THE GUIDE TO WIND TURBINES FOR YOUR HOME
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THE BASICS
In a broad sense, a wind turbine is any device that converts the kinetic energy of the wind into other usable forms of energy. Nowadays, this term usually refers to an electro-mechanical system that converts the wind energy to electricity. If the wind energy is converted to mechanical energy that is used directly by machinery, such as a pump or grinding stones, such turbine is more often referred to as a windmill.
TYPES OF WIND TURBINES
There are two main types of the turbines based on the axis on which they rotate: horizontal axis (propeller-style) and vertical-axis ("egg-beater" style). The main advantage of horizontal axis wind turbines (HAWT) is their higher efficiency. Their main disadvantages are the requirement of tall towers, which are difficult to transport and install, and the necessity to constantly align in the direction of the wind. Vertical-axis wind turbines (VAWT) do not have to be pointed into the wind, and their alternator and gearbox can be placed at the bottom of the tower. A VAWT can also be mounted on a flat rooftop, since the building generally redirects an air flow over the roof and this can increase its wind speed at the turbine. However, because of lower efficiency of vertical-axis systems, HAWTs are the more common nowadays. The rotors of most HAWT are located upwind of the tower.
HOW WIND TURBINES WORK
The diagram below shows the inside of a wind turbine. It is an upwind HAWT. The main rotor shaft and alternator at the top of a tower must be pointed into the wind and changes orientation when the wind changes direction. To support the turbine as it swivels on its tower such systems use sealed bearing called the yaw bearing (the swiveling motion is called yawing). Upwind turbines have yaw drive to orient the rotor into the wind. Downwind turbines don't need a yaw drive: the wind itself turns the rotor away from moving air. To connect the output terminals of the moving alternator to the fixed cables that carries the electric current down the tower, most turbines use copper alloy slip rings. The moving part of the turbine has a set of graphite brushes that ride on the slip rings as the turbine yaws.
The anemometer monitors the wind speed and sends the data to the controller, which starts up the system at certain minimum wind speed (typically 8 to 16 mph) and shuts it off the machine at high winds (typically above 50 mph). The rotor assembly includes the blades that convert kinetic energy of moving air to rotational shaft energy. Most modern turbines have three blades. The rotor is connected to the main shaft of the alternator. The alternator converts the rotational energy of the spinning shaft into electrical energy. It contains one or several magnets that spin inside or around stationary coils. When the wind turns the blades, they spin the shaft with magnets, which creates an alternating magnetic field in the coils. The alternating magnetic field in turn induces voltage in the coils according to the Faraday's induction law. In small wind generators for home use, the blade rotor drives the magnet rotor directly. Higher power systems (>10kW) usually have a gearbox that converts the low-speed high-torque incoming rotation of the blade rotor to high-speed low-torque rotation of the alternator rotor.
Most turbines have some means of stopping it. The drawing above shows a mechanical brake. Many turbines have "electrical braking," where a switch disconnects the load and shorts all the three phases of the alternator together.
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