The motorcycle engine Stator is the heart of the electrical system, providing electrical current for your engine for battery charging, lighting, and the ignition system. This section has descriptions of the common types, and troubleshooting information.
There are a few types of stators commonly used on motorcycle and powersports engines. This document will have descriptions of the main types, with troubleshooting information at the bottom. There have been some improvements and changes over the years, but the common basic motorcycle stator has been the same since the 1970's.
Permanent Magnet Flywheel System
Permanent magnet flywheel systems are the most common, and simplest form of generating power for a motorcycle electrical system. The permanent magnet flywheel consists of a cupped metal flywheel that mounts to one end of the crankshaft in the engine. The crankshaft end and the inside of the flywheel share a taper, that allow for a tight, high-friction fit. The flywheel is then secured to the crankshaft with a nut or bolt (depending on internal or external threads). This is usually on the riders left side of the engine (most common Japanese models), and spins consistent with engine RPM. The flywheel has timing notches on it's outer diameter that work with the pulser coil (see pulser coil section for timing and ignition system information). The timing notches on the flywheel are keyed to Top Dead Center location of the pistons by means of a slot in flywheel taper, and a key (called a 'Woodruff key') pressed into the crankshaft end.
The flywheel is constructed with internal magnets along the walls. The magnets are alternating North & South (Positive & Negative) polarity magnets. These magnets are spaced according to the poles (arms) of the stator. As the flywheel rotates on the crankshaft, alternating positive & negative polarity magnets cross poles for the same coil of wire (wound in opposing directions). This magnetic field crossing in close proximity to the stator coils produces an alternating current in the stator windings. The positive side of this system is it's simplicity and reliability. The negatives are excessive heat buildup in the stator windings, as the magnetism is constant, and the stator produces maximum possible current at all times; the only variable being engine RPM.
DC Ignition System
DC ignition systems are common on modern sport & street motorcycles. This type of system means the entire output of the stator is current intended for charging the motorcycle's battery. The stator is wound in either a single or three phase configuration with large diameter wire, to produce relatively high current and low voltage. It is configured to produce as much current as possible across the entire RPM range of the engine. The ignition system is powered off of the battery, which is capable of providing a very powerful spark. This also means it is critical for the motorcycle to have a fully charged battery, and the bike will not run with a discharged battery.
Single Phase Charging Stator
Single phase charging stators consist of a single coil wound among all the poles of the stator. The coil alternates winding direction on each pole. Some stators will have one end of the coil grounded by attaching to the stator core. This simplifies the wiring harness, as only one wire needs to be attached to the bike's wiring, as the other end of the coil shares a chassis ground with DC side of the system. However other stators have the coil 'floated', and have 2 wires, for both sides of the coil, exiting the stator core. Single phase charging stators are simple and reliable, but produce less current and are much more dependent on RPM than the three phase alternatives.
Three Phase Charging Stator
Three phase charging stators consist of three separate coils wound alternating every third pole of the stator. Each coil alternates winding direction on each pole. There are two variations of three phase stators, called Wye, and Delta configurations. These describe the way the three coils are connected to each other. It is out of the scope of this overview to go into too much detail on each configuration, but both types are commonly used in three phase motorcycle stators, depending on the engine design and charging needs. A Delta configuration has one end of each coil connected to another in series, looking like a Delta symbol (triangle) when drawn. A Wye configuration shares one end of each coil connected together in a common spot, and the opposite end of each coil being the output. This looks like a Y when drawn. Delta connected stators generally produce a higher current flow at RPM's, and are ideal for many three phase battery charging stators. Wye connected stators are capable of producing higher voltage at lower RPM's. Three phase stators overall are much more efficient than their single phase counterparts. They produce more current at lower RPM's, and overall much more across the entire RPM range than a comparable single phase stator, making them ideal for most street motorcycles (and many larger dirt motorcycles with batteries).
AC Ignition System
AC ignition systems are common on dirt/off-road motorcycles, where reliability & weight is of the utmost importance. An AC ignition system does not require a battery, rather powering the ignition system directly from the stator. These systems operate with a permanent magnet flywheel like the description above. The stators are generally very small, often with a single a coil providing a single phase alternating current output to power the ignition system. These systems are very reliable, as the bike always be kickstarted or push-started easily, without having to worry about whether a battery is charged to power the ignition system. They are also used on motocross and racing model since a battery is not required, and weight is kept to a minimum by eliminating a heavy battery and regulator-rectifier unit.
AC ignition only systems are the simplest form. They are commonly used in motocross and racing models since they do not require a battery or other components. The stators are physically small, and very light. They can include single, or multiple coils (always single phase output) that power the ignition system directly. The coils are generally wound with many turns of extremely fine (small gauge) wire, to produce very high voltage and low current output.
