Normally, in a DC generator the stator (outside) produces the magnetic field and the output is collected from lots of coils in the rotor as it rotates.
That means that brushes are needed to collect the current and that the current in each rotor coil reverses direction twice each rotation because the magnetic field is fixed and at any point the rotor coil is one way on to the field and 180 deg later its the other way round. The brushes run on a series of copper segments arranged in a circle (the commutator) with the opposite ends of each coil connected to segments opposite each other so that as the coil current reverses so do the electrical connections and DC out is the result. That reversal is called commutation The brushes thus handle the full output current of the machine and sparks occur at each commutator segment leading to wear. Brush sparking is also a problem at high altitude, but I can't remember why.
In an AC machine the magnetic field is in the rotating part and the output current is collected from the stationary coils. The current into the rotating bit to make the magnetic field is quite small compared to the output and there is no need to reverse its direction. This makes life for the brushes much easier.
The AC generating machine can also be more mechanically robust so it can go round faster than the DC machine.
Last edited by Bengo; 28th Apr 2013 at 12:24.