How Does An Automotive Alternator Work?...

INDUCTION (to induce),
Induction is the basic electrical principal used by mechanical style generators to produce useable electrical current.

When a electrical conductor passes through a magnetic field, and electrical current will be generated.
The reverse is also true,
When an electrical current passes through a conductor, a magnetic field is generated.

In the case of an automotive style electrical generator, both sides of the induction principal are used...

Batteries produce True DC that doesn't waver as illustrated in the graphic below.
Batteries also like to have DC returned the same way for charging, so getting Generated DC as close to flat line as possible is manditory.

Very early generators were nothing more than permanent magnets being spun in a coil of wire.
As the Poles (North & South) of the magnet passed by certain parts of the wire, the current changed polarity.
This very rapid current polarity change is called 'Alternating Current' or AC for short.
This 'Generated' AC (Alternating Current) and was a scientific curiosity and parlor trick, but not much more.

Along came a guy that figured a way to put the magnets on the outside of the coils of wire, and spin the coils inside the magnets.
As the coils neared the Magnetic Pole change, the current path was interrupted, and the next coil of wire was connected.
By doing this, the current produced didn't change polarity...
Electrical Current that doesn't change polarity is called, 'Direct Current' or DC. By spinning the coils of wire inside of a stationary magnet, and disconnecting the coils when they were about to change current, the 'Mechanical Generator' produced a useable 'Direct Current' (DC) flow.
The spinning mass was called an 'Armature', and the Generated current was output through a 'Commutator'.
These early mechanical generators soon used coils of wire to produce the magnetic field instead of fragile and expensive permanent magnets.
Output is still dependant on how fast you can safely spin the Armature and Commutator assembly.

The limiting factor in this mechanical generator was the 'Commutator'.
The 'Commutator' is a set of metal strips attached to an electrical insulator, and electrical contact is made by a set of electrically conductive 'Brushes'.
(If you have ever taken a small DC electrical motor apart, you have seen the commutator and brushes.)
The Commutator is the weak link in the in the Direct Current Mechanical Generator.
Spin the rotor too fast, and the Commutator comes flying apart!
About 1,500 RPM was the limit for the generator.
As electrical demands increased, the ability of the commutator to transfer the Generated current through the brushes was reaching it's limits.

Three things came about that made the modern Alternating Generator possible...
1. With Commutators limiting RPM (and output current production), it was obvious the Induction coils had to be moved from the Armature (rotating parts) to a stationary position on the outside of the rotor.
This really wasn't a big deal, Alternating Current (AC) generators had been using Stator windings for years, but vehicles require Direct Current (DC), and there was no practical way of converting the AC to DC.

2. Invention of the Semi-Conductor.
Semiconductors have the ability to switch currents nearly instantly with no moving parts.
With a Semiconductor called a 'Diode', Rectifying (converting) AC to DC is nearly seamless.
A Diode will allow a single polarity to pass through. A Diode allows all positive current to pass, while stopping all negative current, effectively Rectifying the AC current flow to DC.

3. The need for more current from automotive applications far exceeded the ability of mechanical direct current generators, and the trend towards 'power everything' hasn't let up yet.
Automotive electrical systems have gone from 4 volts, to 6 volts, to 12 volts and now 48 volts.
Generators that could barely produce 2 Ampers at 6.5 Volts, can now routinely produce 200 Ampers at 14.5 Volts.
Unregulated, the voltages can reach 250 volts and 250 Amps!

The modern Alternating Generator produces voltage in the form of 3 Phase Alternating Current. Two sets of Diodes Rectify the Alternating Current To Direct Current so your vehicle can use the current produced.

The process begins with a 'STATOR'.
A Stator is usually a metal ring around the outside of the alternator, and is laced with coils of wire.
As the magnetic field rotates inside of the Stator's windings, it will INDUCE an electrical current in the wires.
The windings are spaced so when the magnetic field from the ROTOR comes around, it induces three separate currents 120 degrees apart.
This is the reason the AC is called 'Three Phase'. There are three different sources of current being produced every time the Poles of the magnetic field change as the Rotor spins.
This current is delivered to Rectifier. More on that later.

The ROTOR is a rotating Electro-Magnet.
Instead of spinning natural magnets, a coil of wire is charged with Direct Current, and created a strong magnetic field.
This Electro Magnet is powered by a set of 'Brushes' (electrical contacts) through 'Slip Rings' mounted on the rotor it's self.

By controlling the amount of current going to the rotor, you can control the strength of the magnetic field, and that will limit the amount of current produced in the Stator.

Once you have the current produced in AC, you have to make it useable to recharge your battery, and power all of those DC gadgets in your vehicle.
To do this the modern 'Alternator' uses a 'Diode Rectifier'.
A diode is a 'SemiConductor', and has no moving parts.
Think of a diode as a one way gate valve for electricity.
Depending on the way it's installed in the circuit, it will only let Positive current through.
If the 3 phase AC current is getting through, it would distroy your battery in a matter of minutes.
What a Diode Rectifier does is let only the Positive current pluses through.
This way, you will be able to use the output of the alternator.

Now all your electrical system will 'See' is the 'Peaks' of the Positive Pluses.
This is called the 'Ripple Effect'...

The 'Ripple Effect' can be smoothed out to nearly perfectly smooth DC with the use of a simple electrical device called a 'Capicator'. It too is solid state with no moving parts like the Diode.

The four primary parts of a modern Alternating Generator (Alternator) are...
The 'STATOR', The 'ROTOR', The 'RECTIFIER', and the 'REGULATOR'.

Now that you know the basics of the electrical production, Click Here for the basics of automotive alternating generators.