Alternator Tutorial - Motorcycle DIY Article - STILL UNDER PREPARATION !

This DIY article is a tutorial about typical motorcycle alternators like those on Honda CBR1000RR Fireblades and Honda CBF1000, how they work and how they fail.

On the various forums, there is a lot of fuzz going on about alternator troubles on these motorcycles. I have seen many posts with incorrect explanations (some of them even made by myself ;-). I therefore hope this article will help people to get a better understanding of this subject.

The fundamental principles of the 3-phased alternator itself are pretty simple and not difficult to understand. They are explained in detail in the following section.

The way to control the alternator - that is the regulators job - is however quite complicated to implement and also somewhat difficult to explain in simple terms. Two fundamentally different regulation principles are used:

My DIY articles are usually based on first hand experience, but this article is an exception. It is based on my general understanding of physics and electronics and on reports from others. It further includes some speculative sections written in blue font to emphasize that I have not - yet - got 100% confirmation of correctness.

The 3-Phased Alternator - Principle of Operation

The following description is brief, so pay attention ;-)

  1. A large cup-shaped permanent magnet is mounted on the cranck shaft and creates a turning magnetic field when rotating.
  2. The stator - comprising a number of iron cores wound with laquer isolated cobber wire - is mounted in a static position inside the rotating permanent magnet.
  3. The many cores and their windings are combined into three sets of windings each set called a 'phase'.
    The three phases are typically coupled together in a delta coupling, resulting in 3 wires fed out from the stator.
    Notice there is no ground wire and no 0 Volt wire. The stator windings are hence 'floating' relative to ground.
  4. The magnetic field rotating around the stator induces an alternating voltage in each phase with a value proportional to the engine rotation speed.
    At 6000 rpm the alternator will typically be designed to put out arround 65 Volt AC (measured accross each phase).
    At 1000 rpm Vg then equals 65 Volt * 1000/6000 = 11 Volt AC and at 12000 rpm Vg = 65 Volt * 12000/6000 = 130 VAC.
  5. The 3 phase output from the stator is fed to the rectifier/regulator where it is rectified into a DC voltage and subsequently regulated by a a switch mode DC to DC converter to the proper battery charging voltage, typically around 14 Volt.
  6. The currents flowing in the stator windings depends on the power drawn by the entire 12 VDC system on the bike.
    A good DC/DC converter of the non-standard serial type would operate with > 90 % efficiency and draw 111 Watt from the stator if it has to deliver e.g. 100 W to the 12 VDC sytem. This is 111/3 W = 37 W from each phase.
    The stator current 'I' will hence depend on rpm so we get:
    - at 1000 rpm: I = 37W / 11V = 3.4 Ampere.
    - at 6000 rpm: I = 37W / 65V = 0.6 Ampere.
    - at 12000 rpm: I = 37W / 130V = 0.3 Ampere.
    In theese simplified calculations I have neglected losses due to resistance in the stator windings and other factors. Another simplification is the assumption of using a non-standard serial type regulator, which is technically easier to understand than the standard shunt type. See the later section about the regulator types for further details on this.
  7. The resistance in the stator windings will cause power dissipation inside these.
    If the resistance is e.g. 0.5 Ohm, the voltage drop will be 3.4 A * 0.5 Ohm = 1.7 Volt, and the power dissipation in the stator will be 3 * 1.7 V * 3.4A = 17 Watt !

Here follows more details about some of the above points:

ad 2) The picture below shows a fried stator from a CBF1000 2010:

friedstator.jpg - photo by arizonarocket

ad 4) This is a fundamental physical behavior which exists in our universe as we know it: A changeing magnetic field in a coil will induce a voltage in this coil which is proportional to how fast the magnetic field is changeing, and proportional to the number turns in the coil.
The voltage vaweforms for the 3 phases are shown on the graph below for an idealized and unloaded system. Notice how the 3 phases are shifted 120 degrees relative to each other.

3phase01.jpg

ad 6) The currents flowing in the stator windings will create their own magnetic field which will counteract the field from the rotating permanent magnet to a certain degree, and hence reduce the stator output voltage. This is neglegted in the above simplified calculations.
This winter I will see if I can make a brake-out connector for the stator output, so I can measure voltages and currents under various conditions with my oscilloscope.

ad 7) The induced voltage will be available at the coil wire ends, except for a voltage drop due to internal resistance of the coil and the current flowing in it. The size of this voltage drop is equal to R * I where R is the resistance of the coil and I is the current. It is this drop which dissipates heat inside the stator.

