What are the differences between dynamos, magnetos and altenators?

Answer 1

I imagine you are thinking about motor vehicles. A dynamo has fixed 'field windings' to produce the magnetic field and a rotating armature in which the current is generated. The generated current is kept at the correct polarity by using a commutator, with carbon brushes pressing on it to pick up the current. However, the use of a commutator and its brushes tends to limit the amount of current that can be produced.

An alternator uses the rotating armature to produce the magnetic field in static armature windings. This allows much larger currents to be generated, but they are AC, so must be rectified before going to the battery. This is easy to do with modern solid-state diodes. With an alternator, the control electronics are often part of the alternator unit, whereas with a DC dynamo, they are usually separate. In practical terms, this makes the alternator capable of producing much higher power and is more reliable than a dynamo. Note that both a dynamo and alternator produce DC at the output terminals (in vehicle use).

A magneto is used to produce high-tension (high voltage) current for the ignition system, and is usually used on motorbikes, lawnmowers, etc, that don't have a battery. It has a permanent magnet to produce the magnetic field, rather than a field winding, hence its name.

Answer 2

A dynamo produces Direct Current (DC), and Alternator produces Alternating Current (AC) and a magneto is a type of dynamo usually found in old aircraft or motorcycles which produces low voltage DC for lighting etc. and high voltage DC for spark plugs in an internal combustion engine.