In some cases, a gearbox is warranted, to increase the shaft speed from the blades to a higher shaft speed for the generator. On home-designed machines, this is sometimes a relatively inefficient belt and pulley arrangement, which will waste a lot of energy. On manufactured machines, a gearbox is used to increase the rpm for the generator. A gearbox adds to the mechanics of the system, which means more wear and maintenance—regular oil changes and, eventually, gear replacement. But it may be a worthwhile trade-off—if you have a good wind resource—to get good matching and for using conventional and economical generators, designed for higher speeds than we want our blades spinning. (Larger-diameter blades need to spin slower so they don’t self-destruct from centrifugal force.) Although mostly we see direct drive in home-scale turbines and, historically, more gear-driven utility-scale machines, more home-scale wind generators are using gearboxes.
A generator or alternator moves magnetic fields past wire coils or vice versa, which makes electrons move in the wire. You may remember doing some science experiments as a child that showed this—it’s not hard to measure voltage while waving a magnet past a loop of wire.
With generators, the goal is to move lots of magnetism past lots of wire to move lots of electrons. In one way or another, this is what generating devices are doing. There are three primary configurations, each of which has its own permutations and variations.
Wound-field alternators have a set of copper coils that are typically fixed (the “stator”)—these are the coils we tap to harvest the electrical energy. They have another set of copper coils that are an electromagnet, and these coils typically spin past the fixed coils. These alternators are not very common today—we find them mostly in older designs. However, if they are well-designed, the magnetism (“field” or “excitation”) can be very well matched to the wind speed. This means that the loading of the alternator can be matched with the available wind. One drawback of this design is that some of the wind energy is used to induce magnetism, so that energy is not available to make electricity.
Permanent-magnet (PM) alternators follow the principle of moving magnetism past copper coils, but use permanent magnets—metallic materials that have stable magnetic properties. Older turbines use ferrite magnets; newer machines tend to use magnets of neodymium, a strongly magnetic material found primarily in China that commands very high prices.
PM alternators can be configured in a variety of ways, with coils spinning around magnets, magnets spinning around coils (most common), or in “axial” designs where the magnets and coils face each other in a disk-like arrangement, and the magnets typically spin.
Most home-scale wind turbines use PM alternators, which are simple, reliable, and economical. One minor drawback is that the magnetic strength is fixed, and not optimized for maximum power production at each wind speed. But lately, wind electronics engineers have been experimenting with voltage converters that adjust the balance between wind energy in and loading/generation, which overcomes this drawback somewhat to maximize production.
Induction generators use a cage, or conductive bars, spinning relative to groups of coils. A rotating magnetic field is created by feeding the stator coils with alternating current from the grid. This field interacts with the cage in the rotor to produce currents that make an opposing magnetic field, setting the rotor in motion. If the rotor is forced (by the wind) to turn faster than the magnetic field produced by the grid, then instead of drawing power, the device sends energy out to the grid.