How bicycle dynamo systems work.

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This is an in-depth look at how bicycle dynamo systems work. There is more information here than you need to set up a dynamo system, however, understanding how they work will certainly help with installing and maintaining a system. If you’re just looking for an installation guide you can visit this page.

The dynamo

As the source of electricity within the system the dynamo is a logical place to start.

Electromagnetic induction

Electromaganetic induction is the effect used to convert the energy in the movement of the dynamo into electricity. It describes the effect such that when a magnetic field applied to a conductor (such as a wire) changes, electricity is generated in that conductor. Consider a wire moving relative to a magnet (or vice versa), as illustrated, and we have a simple electrical generator.

The theoretical dynamo (magneto)

To use electromagnetic induction to generate a continuous electrical output, a bicycle dynamo consists of magnets and conductors rotating relative to each other. The exact arrangement depends on the type of dynamo, but typically they are based on rotating magnets with stationary conductors. The general term for this sort of device is a magneto. (Confusingly, in the non-cycling world, a dynamo is a different type of electrical generator) The illustration below shows a simple magneto, which is similar in design to a bicycle hub dynamo. It consists of wire coils inside a ring of magnets.

The most notable feature is that the output is not constant. This is what causes dynamo lights to flicker at low speeds. The output from a dynamo is alternating current known as AC, which differs from direct current, known as DC. The diagram below shows the difference between AC and DC outputs.

Dynamo electrical output

Examining the dynamo-like magneto above, we can see that the frequency of the AC output from a dynamo is determined by the speed of rotation and the number of magnets, known as poles. However, predicting the precise power output from a particular dynamo at a particular speed is not trivial, as the electromagnetic interactions between the conductors and the magnets are extremely complicated. The power outputs of many dynamos have been measured experimentally however, and you can read about that here. In general, the higher the speed of rotation of the dynamo, the larger the electrical power output will be.

Regulating the voltage

The variation in power output with speed is not ideal for running electrical devices such as lights. In particular, the increased power output at high speed could damage devices. For this reason dynamo systems often contain some sort of voltage regulator. A voltage regulator will limit the maximum voltage delivered to any devices. The voltage may be regulated at the dynamo, lights or a dedicated regulator. Most modern dynamo lights contain a built in regulator, so no specific consideration of regulation is required when installing a system.

Design voltage and power

The vast majority of dynamos are designed to operate at 6 volts with a total power output of 3 watts. There are also systems which operate at 12 volts and 6 watts, and 6 volts and 1.5 watts. The table below shows the different types of systems which exist.

Type Power (Watts) Voltage (Volts)
Standard system 3 6
1.5 W system 1.5 6
Busch and Müller 12 V (discontinued) 6 12

This is a good point to mention that it is useful (but not absolutely necessary) to know some basic electrical rules to understand dynamo systems. In particular the relationship between voltage, current and resistance, known as Ohm’s law, and related relationship between power, current and voltage.

Ohm’s Law:
$$I = \frac{V}{R}$$

Power Equation:
$$P = I V$$

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