Let me take a stab at this . . .
AC Motor, DC Motor, Alternator, DC Generator.
They all have common elements:
The rotor (the part that turns), The stator (the part that doesnt turn), and some way of making magnetic fields interact with each other inside.
Then it branches off, motors and generators (an alternator is a generator that produces AC, and just the term generator by itself generally refers to DC)
Motors produce mechanical output from an electrical input. Electrical current flows through the coils of wire, make a magnetic field, which interacts with another magnetic field inside the motor, making the rotor turn.
Generators produce electrical output from a mechanical input.
Physical operation is identical, in principle, which is why a motor can be used as a generator.
It is HOW the magnetic fields interact and how the electrical energy is extracted that makes them different.
DC Generator:
In a PM DC generator, the magnets (usually) are stationary, and the coils of wire are on the rotor (or armature). As a coil begins to pass by one of the magnets, it is switched 'into' the circuit by what is called a commutator. An identical process happens with another coil, with opposite magnetic field (and subsequently the opposite electrical potential), which is collected at the other side of the commutator, and brought out to the other terminal. These coils are connected together, and work additively to create the output that you get at the terminals.
At the point in rotation that the coils are no longer producing usable power, the contacts for those coils on the commutator are also leaving the brushes (the stationary part of the commutator that connects to the terminals).
Think of a commutator as just one big automatic rotating switch. Each coil is physically and electrically arranged inside the generator so that a coil is only connected as it passes by a magnet, and each one that gets connected has current flowing through it only one way. The result is the output power flows in only one direction, dependent on the direction of the rotation of the rotor.
There is little physical difference (for these purposes) between a DC motor and a DC generator. The difference that does exist is primarily in timing of the commutator, something you don't really need to worry about to understand the theory.
AC Alternator:
In a PMA, the coils are stationary instead, and the magnets move past the coils. There is no mechanical switching to connect and disconnect the coils at the appropriate time. To get DC from an alternator, you must use rectifiers (AKA diodes) to get the current flowing one way. Usually, to maximize efficiency, a set of diodes arranged in a special way (called a bridge rectifier) is used. This utilizes both the positive and the negative current flow in the coils, and converts them both to positive.
In the case of an alternator, the 'switching' is done electronically, rather than mechanically, and can be done anywhere (that it is practical) in the circuit. This is one of the reasons (among many) that they are preferred for wind - The components are out in the elements. Mechanical devices complicate maintenance and lead to early failure. A PMA alternator only really has 1 moving part (if you exclude the bearings); the rotor.
Hopefully this helped you understand a little better. If you have questions about an individual component mentioned here, punch it into Google. These are all basic concepts that are well explained all over the internet.
Steve
