"Fundamental Problems for Permanent Magnet Alternators-
There are fundamental physical difficulties in constructing a generator with permanent magnets, that has to operate over a large rpm range. Hence all manufactures using this principle fight with the following issues:
•Very large currents cause very high temperatures inside the windings of the stator.
•Very high voltages at high rpm cause breakdown and shorts in the insulation materials.
•Temperature cycles inside the stator cause mechanical expansion and compression. On top of this comes vibrations, so over time the insulation materials will crack and deteriorate.
•The subsequent voltage regulator must be able to convert a very large input voltage range to the output 13.5 Volts.
On top of these problems, Honda and many other manufactures unfortunately use a very unelegant rectifier/regulator design called a shunt regulator which causes much higher currents in the stator windings than a more ideal design would. The shunt type deliberately shorts some of the stator windings when the output voltage exceeds the desired level, thereby allowing huge currents to flow in the windings. These currents create a rotating magnetic field which counteracts the rotating magnetic field from the permanent magnet, and thus effectively reduces the induced voltage. These shorting currents do not dissipate much power outside the alternator as the shorting voltage is low (it is a thyristor or a FET which creates the short), but they cause extra heat dissipation and hence extra temperature rise inside the stator windings.
The Shunt Regulator-
The shunt regulator is called 'shunt' because it literally puts a shunt across two stator outputs each time the voltage exceeds a certain limit. The first time I heard about this principle, I refused to believe that anything so stupid had been designed, but I was proven wrong. The shunt regulator can be constructed with lower production cost and has hence been chosen as standard, even if it means much higher current loading on the stator windings.
The shunt regulator uses high shorting currents in the stator windings to create an extra rotating magnetic field counteracting the rotating field from the permanent magnet. The resulting magnetic field is hence reduced and so are the induced voltages. The high shorting currents cause extra heat dissipation in the stator windings and are probably the reason for having high failure rates on this component.
The Serial Regulator-
A rectifier/regulator design of a different type called a series regulator uses disconnection rather than shorting to obtain regulation. It therefore has inherently lower current load on the stator windings with potentially lower stator failure rates.
Symptom: Battery runs flat and bike wont start-
This happened for me with my one-year old Honda CBF1000A. As it was still under warranty, I simply took it to the Honda dealer where the stator was replaced.
According to what I have read on the forums, Honda is often (but not always) replacing stators free of charge (you see the ambiguity of this expression in this context ? ;-) even if the bike is no longer under warranty.
It could also have been the battery which was not working properly, but a charging test will pretty easily reveal if this is the case.
It might also be due to a failing rectifier/regulator, but it is not my impression that this is often the reason. To get a better feeling of this, I would like to hear from you if you have positively experienced a blown rectifier/regulator unit on any of these bikes.
Failure: Stator Shorted to Ground-
A stator with a winding shorted to ground on a CBR600 will not charge the battery, even if putting out 65 VAC from each phase at 6000 rpm. The regulator is simply not able to operate correctly with a non-floating stator.
Failure: Blown Diode or FET in Rectifier/Regulator-
In case one of the diodes (or one of the FETs if such are used instead of diodes) in the rectifier/regulator has been blown, only two of the three windings will have to deliver all the required power, with consequent overheating of the windings and a fried stator as result.
By constructing a break-out connector with 3 small resistors e.g. 0.1 Ohm each in series with the stator outputs, it is possible with a simple AC voltmeter to verify proper symmetrical current load on all three windings."