Absolute humidity

 

in air ranges from zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C. The humidity of a volume of air affects the density of that volume and therefore its wind power potential.

 

Air density

 

The mass of air per unit volume of Earth's atmosphere. The wind power potential of a moving volume of air is proportional to its air density.

 

Air Gap

 

is the distance between the rotor (rotating part) and the stator (staionary part) in a permanent magnet alternator. The air gap has important effects and is generally as small as possible, as a large gap has a strong negative effect on the performance of an electric motor.

 

Airfoil

 

The shape of the blade cross-section, which for most modern horizontal-axis wind turbines is designed to enhance the 'lift' and improve turbine performance.

 

Alternator

 

A device that produces Alternating Current from the rotation of a shaft.

 

Anemometer

 

An instrument designed to record wind speed. Commonly used to measure observed wind conditions at a location as a means of assessing its potential for wind power production.

 

Angle of Attack

 

The angle of relative air flow to the blade chord (blade cord is the imaginery straight line between the leading and trailing edges of the blade).

 

Area of a Circle

 

Pi (approx 3.14) multiplied by the Radius squared.

 

Armature

 

The moving part of an alternator, generator or motor. In many Permanent Magnet (PM) alternator designs, it carries the magnets and is attached to the blades and hub. Also called a Rotor.

 

Average wind speed

 

The mean wind speed over a specified period of time.

 

Axial Alternator

 

An alternator design where a flat disc carrying magnets on the face (the Armature) rotates near a flat disc carrying coils (the Stator).

 

Axis

 

The centre line of a rotating object's movement.

 

Balancing

 

Adjusting the weight and weight distribution through 2 axes of wind turbine blades so that all blades are the same. Unbalanced blades create damaging vibration.

 

Battery Bank

 

An array of Batteries connected in series, parallel, or both.

 

Bearing

 

A device that transfers a force to structural supports. In a wind generator, bearings allow the Shaft to rotate freely, and allow the machine to Yaw into and out of the wind.

 

Belt

 

A device for transferring power from a rotating shaft to a generator. Allows the use of Pulleys to change the ratio of shaft speed to and from the generator.

 

Betz Coefficient

 

59.3%. This is the theoretical maximum efficiency at which a wind generator can operate, by slowing the wind down. If the wind generator slows the wind down too much, air piles up in front of the blades and cannot be used for extracting energy.

 

Betz' Law

 

The theory advanced by German physicist Alfred Betz that no more than 59.3% of the kinetic energy of a parcel of moving air (wind) can be converted into mechanical energy via the rotors of a wind turbine.

 

Blades

 

Most turbines have either two or three blades. Wind blowing over the blades causes the blades to 'lift' and rotate.

 

Brake

 

A disc brake that can be applied mechanically, electrically or hydraulically to stop the rotor in emergencies.

 

Brakedrum Windmill

 

A home-built wind generator design by Hugh Piggott of Scotland.

 

Braking System

 

A system (electrical or mechanical) used to slow the blade rotation of a wind turbine.

 

Bridge Rectifier

 

An array of diodes used to convert Alternating Current to Direct Current. Single-phase bridge rectifiers use 4 diodes, 3-phase bridge rectifiers use 6 diodes.

 

Brushes

 

Devices for transferring power to or from a rotating object. Usually made of carbon-graphite.

 

Ceramic Magnets

 

See Ferrite Magnets.

 

Chord

 

The width of a wind turbine blade at a given location along its length.

 

Coercivity

 

The amount of power needed to magnetise or demagnetise a permanent magnet. Measured in MegaGauss Oersted (MGO)

 

Cogging

 

The cyclic physical resistance felt in some alternator designs from magnets passing the coils and gaps in the laminates. Detrimental to Start-up.

 

Commutator

 

The rotating part of a DC generator.

 

Concave

 

A surface curved like the interior of a circle or sphere.

 

Controller

 

The controller starts the wind generator at wind speeds of around 8 to 16 miles per hour (mph) and shuts them off at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because of possible damage by high winds.

 

Converter

 

See Inverter.

 

Convex

 

A surface curved like the exterior of a circle or sphere.

 

Cowling

 

See Nacelle.

 

Cut-In

 

The rotational speed at which an alternator or generator starts generating enough electricity to make it flow in a circuit.

 

Cut-out wind speed

 

The wind speed at which a wind turbine ceases to generate electricity.

 

Cyanoacrylate

 

A fast-setting, hard and brittle adhesive. See Superglue®.

 

Cycles per Second

 

Measured in Hertz. In electricity, it is the number of times an AC circuit reaches both minimum and maximum values in one second.

 

Darrieus

 

A Vertical Axis Wind Turbine design from the 1920s and 1930s by F.M. Darrieus, a French wind turbine designer.

 

DC

 

See Direct Current

 

Decibel (dB)

 

A unit of measurement used to compare the magnitude of one physical quantity to a reference quantity. Commonly used to express the sound levels associated with a wind turbine. Because this uses a logarithmic scale, a sound that is, for example, 3 dB louder than another one is approximately twice as loud as the reference source.

 

Delta

 

A 3-phase alternator wiring configuration in which all phases are connected in Series.

 

Density

 

Mass per unit of volume.

