An alternator is a device which generates electricity to charge the battery on your bike.
It does this by either spinning a coil of copper wire through a magnetic field or by spinning a magnetic field around a copper coil or series of coils connected together.
This magnetic field can be produced either by permanent magnets or by passing a current through a copper coil.
In the second case the amount of current passing through the coil determines the strength of the magnetic field.
What I have just described is the basis for all alternators and generators.
Standard XS650 Alternator
The XS650 has an alternator which uses copper coils for both the rotor and the stator, the rotor being the bit that spins and the stator being the bit that doesn't.
How this works is quite simple. When you turn on the ignition a current passes through the rotor via the voltage regulator and the brushes and creates a magnetic field. When the engine rotates this field is spun inside the copper coils of the stator and begins to generate a current in the stator.
As the revs increase so does the current flowing through the stator and the voltage increases.
This output from the stator is Alternating current (AC) and must now pass through a rectifier bridge to be converted to Direct Current (DC) to charge the battery and needs to be around 13.8 Volts. The additional 1.8 volts above the batteries 12volts ensures the battery charges.
When the voltages reaches the 13.8 volts the regulator switches in and reduces the current supplied to the rotor which in turn reduces the strength of the magnetic field and in turn reduces to output from the stator.
If the load increases by turning on the lights etc. the voltage drops to 12 volts because the battery is now supplying the extra current needed , now the regulator increases the current to the rotor to compensate and so increases the strength of the magnetic field which in turn increases the output from the stator. When the voltage increases to 13.8 volts again the regulator steps in to decrease the current to the rotor and stabilizes the output from the stator.
This process is continually adjusting itself while the engine is running.
Earlier models of the XS650 use a regulator which achieves this by means of relays and contacts continually switching the current to the rotor on and off.
Later models use transistors to do the same job and are referred to as solid state regulators.
All this is good solid engineering with few problems or weaknesses and gives good service for many years.
Some perceived weaknesses are that the battery must be in good condition or there will be insufficient voltage/current available at start up to begin the process of generating a charge and the bike will not start.
Also when the brushes are worn out the alternator can stop charging with little or no warning.
Alternative Alternator
The other type of alternator that can be adapted to the XS650 is a permanent magnet type, by substituting permanent magnets in the rotor you eliminate the need for a good battery at start up, in fact you can eliminate the battery all together if you wish.
By using the PM alternator you also eliminate the brushes from the system. As soon as you start to spin the motor the PM alternator begins to generate a charge because the rotor's magnetic field is already at full strength and inducing a current flow in the stator.
The regulator is the only major difference between the two types as it works in an entirely different way.
Because with permanent magnets you cannot regulate the rotor, you must now regulate the output from the stator and this is done by shunting off excess current the battery doesn't need through a zener diode and some resistors to earth.
This excess energy is dissipated in the form of heat from the regulator. With a PM alternator you are not regulating how much energy it produces because it produces it's maximum output all the time, what you are doing is getting rid of what you don't need via the regulator and dissipating that excess energy by converting it to heat.
I know it sounds like black magic but most of the English bikes fitted with alternators did it this way.
Conclusion
I hope this has explained to you the differences in the two types of alternator in a way you can understand so you can make an informed decision whether or not to change.
The conversion
This conversion is specifically for fitting a Yamaha RZ250 alternator to a Yamaha XS650 motor
I finally got my hands on a RZ250 stator and rotor and as you can see they are in excellent shape.
These will be the basic components of my permanent magnet alternator conversion. As I go through the process of converting this to suit the XS650 I will take pictures and measurements and post them on the website for everyone to see so you can use it as a guide for your own conversion if you decide to do one.
Here's the 130mm OD X 85mm ID X 5mm thick plate with 6mm holes drilled at the sides for mounting to the engine. The three holes for mounting the stator windings have not been drilled and tapped yet.
Note the small cut-out at the bottom for the locating pin.
Here's the design for the plate including the three holes at 60° apart and 100mm centres for mounting the RZ alternator to the plate. Some of these dimensions are only approximations but close enough. The three holes 60° apart are tapped to 6mm Metric thread and the other two holes for mounting the ring to the engine are 6mm diameter.
If you click on the image you can download a .pdf file, if you don't shrink it when you print it you can use it as a template as it will print the exact dimensions. I used a printout like this to centre punch the three 6mm threaded holes for mounting the RZ alternator and it worked out great, just overlay the printout over the plate accurately and centre punch the location for the holes through the printout. Then just drill and tap them to 6mm metric thread.
There are still some modifications to be done to the RZ alternator mounting bracket before fitting and I will show these when I do them.
There is still some trimming to do at the bottom where the ignition pickup mount needs to be removed and a hole drilled for the wiring loom to exit behind the mounting ring but apart from that it's all but finished. Now all I have to do is transfer it across to another motor I have with a crankshaft to make sure it all fits together properly and rotates as it should.
If you look at the above left picture and compare it to the one below left you can see where the mounting boss for the ignition pickup was removed, also you can see I have now transferred the alternator to another motor and fitted the rotor to the crankshaft. It all fits and turns nicely. In the above right picture at the bottom you can see a pointer near where the wiring exits, this pointer could be used with some new marks on the rotor for TDC on both cylinders, unfortunately the existing mark cannot be used as it is 180° out for the XS crank.
As you can see it also fits neatly inside the cover so when it is all buttoned up no one will ever know it has been modified.
I will explain the wiring differences and which ones to remove when I get to it on my conversion. I have yet to modify the woodruff key as the slot in the crank is narrower than the slot in the rotor so the key will need to be stepped, more on that later when I have located the right key as it wasn't supplied with the alternator.
I bought a standard RZ250 woodruff key from Yamaha ($9.50) and had it stepped at a local engineering workshop ($30) so my conversion is now mechanically complete, just the wiring to finish.
I intend to use a Honda regulator/rectifier for the conversion which was left over from the Honda CB250N fiasco. More on this later when I actually get around to wiring it together.