Ignition with Single Phase Charging
These systems operate the ignition system as described above in the Ignition Only section, but are generally slightly larger stators. They include a single phase charging coil wound on the majority of the stator poles. This coil is used in conjunction with a single-phase regulator-rectifier unit to charge a small battery. They are often seen on enduro type off-road bikes (example: Yamaha WR models) with an electric starter and/or lighting system that are powered from the battery.
Ignition with Three Phase Charging
These systems operate the ignition system as described above in the Ignition Only section, but are generally full size stators. They include at least one single phase coil to power the ignition system. They also have a three phase charging output wound on the majority of the stator poles. This output provides a great deal of current to charge a larger battery. These systems are often used street-legal off-road or dual-sport bikes that require a full lighting system with headlight, taillight, turn signals, horn, etc. for street registration requirements. The ignition is still powered directly off the stator for off-road reliability (a fully charged battery is not required to run the ignition system). Another feature of this system is being able to power the lighting system from the battery while the engine is off, which is generally a requirement for street legal motorcycles. A good example of this system is Suzuki DR-Z400 models.
General Stator Failure Information
Stators fail in multiple ways, but are generally simple to troubleshoot. A stator almost always fails due to heat buildup causing an insulation failure of the wire. Keep in mind that a stator is composed of hundreds of feet of very fine wire, with very thin insulation, wound at relatively high tension around a metal core. The stator lives a high stress life, always surrounded by heat. The stator itself produces heat, as it is a by-product of current generation as the magnetic flywheel spins around it. On most engines (specifically 4-stroke's) the stator lives in a bath of oil from the crankcase. While this oil does provide cooling functions as the stator is immersed in it, the ambient temperature is very high as the oil is heated by engine operation.
The stator generally fails from a hot-spot of the wire's insulation wearing through, resulting in two types of short-circuit failures. If the insulation fails somewhere in the middle of a winding, a short circuit occurs in the coil. The remaining wire in the coil after the short is no longer in the circuit, and the winding's output drops by the percentage of the coil that is bypasses. On a charging coil, this is often not immediately apparent, as the stator may still produce enough current to keep the battery marginally charged for a time, but it will start to be noticeable as the battery is never fully charged. The short will often get worse, or occur at other locations on the winding, continually reducing the output. Taking a resistance reading of the coil (described below) and comparing it to the technical resistance specifications of the stator windings (available on all of our product description pages) will help you troubleshoot this type of failure.
The other common type of short-circuit failure on a stator is a short to the stator core, engine case, or commonly called 'ground'. This type of failure occurs when a wire's insulation melts, or is worn through, and allows the wire strand to touch the stator core. The stator core is grounded to the motorcycle chassis through it's mounting bolts. This type of short generally completely destroys the stator coil's output, resulting in no charging or ignition current output. It will often be noticeable as a dead battery, or no spark produced by the ignition system. Keep in mind that the wires exiting the stator to attach to the bikes wiring harness can also have the insulation nicked, or melted, and cause the same kind of failures outside of the windings on the stator poles.
General Stator Troubleshooting Steps
A Digital MultiMeter (DMM) is the most important tool for troubleshooting a motorcycles electrical system. We recommend a reasonably priced, but high quality Fluke model. Lots of different makes and models are available on Amazon.com, and many other online retailers. To detect either of common stator failure modes described above, a simple resistance check is all that is required.
Coil Resistance Check:
Turn on DMM.
Set DMM to resistance (ohms setting), set to lowest range.
Attach DMM leads to each pair of wires for each stator coil.
Compare resistance measurements to technical data (available in OEM shop manual, or our product description pages).
If resistance matches reading, coil is most likely fine.
If DMM shows 'OL' (overload) or infinite resistance, coil could be broken, wire not internally connected.
If DMM shows 0ohms or continuity (dead short), a short could exist between output wires of stator or in the connector.
Coil Short to Ground Check:
Turn on DMM.
Set DMM to resistance (ohms setting), set to lowest range.
Attach one DMM lead to a good chassis ground point (or negative terminal of battery).
Attach one DMM lead to each output wire from stator, in turn.
On most models and wires, correct measurement should be 'OL' (overload), or infinite resistance.
If resistance is measured, or no resistance (dead short), stator winding is internally shorted at some point to stator core or chassis.
In either case, stator most likely needs to be replaced.
These descriptions and troubleshooting tips should give you a starting place to understand your motorcycle stator, and electrical system. We are always available to help with troubleshooting, and would be happy to help you diagnose your problem before ordering from us. We want to make sure you get the right part you need, not spend money unnecessarily on electrical components that are not bad.