Motorcycle Electrical Systems

This chap on our right is Michael Faraday, who was born in 1791 in London and, without any formal education other than a persistent and inquiring mind, became one of the foremost scientists of the 19th century. One of the things Faraday explained to the world was the principle of electromagnetic induction. Electromagnetic induction is the production of a potential difference (a voltage) across a conductor (a wire) when it is exposed to a varying magnetic field.

Faraday's principle underlies most of the ability of our machines to run and generate light.

Power Generation

Electrical generators, and let’s not confuse the situation by calling them alternators or dynamos for the moment, operate by the use of energy from the engine to rotate a coil of wire relative to a magnetic field, thus, by the understanding provided by Mr. Faraday, inducing a voltage in the wire. In the simplest installation (including Bantams) a fixed coil is mounted within a rotating drum; the drum is provided with one or more permanent magnets in its periphery.

Because the magnets rotate around the coils, passing them with each rotation of the engine, the current that flows in the coils changes according to the proximity of the magnet (i.e. the strength of the magnetic field) and can flow in either direction according to the polarity of the field. We say the current alternates. The fixed coils make it very easy to connect to the vehicle electrical system. In fact, we’ll connect each end of our coil to each end of a light bulb filament

And that’s it. With some wires and switches, we can connect the coils to the light bulbs and ride at night. Bulbs don’t mind if their current swings both ways.

The other interesting thing that happens is that the current that can be induced to flow in the coils increases with the speed of the passing magnet – thus, if we increase engine speed, we can have more voltage and more current.

This starts to throw up the first problem with our simple system – when we ride too fast in the dark, the bulbs blow…

Voltage Control & Rectification

All Bantams, battery lighting or otherwise, use a form of voltage control common to small bikes of the period. Essentially, the generator coils (and there may be two or three lighting coils depending on the type) are switched in and out of circuit according to the lighting load. If all the coils are in use, and the lights are switched to ‘sidelights’, increasing the speed of the engine will generate considerably more than 6 V and will blow the bulbs; thus, when only sidelights are in use, only one or two lighting coils are used, the system brings the other into service when the lighting switch is moved to the headlamp position. This switching of coils is achieved in the lighting switch in the headlamp.

Battery lighting Bantams have another need, beyond voltage control. The current coming from the generator is alternating current – accepted happily by the bulbs, but no good for battery charging. The battery requires direct current – and thus the alternating current has to be rectified. Bantams had an early Selenium solid state rectifier, a step up from the glass valves they replaced but not as efficient as a modern silicon rectifier, and rather prone to temperature related failures.

So what we need to do is take our generator coils and connect them to the rectifier before we hook them up to the bike. What we need though is to make sure the current can only flow in one direction, so we need a one way ‘valve’. Electrically it looks like this:

The alternating current from the generator is provided at the input to the rectifier, and plotted on an oscilloscope looks like the waveform at the top. Notice that the waves are alternately positive and negative. The rectifier is made of four devices called ‘diodes’ – labelled D1 to D4, which pass current in one direction. When the waveform is positive the input current passes to the output through diodes D1 and D2, but cannot pass through D3 and D4; so effectively D1 and D2 only give us the positive voltage peaks from the waveform. Conversely, when the waveform is negative the input current passes to the output through diodes D3 and D4, but cannot pass through D1 and D2; so effectively D3 and D4 only give us the negative voltage peaks from the waveform but since both pairs of diodes are connected to the same output, all voltage peaks appear as positive, giving us the waveform at the bottom.

Now, this is not true DC current, but since all peaks are positive, it will charge a battery.

And sparks…

Sparks are provided by another neat little Faradaien trick – it’s really the same one again, our friend electromagnetic induction, but there is nothing moving here except a magnetic field.

A clever American chap, by the name of Charles Kettering, invented a system to replace the hot tube ignition systems common in the first road vehicles.

Two coils of wire are arranged around a common core – that is, they are wound onto the same bobbin. One coil has a few turns of thick wire (called the primary winding); the other has tens of thousands of turns of thin wire (called the secondary winding). We arrange for a current to flow in the primary, from a battery (or perhaps a magnet rocking by, in the rotor of a generator), and we end up with a magnetic field in the core of both the primary and the secondary winding. Now, we thoughtfully arrange for an engine driven switch, the ‘contact breaker’ to control the flow of current in the primary winding. When we open the switch, the current in the primary no longer flows and the magnetic field disappears.

Now, remembering how movement of coils of wire in magnetic fields induces current in those wires due to electromagnetic induction, so does movement of magnetic fields in coils of wire induce current in those same wires. So when our field collapses, because we switch off the current in the primary, a current is induced in the secondary. Now, because we have arranged for the secondary to have many, many more turns than the primary, the voltage is very high (but there is not much current, which doesn’t matter.) But where does it go? It jumps the shortest distance it can to earth, as a fat spark.

All we need to do now is arrange for that spark to occur inside the engine (with a special device called a spark plug), and to get the voltage to the plug (with some cable capable of passing a high voltage without losing it), and we are home and dry.

Except for one small thing – remember we induced a current in the secondary when we collapsed our magnetic field? Well guess what. The same thing happens in the primary – it’s only small, but the spark it produces occurs at our contact breaker points and will destroy them if we don’t address the problem. To fix that, Kettering added a small device called a condenser.

The Condenser

An automotive condenser is basically two strips of foil, with an insulating strip between them, rolled up and placed in a tin. The wire is connected to one strip, the tin case to the other and putting a voltage across the two strips causes an electrical charge to be set up, rather like the static charge set up when you rub a balloon on the carpet.

They behave a bit like a battery in that you can charge the two foil strips, much like the two plates in a battery. When the points are closed, the condenser is shorted out and there is no charge across the strips. When the points open, the energy in the primary coil circuit has to go somewhere – it either cascades across the points, eventually burning them out, or it charges up the condenser. The energy is lost to earth when the points close.