This post will cover the basics of these electromechanical devices.
What is a magneto?
A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of current.
The idea behind a magneto is simple. It is basically an electrical generator that has been tuned to create a periodic high-voltage pulse rather than continuous current. An electrical generator (or a magneto) is the reverse of an electromagnet. In an electromagnet there is a coil of wire around an iron bar (the armature). When you apply current to the electromagnet's coil, the coil creates a magnetic field in the armature. In a generator, you reverse the process. You move a magnet past the armature to create electric current in the coil.
A magneto consists of five parts:
An armature. In the above magneto, the armature is shaped like a capital "U". The two ends of the U point toward the flywheel.
A primary coil of perhaps 200 turns of thick wire wrapped around one leg of the U
A secondary coil of perhaps 20,000 turns of very thin wire wrapped around the primary coil
A simple electronic control unit that commonly goes by the name "electronic ignition" (or a set of breaker points and a capacitor)
A pair of strong permanent magnets embedded in the engine's flywheel.
When the magnets fly past the U-shaped armature, they induce a magnetic field in the armature. This field induces some small amount of current in the primary and secondary coil. What we need, however, is extremely high voltage. Therefore, as the magnetic field in the armature reaches its maximum, a switch in the electronic control unit opens. This switch breaks the flow of current through the primary coil and causes a voltage spike (of perhaps 200 volts). The secondary coil, having 100 times more turns than the primary coil, amplifies this voltage to approximately 20,000 volts, and this voltage feeds to the spark plug.
Many riding lawn mowers do have a battery if they have accessories like headlights and electric start. Even so, the engine may use a magneto because the magneto is simple and reliable.
How an Induction Coil Works
An induction coil produces a pulse of energy according to Faraday's Law, rather than DC or AC. It basically works the same way as a magneto, however it's initially energized by a battery instead of the rotation of the engine's flywheel. Like a magneto, it produces a pulse of electricity that is used to power the spark plug.
How an Alternator Works
An alternator produces AC electricity by rotating a coil of copper wire wrapped around an iron core through stationary coils of copper wire on the stator. The rotor is attached to a shaft via slip rings that are in contact with carbon brushes held in place by springs. When an electric current is initially applied to these brushes, it induces an electromagnetic field in the copper windings of the rotor. Two iron claws are wrapped around the copper wire that enhance the magnetic field. One end becomes the north pole and one end becomes the south pole.
As the shaft of the alternator is connected to the engine via a belt, the rotor rotates which causes the magnetic field to rotate inside the copper windings of the stator thereby producing a 3-phase AC current. Once the alternator is spinning, it can generate enough electricity to maintain the induced electric field by itself. But in order to do this it must convert the the 3-phase AC power to DC.
Alternators use a rectifier (diode trio) to convert the 3-phase AC power to DC. Since an alternator will produce a higher voltage the faster it spins, it uses an integrated circuit called a regulator to control the output voltage. A regulator controls the output voltage of the alternator by varying the current flowing through the rotor's electromagnet to control its strength. A capacitor then ensures that the DC power is clean and uniform.
AC vs DC Generators
Generators can produce either AC or DC current depending on their construction. AC generators work similarly to an alternator, explained above.
DC generators use a permanent magnet stator with an armature of copper wire as the rotor. The output shaft of the rotor has two split rings mounted on it which rub against brushes connected to the output wires. Since these split rings are not connected, they alternate which brush they are in contact with every half rotation of the rotor. This system of split rings, called a commutator, is how the generator can produce DC. The split rings reverse the direction of current flow at the same time that the current direction induced in the rotor are also reversed, thereby making the current all flow in the same direction. The advantages of DC generators include simple design, simple operation, good reliability, constant voltage capability, and easy speed control.
An AC generator basically works exactly the same way, but without the slip rings connected to the rotor shaft, so the current isn't reversed. The natural rotation of the armature in the magnetic field will induce an alternating current. In short, AC generators do not have a commutator, DC generators do.