Alternator Design

The field coils are not stationary in an alternator. They are wound around a large bobbin and surrounded by two interlocking 'clawfoot' iron shells (extend your fingers and mesh them together to see what I'm talking about). The coil is attached to two copper slip rings which the regulator applies power to through two copper and carbon brushes to control the charging of the alternator. The whole rotating assembly is known as a 'rotor'. When the rotor is energized, the effect produced is that the top shell becomes a magnetic 'north' and the bottom shell becomes a magnetic 'south'. In the alternator, there is a stationary set of windings wrapped around a steel core known as the 'stator'. The rotor spins inside of the stator. As the rotor spins, the alternating between north and south poles (remember the 'clawfoot' design of the rotor!) induces a current in the stator. However, the current induced is alternating current - hence the name 'alternator'. The output current from the stator then passes through a set of six to eight parralel-wired diodes - three or four positive diodes and three of four negative diodes, which polarize (or 'rectify', hence why the assembly which hold the diodes is called a 'rectifier') the AC output into distinct positives and negatives. The negative bridge of the rectifier connects directly to the alternator housing, which is of course grounded to the engine block, while the positive bridge connects to the output stud, which then goes back to the battery.
There are several advantages and disadvantages to alternators. Because of the nature of their design, alternators can produce much higher output than generators - current designs on luxury cars produce upwards of 140 amps. Since the brushes on an alternator only need to carry the current necessary to power the rotor (about 7A maximum) and the slip rings that they ride on are continuous and smooth, they last exponentially longer than generator brushes do - about 120-150,000 miles. There are some disadvantages to alternators, though. The Lundell alternator (the technical name for a clawfoot-rotor alternator) requires two watts of energy(power? my terminology is slipping) to spin it for every one watt that it produces. It is not an incredibly efficient way to produce electricity. Alternators also rely on diodes to rectify their AC output. These diodes only last so many thermal cycles and some of the newer zener ('avalanche') diodes are designed to be self-sacrificial by shunting large output spikes to ground to help save the sophisticated on-board electronics found on today's vehicles. Today's alternators also produce high output from a very small package, which means that there is not a large enough case to act as a heat sink. The high heat helps shorten the life of the diode chips significantly.
There are several new alternator designs out there - General Motors has gone to water-cooled alternators (yes, there are 5/8" hose nipples on the back of the alternator) to keep the diodes cool on the newer model 4.6L Northstar V8s. BMW has designed a combination starter/alternator which is built into the engine's flywheel. The expected switch from 12V to 36V or 42V systems, as well as single-wire multiplexed vehicle systems in the next couple of years also promises to push alternator technology ahead further.