In simple terms, a stator can be defined as the stationary part of an alternator. In order to understand what the stator does, it is helpful to know how a basic electrical system works.
The electrical system on most vehicles includes a way to generate, store, and use electricity. An alternator generates power by producing AC (alternating current). Alternating current is often used exclusively to power halogen bulbs, tail lights, and some ignition systems on mopeds, dirt bikes, and ATVs. Any motorcycle that has a starter motor must also have a battery for powering it. Batteries cannot store AC current; therefore current must be converted into a storable form known as DC (direct current). A component known as the regulator/rectifier performs this conversion. The rectifier actually converts the current from AC to DC, while the regulator keeps the power level (voltage) from going above the 13.8 -14.5 volts needed to power a standard 12 volt battery. It is important that the stator provides more power than needed by the motorcycle so that the battery will stay charged. If the motorcycle uses more power than the stator can provide, the battery will start to drain. This will typically happen if the voltage drops below 13 volts.
A traditional alternator uses a coil that uses DC power from the battery to produce a magnetic field. This coil is wound on a spool and is sandwiched between two metal flywheels with fingers of metal that alternate from one side to the other. When this coil is powered, it makes each finger have a magnetic field. These alternate with each finger between north and south pole. As this assembly spins, the alternating poles of magnetism energize the outer windings of the alternator to produce AC power. The benefit to this type of system is that it can produce more power at lower engine speeds which is why most cars and trucks use this system. The down-side to this system is that it requires a charged battery to work. Motorcycles often require a lighter weight system that can produce power with no battery installed or with a small battery. This requires a slightly different way of making power.
The alternator on a motorcycle or ATV converts kinetic energy (power of motion) into electrical energy to charge the battery. An alternator is made up of two parts; the stator and the magnet rotor, also known as the flywheel. The rotor or flywheel contains permanent magnets and spins around the stator to produce energy. Energy use varies from motorcycle to motorcycle and as a result, individual stators may differ. Some stators produce AC power for the ignition system only while others produce AC which is then converted to DC for powering lighting, ignition computers, fuel injection, etc.
Stator designs vary widely but all work on the same principles. When a magnet is moved past a coil of wire, it causes the coil to produce a burst of electricity. A stator has between four to eighteen spokes or poles arranged in a circle similar to the spokes on a wagon wheel. These spokes have an iron core made of a number of thin plates stacked on top of one another. Each spoke has copper magnet wire wound around it. Some stators use a single coil spread across a few poles while others use a few coils with different windings and wire sizes on a single stator. Still others are split into three evenly divided sections called three-phase, used for high powered output to be converted to DC. Three-phase stators are easily recognizable by three yellow or white output wires. The winding, length, and diameter of the copper wire is customized to suit the power requirements of each motorcycle, as well as optimize the ignition system’s power output and starting capability.
Some stators include a pulser or pickup coil. This small coil is mounted separately to the motor or to a mounting plate, allowing it to sense the crank angle via one or more small bumps on the outside of the flywheel. The coil must be placed between 0.02 and 0.50 mm away from the highest part of this bump or “trigger”. The input from this coil is sent to the ignition computer (CDI), where the spark timing is calculated. Some CDIs are powered with AC power from a specific coil, called a source coil, which is wound with fine wire. Other CDIs may be powered by DC current produced by a stator, regulator/rectifier, and battery combination. The current from the CDI is sent to the ignition coil and magnified to over 10,000 volts. It is then sent to the spark plug.