In pursuit of real performance

In its simplest form, a wind turbine’s blade requires only a five-degree pitch down its entire length to perform adequately. But as soon as you give the blade some taper, and make its pitch greater at the root than at the tip, you start getting real performance.

The length of the blades also determines performance levels: for example, if they are too short, they will be unable to turn the generator fast enough to produce efficient power. Conversely, if they are too long, they can overpower the generator, causing it to burn out.

Blades are mostly carved out of wood, but some enterprising individuals have fashioned simple but effective blades from PVC piping. Becker chose balsa for his because he lives in suburbia and likes being polite.

“Balsa is light and strong, and if a blade is going to fly off in high winds, I didn’t want it to break any windows or cause injury,” he explains.

He began by carving the required profiles out of solid balsa, but once he started work on his present turbine, his curiosity got better of him.

“When making these blades, I used 1 mm balsa sheets because I wanted to see how long they’d take to break... and they haven’t.”

He uses wedges to set the blade’s pitch at an angle of about 30 degrees at the root and about 6 degrees at the tip. This gives the turbine an added boost during start-up as well as enhancing high-speed performance. The root area, where it’s fixed to the turbine’s hub, is reinforced with extra balsa and glass fibre, and a strip of pine adds rigidity to the leading edge.

Finally, the entire blade gets a liberal coating of epoxy inside and out. They taper from 100 mm at the root down to 50 mm at the tip, and are 1 m in length (there is no arcane reason for this measurement; the length is limited solely by the availability of suitable balsa from Becker’s local hobby shop).

He used 5 mm-thick mild steel for the two 250 mm-diameter rotor discs, explaining that these helped contain and intensify the magnetic field in the air gap between them.

To ensure they mirrored each other perfectly, Becker had them precision-cut by experts. He also had a template made so that the rotors could be accurately drilled and aligned. Another template ensured the accurate positioning of the 16 magnets on each rotor.

Since the strength of the magnets determines how much power your turbine can produce, Becker opted for the best — namely, rare earth (or Neodymium-Iron-Boron) magnets. Measuring just 20 mm x 50 mm x 8 mm, they seem rather benign when examined individually — but they can drive you crazy when assembled in a small space.

Becker explains: “When you put them down, they have to be at least 15 cm apart or they are forcefully attracted to each other, causing them to chip. By the time you’ve found a place for all 32, you have no space left in your workshop — and you’ve still got to find a safe place to put your screwdriver!”

Becker coaxes the magnets into position by first attaching his template to the steel base and then gently sliding them into position from the open end. The magnets are placed with their poles alternating north and south to shunt the electrons in a steady stream along the coils on the stator. The faster the alternating north and south poles pass the coils, the more power is produced.

Once the magnets are in position, the template can be safely removed. Epoxy is then poured between them to hold everything in place.

According to Becker, making the stator is by far the most complicated part of the construction process. Fixed in position between the two rotors, it encapsulates nine coils (three coils per phase) to produce three-phase power.

Says Becker: “I wanted three-phase power because it allows me to squeeze more power from the generator, and the current is more stable. In a single-phase design there are always periods were no power is produced.”

If all else fails, try meditation

Enamelled copper winding wire was used for the coils because of its thin insulation and superior heat resistance.

There are a number of factors to consider before winding the coils. For example, thinner wire allows for more windings per coil and delivers better performance at low speeds than a coil comprising fewer windings of thicker wire. But as soon as the wind speed increases, the thinner wire becomes less efficient.

After much experimenting, Becker settled on wire with a diameter of 0.8 mm and 80 turns per coil for his latest design. Even with his specially devised coil winding jig, getting it right was an excruciating task.

“You almost have to sit in a meditative state while counting out the turns, because it’s critical that you make them exactly the same length.”

They also have to be wound tightly and as flat as possible, to keep the air gap between the rotors as narrow as possible, and they must be exactly the same shape and size to ensure they are correctly aligned with the magnets.

The completed coils are then placed in a mould and connected in threes to produce three-phase power. Finally, the coils are encapsulated in epoxy and the stator’s square corners are reinforced with glass fibre, giving it the strength to be securely attached to its mounting bracket by four long bolts.

Connecting the components isn’t complicated, says Becker, but it does require a degree of finesse. When assembling the turbine, he treats the two rotors with great respect, and with good reason — their combined magnetism is sufficiently powerful to crush fingers if they snap together.

Using wooden levers and spacers to keep them apart, he carefully tightens the four bolts until the two rotors are parallel.

The secret lies in getting the magnets as close to the coils as possible without touching the stator. Once it’s fully assembled, the wires from the stator are fed into four diodes (each rated at 35 amps) to turn the three-phase power into direct current and feed the batteries. The diodes, in turn, are mounted directly on to the mounting bracket to dissipate heat.

Becker put a lot of thought into the turbine’s mounting bracket and connection points, being aware that powerful forces come into play when the turbine is spinning under load.

The four 12 mm stainless steel bolts connecting the rotors distribute the torque and take the pressure off the stationary and decidedly weaker central axle, while two sturdy automotive bearings keep everything running with minimal friction.

The free-swinging windvane acts like a shock absorber, preventing the turbine from snapping around in strong gusts of wind and causing the blades to flex.