make a coil of wire that measured one mile from end to end, and then you passed a one-foot-long magnet through the middle of that coil from one end to the other. The magnet would "carry" electrons in a single direction, away from it's starting point (the negative side) and toward the end (the positive side). This one-way flow of electricity -- known as direct current or DC -- is simple to use, but a mile's worth of coil wouldn't fit in your car. Now, imagine instead passing that foot-long magnet back and forth through a 2-foot-long coil of wire. Now, instead of pushing the electrons in one direction, you're "shaking" them back and forth in the coil. You're still imparting movement energy on a subatomic scale, but now you have a switching or "alternating" current.

How Alternators Work

While the above-described coil-and-magnet assembly would work as a functional alternator, it's not really practical for automotive use. The back-and-forth magnet motion would have to come from somewhere, and that's a problem because most things in your engine spin around in circles. Manufacturers get around this by installing the coil in loops along the inside of a cylindrical case and spinning electromagnets around inside of them. The but the magnets alternate in orientation; the north pole of one triangular magnet faces the front of the alternator, and the north pole of the adjacent magnet faces the back of the alternator. This alternating north-south-north configuration is what "shakes" the electrons on the coils and produces AC power.

Rectification

The problem with using an alternator in a car is that cars use DC power from the battery; the AC power from your alternator is incompatible with the car's electrical system. Alternators use a series of diodes -- one-way valves for electricity -- to convert AC to DC power. The diodes on the alternator's negative or "ground" output face toward the alternator, and allow electrons to flow into the alternator, but not back out. The diodes on the positive side allow electrons to flow out, but not back in. This approach cuts the alternator's output in half (since the dammed-up electrons turn into heat energy), but will effectively convert a back-and-forth AC signal into a one-way DC signal. That DC power leaves the alternator, recharges the battery and the rest of your car draws its power from the battery.

Fun With Thermodynamics

The alternator is one type of transducer, but it's only one link in a very long chain. The alternator gets its power from the engine, which gets its power from gasoline, which is made of dead dinosaurs. Those dinosaurs ate plants, which got their energy from the sun, which gets its energy from mass and gravity, which both started as one big lump in the center of the universe. When you drive your car, you're using it to facilitate your own existence, to spare the calories in heat energy that you would have spent walking. Sooner or later, you and the car are going to stop permanently, becoming inert mass in the Earth and increasing its gravitational field strength. So, you, your car, its alternator and indeed the Earth itself exist primarily to transfer mass and gravitational energy from the center of the universe to another place further out -- a corollary to the Second Law of Thermodynamics known as "entropy."

References

• "How to Diagnose and Repair Automotive Electrical Systems; Tracy Martin; 2005
• "Foundations of Electrical Engineering"; John Cogdell; 1995
• "Fundamentals of Engineering Thermodynamics"; Michael Moran; 2007