OK. An alternator has three output coils which are static, and spaced 120 degrees apart around the rotating field coil. The field coil is a rotating electromagnet, powered via an external direct current source, via slip rings and brushes. The voltage and therefore the strength of the field coil can be varied to control the output of the alternator, which is 3 phase alternating current.
If an aircraft with AC alternators needs DC voltage, it uses so-called "transformer-rectifier" units. The transformer changes the AC voltage, which is usually 115 volts from aircraft alternators down to 28 volts using a transformer, (which is simply two coils wound around an iron core). The rectifier part uses diodes, (which are electrical "one way valves") to change the alternating 28 volts into direct 28 volts.
A "DC alternator" is presumably like the one in your modern car, which has the diode pack element of a transformer-rectifier built in to channel the AC output of the three coils into one feed of DC. (in your car, the transformer part is not needed since the AC output is designed to be 12 volts).
(an "invertor" is a device that changes DC into AC, {not to be confused with a transformer-rectifier which does the opposite}, essentially by using direct current to run a high power oscillator to produce 115 volts AC at 400 Hz)
Last edited by Uplinker; 8th May 2014 at 20:01.