In one of my previous articles we discussed the construction of a windmill generator using an ordinary motor. In the article I recommended the use of the anemometer principle while designing the propellers. The reason behind this was to avoid the inclusion of cumbersome rudders and complicated pivot mechanism. The propellers associated with anemometers do not depend on wind directions and are capable of producing consistent, unidirectional, high speed revolutions under most wind conditions.
Although the use of a stepper motor has an advantage of driving the load (charging batteries) directly, without the need of an external circuit, the device is costly and requires higher wind speeds for optimum saturating levels. Moreover, new electronic enthusiasts often get confused with the specifications while procuring them.
Here, the use of a bicycle dynamo makes the procedures very simple, however since the unit generates electricity at relatively lower levels, it may require an external voltage booster circuit for driving a load consistently. In the present application we are able to use the generated voltage for charging many Ni-Cd cells (1.2 V each) in series.
The mechanical set up illustrated in the diagram is self-explanatory and quite simple to construct. As per the law of the moment of inertia, longer propeller shafts will help to generate greater torque and consequently higher currents from the dynamo.
Practically though, you will find, only at optimum speeds, the generator produces voltages above 4 volts. Therefore, in order to get this voltage, which is ideally suited for the application even during dull wind speeds, we will require a voltage booster circuit or what is popularly know as the “charge pump" circuit.