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.


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 like the one pictured above. 

The Honda unit I will be using has two connectors, one containing three YELLOW wires. These are the AC wires and connect in any order to the three WHITE wires coming from the Alternator. This is the input side of the regulator/rectifier.

The other connector has a RED wire a GREEN wire and a BLACK wire these being the output side with the RED wire connecting to the Battery +ve and the GREEN wire connecting to the battery -ve. The BLACK wire is a voltage sensing wire and is connected to a +ve 12 Volt source that is switched on by the ignition key. This regulator uses SCR's (silicon controlled rectifiers) to regulate the current that is shunted to earth to maintain a constant voltage.

Most of the additional wiring from the Alternator (other than the three white wires) are not used on the XS650 conversion.
If you use the same alternator as I have you will find the extra wiring consists of a red, green and brown wire all going to a separate connector than the three white wires. These are connected to a lighting coil on the RZ and will not be used so they can be cut, insulated and taped into the wiring loom. Do not connect them together. There is also a blue wire in the connector with the three white wires, just pull it out, it is not used for this conversion. There is also a separate black wire which is earth, it's not necessary but you can connect it if you want to insure a good connection between the alternator and the frame.

There is a fourth connector with two thin white wires, pull these out, they are not used. 

Here is a circuit of how it is wired. The picture of the inside of the regulator I got from a Honda manual and is in schematic form and does not accurately represent what's inside it's just used to explain how it works. 
Basically as I understand it the black wire senses the battery voltage when the ignition is turned on and if it gets too high the internal regulator switches on the SCR's and this shunts away the output from the alternator to earth which will cause the voltage to drop. When the voltage drops below the SCR's threshold value the SCR's turn off allowing the voltage to rise again starting the process all over again. I imagine this all happens several times per second.

This is a layman's description, if anyone knows better please let me know and I'll correct it.

I received this additional information from Nikola Patrikov along with an updated diagram.

The schematics provided by HONDA represents accurately what is inside the regulator. The only thing that is displayed without any details is the regulator logic (maybe because it may vary between different models of the regulator itself).

I’ll put some missing information (like junction points) and give you more information how the schematics should be read (see picture attached):

- Rectifier (green) – transforms alternate current into direct current

- Reg – logic that monitors voltage level between black and green wires

- Switches (blue) – used to control output voltage level

What actually the regulator does:

1. In case that monitored voltage is below some defined threshold (usually between 14 and 15 Volts) the regulator logic does not take any action. All switches are off (opened). There is current flow from alternator through rectifier to your battery and other consumers.

2. In case that monitored voltage goes above defined threshold regulator logic will close all the switches. This will make short circuit between alternator and the ground and will cut of the input to rectifier. The switches will remain closed until the voltage goes down at acceptable limit (usually very short time). This procedure will be executed many times per second (depends on revolutions per second your alternator is driven with).

Advices on diagnostics and ,protection of power regulator:

1. Power rectifiers can be broken very easily at relatively low voltage. (if maximum input voltage is specified to be 30V usually at 50 the rectifier will burn). At the same time alternator without any regulation may rise its voltage up to several hundred volts. Because of that:

2. Never disconnect battery while engine is running

3. Most of regulators have internal connection of regulator logic input to rectifier output. This reduces battery retention in case that you not use your bike very often, but eliminates the risk of regulator and other electronics damage in case of main fuse or ignition switch failure while the engine is running.

Nikola Patrikov

It is now possible to buy a complete permanent magnet alternator conversion from several aftermarket sources like Hugh's Handbuilt, look them up online.