Detailed Description of Design

The model was designed using Autodesk Fusion 360 to be accessible for everyone to download and work with as Fusion 360 is available for free for hobbyists to use. It is also widely supported online and plenty of guides are available to allow for the model parts to be manufactured using almost any method.

The idea behind the design

The inspiration for this design comes from the way helicopter blade pitch mechanisms are designed. Helicopters are controlled almost entirely by changing the orientation of the main rotor. To go forward and backward, the whole assembly is tilted to direct the generated thrust as required. This can also be done to fly sideways, although turns are coordinated by using both the main and the rear rotors. The amount of overall lift is adjusted using the collective, which changes the pitch of the blades while keeping the throttle application unchanged. This has the effect of pushing a larger volume of air through the rotor 'disk'.

To use this idea on a wind turbine, we need to simply change our frame of reference. Let us examine the requirements for maintaining a hover with a helicopter. If the air is perfectly still and uniform, all we need to do at a certain throttle setting (a fixed throttle setting assumes a constant speed of rotation regardless of the pitch setting for simplicity) is to find the blade pitch that pushes enough air to counteract the weight of the helicopter. If we were to increase the throttle, thus increasing the rotor speed, the veolume of the air pushed through the disk would increase as well. To keep the helicopter in a hover, the pitch would have to be decreased to keep the volume of air per unit time constant.

In a wind turbine, the speed of the air comes from the wind, not the rotor, and the turbine is fixed so it obviously cannot hover. But pitch control of the blades is still crucial, particularly at high winds, when the turbine is at the limit of its generator capacity. At the limit, if the generator spins any faster, it could lead to the failure of the turbine. Thus, the speed of rotation has to be kept constant. With the turbine fixed, the only was to do this is by changing the blade pitch.

Another important task of the pitch mechanism is to reduce stress on the turbine tower if the wind speed gets too high. In this case, the rotor brake is engaged to stop the turbine, while the blade pitch is adjusted to provide the least possible resistance to the wind (lowest possible lift), thus minimising the forces exerted on the tower structure.

Rotor

The most crucial reguirement for the rotor is that it must be able to adjust the pitch while still for the rotation of the blades. In helicopters, this is solved by using a swash plate, which is essentially a bearings, poth parts of which are independent from the main rotor shaft. One part rotates at the same speed as the shaft, but crucially, can slide along it to adjust pitch. The other part does not rotate with respect to the main helicopted body and is connected to the servos that actuate the pitch mechanism. This is a requirement as there would be no sensible way to have the servos rotate with the blades.

The swash plate in our design uses a single ordinary ball bearing, onto which laser cur pieces are attached, one onto the stationary side (outside of bearing) and one to a copper pipe attached to the rotating side (bearing inside). The exploded view of the swash late can be seen below.

The rotor pitch is adjusted using a single S3003 servo linked to the stationary side of the swash plate. The swash plate is then connected to the turbine hub by miniature ball joints and steel rods used primarily for radio controlled cars and planes. The hub allows the blades to rotate around only one axis. There are three miniature roller bearings withing the hub that accommodates this.

The full pitch assembly can be seen below. The casing is removed to more clearly visualise the mechanism.

The blades

As the project aims to be simple and low cost, the blades do not have an aerodynamic shape like their real counterparts. Instead, they are laser cut from 1 mm basswood so they are very light, and therefore the inertia of the rotor is minimised. The flat shape has a much higher drag but still provides enough torque for the model to function.

Generator

The last main piece of the assembly is the 'generator'. As this model is only intended to show the principles of wind turbine design, a small DC-motor is used. As a permanent magnet generator is roughly the same as a motor, only well optimised and used 'in the other direction'. Instead of creating movement from voltage, it generates a voltage, when rotated. Indeed, some real turbines can use the generator as a motor to rotate the hub when attaching the blades during assembly.

As the generator is designed to run at 3750 RPM at 3.3 V, it is geared to a 2:1 ratio with the main shaft. This way, we increase the generated voltage at any giver speed of rotation by two. Although a bigger gear ratio could have been used, it would have increased the force needed to spin the rotor so the rest of the signal amplification is done electronically (see the wiki page on electronics).

The tower and the nacelle

The mechanism described above is constrained entirely to the nacelle of the turbine and is connected to the Raspberry PI by running wires down the tower to the base of the turbine, where the PI is located.The nacelle itself is made entirely from laser cur acrylic sheets. This makes it extremely fast to make if access to a laser cutter is possible. Most of the parts could also be made on a CNC, however, some refinement would be needed afterwards on the concave 90 degree corners where even the smallest machining bit will create a fillet instead of a sharp corner. Most of the structure is made from standard 6 mm cast acrylic and is easy to substitute with plywood or other sheet materials if necessary.

The tower is the simplest part of the design, made from regular hardware store PVC piping. It has an acrylic base which keeps the tower upright, which is also shaped like a turbine for a bit of extra cringe.

The turbine can be taken apart easily for easy transport and storage. All the connections are interference fit so the assembly is completely tool-less.