Alternator Conversion

Fitting an RZ250 alternator to an XS650

Nikola Patrikov was kind enough to share his modification to the XS650 charging system. A detailed description of the modification follows

An Overview of Alternators

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.

The 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.

An Alternative Solution

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.

The Conversion

This conversion is specifically for fitting a Yamaha RZ250 alternator to a Yamaha XS650 motor. I finally got my ands on a RZ250 stator & rotor that were in excellent condition and will be the basic components of the permanent magnet alternator conversion.

Below is the design for the plate including the 3 holes at 60 degrees apart and at 100mm centers for mounting the RZ alternator plate. Some of these dimensions are only approximations but are close enough. The 3 holes are tapped to metric 6mm and the 2 holes for mounting the ring to the motor are 6mm diameter.
The template image will download as a pdf and if you do not shrink it when you print it will serve as a template for your project. Template Link

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.

In the pictures below 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.  The woodruff key needed to be modified as the slot in the crank is narrower than the slot in the rotor. To rectify this the key will need to be stepped. A standard RZ250 woodruff key was purchased from Yamaha for around $9.50 and given to a local engineering workshop who stepped it for $30.00. This completed the mechanical part of the conversion. Next is the wiring to finish the job.

A Honda regulator/rectifier for the conversion. The Honda unit  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.

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 have added some missing information (like junction points) and givne you more information how the schematics should be read (see picture above):
– 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 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).

Advice 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.