The shaft of a wind turbine carries both rotors—the blades and the rotating part of the alternator. The shaft is suspended in bearings, which are usually sealed permanently. They need to be designed to withstand all the stresses of supporting the blade hub and any vibration from the alternator. Bearings typically last five to ten years.
Wind generators yaw to face the wind, but the transmission wiring is fixed to the ground. This can be a recipe for twisted wires, especially at turbulent sites where the yaw bearing is very free-moving. Most modern turbines use copper alloy slip rings to connect the turbine wiring to the fixed wiring. The slip rings are usually mounted in the part of the turbine that is fixed to the tower top. A set of graphite brushes is mounted on the yawing part of the turbine. The brushes ride on the rings as the turbine yaws, and serve to connect the alternator to the fixed wiring.
A wind turbine needs to be able to follow the wind, changing orientation each time the wind changes direction. The sealed bearing that supports the wind turbine as it swivels on its tower is called the yaw bearing, and the swiveling motion is called yawing.
Braking, as distinct from governing, is the ability to stop the turbine when you choose to. This can be useful when there is a problem with the machine, when you need to work on it, or when you simply don’t need the energy. Manual braking is ideal, since it allows you to stop the machine in all conditions. Drum and disc brakes have been used in a few turbine designs, but most turbines use dynamic, or electrical, braking, where a big switch opens the connection to the grid or batteries, and shorts the three phases of the wind turbine together, making it very stiff to turn. Many wind turbines cannot be slowed by a short circuit once they are already running fast in high-wind conditions. Another method of control is manual furling—the ability to manually crank the tail over to furl the machine out of the wind.
Wiring routed down the tower carries the electricity generated by the turbine to the control room, usually as wild (varying voltage) three-phase AC. A few turbines rectify the output at the tower top, and transmit it as DC. Using thick copper wire reduces the risk of fire and limits power lost as heat. Where the wire run is very long, high-voltage AC transmission to a step-down transformer at the control room can be used to save on wire cost and energy losses. Batteryless grid-tied inverters work with higher voltages than most battery-based systems, so they lose less energy in transmission.
Some wind turbines use a plastic cover (cowling) spaced away from the alternator, which protects it from rain but allows airflow over the alternator for cooling. Many turbines have a separate nose cone, which serves an aesthetic purpose, as well as streamlines the turbine.