Example: Have you ever seen how a permanent magnet can float in the air above a superconductor on the magnetic counter-field it is creating by falling towards the super conductor. The approaching magnet is creating an increasing magnetic field at the surface of the super conductor. This induces voltages in the upper layer of the super conductor which in turn drives currents arround in circles in the surface. These currents creates the counter-acting magnetic field, which - when strong - enough keeps the magnet floating. Watch the video to see it.

Video from brucegray666 - thanks (I hope it is ok to use it here brucegray).

And why did I present this experiment from fundamental physichs? I did it so you can see with your own eyes, that a changeing magnetic field from a permanent magnet can induce currents in a conducting material which in return creates a secondary magnetic field that counteracts the primary field. This is exactly what happens inside the alternator when a standard shunt-type rectifier/regulator is used in the charging system. More about this later.



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:

Generators in cars typically uses electromagnets instead of permanent magnets, and can hence adjust field strength to adapt to the actual rpm. This keeps currents, voltages and temperatures inside much narrower ranges and thus stresses materials less. The actual voltage regulation is in this way done inside the generator itself and not in an external DC to DC switching converter.

On top of theese problems, Honda and many other manufactures unfortunately uses 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 creates a rotating magnetic field which counteracts the rotating magnetic field from the permanent magnet, and thus effectively reduces the induced voltage. Theese 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 causes extra heat dissipation and hence extra temperature rise inside the stator windings.

The Regulator - Principle of Operation

This section is still under preparation. It will discuss and compare the commonly used shunt regulator with the serial regulator.

The Shunt Regulator

The shunt regulator is called 'shunt' because it litterally 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 prowen 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.

I will not present any regulator diagrams in this article, but just shortly describe the operational principle of the shunt regulator:
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 causes extra heat dissipation in the stator windings and are probably the reason for having high failure rates on this component.

See figure 5, 6 and 7 in this excellent article from Ducati Club Denmark. The article is unfortunately in Danish, but the diagrams are easy to understand, that is - if you have quite some knowledge in electronics.

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.

I do not yet have any overview of which series regulators are available for our bikes. The only manufactureres of series regulators that I positively know of are:

Be aware that the manufactures descriptions of how their series regulators works are typically not very detailed and at some poins not even correct. I guess they do not want to disclose too much information for competitive reasons.

Measurements, Symptoms and Failures

Testing stator on a 2008 Fireblade:

OBS: I have not done this yet myself, hence I have no pictures to show.

Testing stator on a 2007 CBF1000A:

OBS: I have not done this yet myself, hence I have no pictures to show.

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 easy 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 Valve Cover Gasket due to Stator Failure !!!

It happens that the valve cover gasket is suddenly blown out by a sudden pressure increase inside the valve cover. According to Honda this may be caused by a failing stator igniting gasses in the cranc case, but Honda has - to my knowledge - not revealed further details about this failure mechanism.

What puzzeld me first, was how the gas inside the cranck case could come to a proper mix ratio for burning? There surely will be plenty of oil droplets to feed a fire, but a fire also requires oxygen!

After some speculations the explanation to this mystery became clear to me: The pressure inside the cranck case is off course a ballance between:

When the suction from the air cleaner winns, there is lower than ambient pressure in the cranck case and it will suck air (and oxygen) from any leackage points. This is probably most likely to happen when the engine is warm and has tight piston rings.

When a faulty stator then generates a small electric arc or a melting hot spot it goes BUM ! - a high pressure shock wave flies up through the channels towards the valve cover interior where the gasket is blown out.

I think all this sounds plausible - right?

I have been asked how a spark could be generated inside the stator which is only generating relatively low voltages. Here is the explanation:

When insulation layers fails and eventually allows two parts of the cobber wires to come sufficiently close to each other, a low voltage spark (an electric arc) will be created. In free air arround 2000 Volt per mm is required, so at e.g. 0.01 mm only 20 Volt is needed to start a spark! This should be sufficient to start a fire inside the cranck case if the right mixture of hydro-carbons and oxygen is present. If this spark is not enough, then when the wires actually tuches each other and creates a real short, molten cobber at high temperatures should be able to do it.

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 over heating 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 posible with a simple AC voltmeter to verify proper symmetrical current load on all three windings.

Finished.


You are invited to participate with review, questions and discussion via this thread on www.fireblades.org or this thread on www.cbf1000.com.

See also:
How to DIY (warnings, tips, tools, methods, ... etc.)
First Year with my Fireblade
Superbikes on the Road: BMW S1000RR versus Honda CBR1000RR Fireblade

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Copyright 2011 FireBladerDk - Last updated: 2011-08-11