For thousands of years, people have been harnessing the energy of the wind. A fascination for wind energy has driven both of us to build, buy, install, and maintain our own machines for the last three decades. Chances are that the first wind-energy users were driven by the same maniacal glee that we experience when we grab energy out of thin air.
The design of home-scale wind-electric generators has been through many permutations and variations, with lots of circles and dead ends. But most of today’s modern wind-electric generators are surprisingly similar. This article will help you understand each part of a typical wind generator, and how it functions in the overall design.
“Rotor” is just a fancy word for a wind turbine’s blade assembly—the part that rotates. (There are actually two rotors—the blade rotor, and the magnet rotor in the alternator or generator, which is driven by the blade rotor.) The wind generator’s blades are the energy collectors. After the wind itself, the circular area that the rotor sweeps is the most important factor in determining how much power the machine can generate. The swept area of the rotor depends on the square of the diameter. Compared to a 5-foot-diameter rotor, a 10-foot one will be twice as wide, and twice as high. Doubling the diameter gives access to four times as much wind, and usually results in four times as much power.
Most modern wind turbines have three blades. Blades are usually made of plastic, often in a composite with fiberglass, or sometimes out of wood. Rotors with more than three blades have more start-up torque, but actually produce less power at high speeds. Two-bladed rotors can work at even higher speeds than their three-bladed counterparts, but can be noisy and also vibrate when the wind changes.
Mechanical power is a combination of speed and torque. Wind-electric generators need to spin at relatively high rpm and at low torque, unlike water-pumping wind machines, which need low rpm and high torque.
For home-scale wind generators, the blade rotor drives the magnet rotor directly. This design is the simplest and most efficient way to collect the wind’s energy. Adding belts, pulleys, gearing, or any other indirect transmission will incur losses, as well as require more maintenance.
Most modern small-scale wind turbines employ permanent magnet alternators (PMAs). Electricity is generated when a magnetic field passes a wire. In a PMA, magnets move relative to coils of wire (windings). In one popular configuration (as shown in the diagram above), the magnet rotor is a rotating “can” that spins outside the coils. The magnets are on the inside (facing the coils) and the blades attach directly to the front of the can. These PMAs usually generate wild three-phase AC, which is not usable by appliances directly. Instead, this energy is rectified (converted to DC electricity). After that, it can be used as DC or inverted to AC electricity for AC appliances. Traditionally, the energy has been stored in batteries, but in many modern wind-electric systems, it is fed straight into the household grid supply.
The tail of a wind generator orients the turbine into the wind. When the wind changes direction, it pushes on one side of the tail, swinging the turbine around to face and collect wind energy. In tail-furling designs, the tail is also involved in protecting the turbine from high winds (see Governing Systems), and tail and boom length are designed carefully for weight and area. Changing these parameters can change the way the machine operates—so don’t do it.