The need for this post arises because I have wired the Huntmaster negative earth. The dynamo that came with the kit is probably the original dynamo which of course will be expecting a positive earth system.
What's a dynamo
A dynamo is an electrical generator that provides a direct current using a wire wound armature moving in a magnetic field following a principle discovered by Michael Faraday in 1832. It needs a mechanical commutator to turn alternating current into direct current since the rotating armature generates a current which changes direction as it moves through the magnetic field. An important principle as far as we are concerned is that the faster the armature rotates, the more voltage and current is generated by the machine. Similarly if the magnetic field strength is increased, the voltage and current will also increase.
How does it work
In the motorcycle dynamos that we are looking at, the armature consists of a number of coils of wire wound around a stack of sheet metal shapes. The wires are terminated in a number of copper strips at the end of the machine arranged into a cylinder. The magnetic field is provided by a coil of wire wound around a suitably shaped metal structure fastened into the inside of the machine. The engine rotates the armature and a current is generated providing a magnetic field in the field coil.
You will note that the magnetic field is provided by an electrical coil, and consequently you can imagine that the machine will not work unless that magnetic field is present when the engine starts. This is one of the issues we have to address.
When the dynamo starts it is disconnected from the rest of the electrical system by the cutout in the regulator box and consequently won't start to generate electricity. In practice the field coil pole piece remains magnetized at least partially, after the machine was last used and we call this residual magnetism. This residual magnetism provides sufficient field strength to induce a current in the armature as it starts to revolve. Of course as the armature starts to revolve it generates a voltage which raises the field current which in turn increases the magnetic field, which allows a higher current to regenerated - and so it goes on. The voltage will carry on rising and in practice is limited by an external voltage regulator which takes effect by limiting current to the field coils.
Why do you re-polarise it
In a world where a motorcycle dynamo is perhaps not used every day, or the dynamo is moved from one machine to another perhaps having been bought at an autojumble or perhaps having lain around for a very long time since it was last used, it's functionality maybe compromised by a residual magnetism that is in the wrong direction also weak that it is virtually non-existent and the machine will not work.
Therefore we need to have a method of generating that residual magnetism or changing its direction. Changing the direction of the residual magnetism might be necessary because you wish to rewire the machine for negative earth or perhaps because the machine for which your dynamo or specified rotated the dynamo in the other direction than which you wish to use it.
How do you re-polarise it?
Repolarizing the dynamo is quite straightforward and is just a matter of providing a current in the appropriate direction through the field coils. If you disconnect your dynamo, and provide a temporary cable from the battery terminal (that is not earthed) to the field terminal on the dynamo, you can repolarize the field coil by touching the wire on the field terminal momentarily. That should be sufficient to change the polarity of the residual magnetism or to generate it if it has seeped away.
It may be that the temporary cable is not really necessary. Disconnecting the wires to the regulator reveals the green field coil cable and a brown and white table from the ammeter. This brown and white cable is actually a battery cable, passing through the ammeter from the battery full stop therefore touching this cable to the field coil cable should be sufficient to polarize the dynamo correctly.
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