These “generators” are the same as induction motors, and can be powered up by applying an electrical source to them. This is used in utility-scale and some home-scale machines to spin a turbine up to speed when the sensors and electronics show that there’s enough wind to capture. Once the “motor” is going, the wind applied to the blades then pushes it beyond the amount of energy used just to generate energy.
One beauty of induction machines is that they need no inverter to connect to the utility grid. Electronics are necessary to facilitate and safeguard the connection, but the equipment is simpler and less costly than the inverter needed when you connect a wound-field or PM turbine to the grid.
A wind generator needs to face the wind so that the blades are oriented to capture the wind efficiently. Home-scale horizontal-axis wind generators have a “yaw bearing,” which allows the wind generator to swivel and face the wind—from whatever direction it is coming. This turning is called “yawing.”
Yawing can be either active or passive. Larger home-scale machines and most utility-scale machines are active yaw, using motors and gears to turn the machine head around to face the wind, with wind vanes sensing the wind direction. Most home-scale machines have passive yawing, which uses the structural design of the machine to orient it into the wind.
With upwind machines (blades are upwind of the tower), a tail sticks out behind the machine, and the force of the wind on it pushes the rotor into a position facing the wind. With downwind machines (blades are downwind of the tower), the rotor acts as the tail to yaw the machine properly.
Because a wind generator yaws, but its tower is fixed, we need a mechanism to transmit the electrical output from the rotating portion to the fixed. Utility-scale machines and some home-built machines allow the wires to be twisted, and either a motor or the owner periodically untwists them. To transmit the energy on most home-scale machines, slip rings—two or more bronze rings that are on the fixed portion of the machine/tower, and graphite brushes that ride on these rings—create a connection for the energy and allow swiveling without twisting wires.
Wires relay energy down the tower to the electronics, batteries (if used), home, and/or utility grid. Transmission may be DC, or as wild, multiphase AC. There’s not a great deal of difference between the two in efficiency or cost; it is higher voltage that gives better efficiency. A big advantage to running AC down the tower is that the rectifiers that convert AC to DC are at ground level, where they are easier to troubleshoot and replace if needed.
A well-designed machine must be able to govern—shed wind in some way to reduce high winds’ force on the turbine and tower. Shedding wind energy also reduces the speed of the turbine, avoiding mechanical or even electrical failure. There are three primary methods of governing, with variations in each.
Furling tilts the whole blade rotor either to the side, up like a helicopter, or in a combination of those two, to reduce the area exposed to the wind. This is a very common governing strategy, and is effective and easy to build. They’re a bit of an art to design, and governing systems of this sort can be sluggish in responding to high wind.