 

Diameter

 

A straight line passing through the centre of a circle, and ending on both edges. Equal to 2 times the Radius.

 

Downwind

 

Refers to a Horizontal Axis Wind Turbine in which the hub and blades point away from the wind direction, the opposite of an Upwind turbine.

 

Drag

 

In a wind generator, the force exerted on an object by moving air. Also refers to a type of wind generator or anemometer design that uses cups instead of a blades with airfoils.

 

Dump Load

 

The disconnection of a powered load - the diversion of wind-generated power when the system batteries are too full to accept more power. This diversion is performed by a Shunt Regulator, which can sink current from the load and still stay in regulation.

 

Duty Cycle

 

In a circuit, the ratio of 'off' time to 'on' time.

 

Dynamo

 

A device that produces Direct Current from a rotating shaft. See Generator.

 

Eddy Currents

 

Currents that flow in a substance from variations in magnetic induction. See also Lenz Effect. Laminates are used to prevent eddy currents, which cause physical and electrical resistance in an alternator or transformer, thereby wasting power.

 

Efficiency

 

The ratio of an output quantity to an input quantity, such as the amount of energy leaving a wind turbine as electricty versus the amount of energy coming in to it as wind.

 

Electromagnet

 

A device made of wire coils that produces a magnetic field when electricity flows through the coils.

 

Epoxy

 

A 2-part adhesive system consisting of resin and hardener. It does not start to harden until the elements are mixed together. NOT compatible with Fiberglass® Resin.

 

Excitation

 

Using an electric current to create a magnetic field. See Electromagnet.

 

Fatigue

 

Stress that causes material failure from repeated, cyclic vibration or stress.

 

Ferrite Magnets

 

Also called Ceramic Magnets. 'Ferrites' are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are used to make permanent magnets, ferrite cores for transformers, and in various other applications.

 

Fiberglas® Resin

 

Another 2-part adhesive system, NOT compatible with Epoxy. Often used for making castings, since it is much cheaper than Epoxy.

 

Field

 

See Magnetic Field

 

Flux

 

See Magnetic Field

 

Freewheeling

 

a wind generator that is NOT connected to a Load is freewheeling, and in danger of self-destruction from overspeeding.

 

Furling

 

A passive protection for the turbine in which the rotor folds up or around the tail vane.

 

Furling Tail

 

A wind generator protection mechanism where the rotor shaft axis is offset horizontally from the yaw axis, and the tail boom is both offset horizontally and hinged diagonally, thus allowing the tail to fold up and in during high winds. This causes the blades to turn out of the wind and thus protecting the machine.

 

Gauss

 

is the standard unit used to measure magnetic fields. Higher Gauss measurements mean more power can be induced to flow in an alternator. Gauss readings can be increased by putting steel behind magnets, stacking magnets, or using larger or higher-grade magnets. A typical fridge magnet carries about 50 gauss, and an fMRI machine about 70,000 gauss. is the standard unit used to measure magnetic fields. A typical fridge magnet carries about 50 gauss, and an fMRI machine about 70,000 gauss.

 

Gear box

 

Gears connect the low-speed shaft to the high-speed shaft and increase rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, which is the rotational speed required by most generators to produce electricity. The gear box is a costly and heavy part of the wind turbine and engineers are exploring 'direct-drive' generators that operate at lower rotational speeds and don’t need gear boxes.

 

Governor

 

A device that regulates the speed of a rotating shaft, either electrically or mechanically.

 

Grid-tie inverter (GTI)

 

A type of inverter that converts the DC electrical current generated by a wind turbine into AC electrical current that can be introduced into the regional electricity grid.

 

Guy Anchor

 

Attaches tower guy wires securely to the earth.

 

Guy Radius

 

The distance between a wind turbine tower and the guy anchors.

 

Guy Wire

 

Attaches a tower to a Guy Anchor and the ground.

 

H-Rotor

 

A Vertical Axis Wind Turbine design.

 

HAWT

 

Horizontal-axis wind turbine.

 

Hertz

 

Frequency measurement. See Cycles per Second

 

Horizontal Axis Wind Turbine (HAWT)

 

A 'normal' wind turbine design, in which the rotor shaft is parallel to the ground, and the blades are perpendicular to the ground.

 

Hub

 

The centre of a wind generator rotor that holds the blades in place and attaches to the shaft.

 

Induction

 

The production of a magnetic field by the proximity of a electric charge or the production of a magnetic field by proximity to an electric charge.

 

Induction Motor

 

An AC motor in which the rotating armature has no electrical connections to it (i.e. no slip rings), and consists of alternating plates of aluminum and steel.

 

Inverter

 

A device that converts direct current (DC) to alternating current (AC).

 

Kilowatt (kW)

 

A measure of power (energy per unit of time) equal to 1,000 watts of energy. The kilowatt typically expresses the output power of engines and the power of electric motors and machines as well as the electromagnetic power output of broadcast radio and television transmitters.

 

Kilowatt-Hour (kWh)

 

A measure of energy equal to 1,000 watts of energy per second for a one hour period, equivalent to 3.6 megajoules. Commonly used to quantify the amount of energy generated (or consumed) over a specifed period of time.

 

Laminar flow

 

The flow of air (or fluid) that is 'smooth' and without signficant variation in speed or direction. Laminar air flow is steady and consistent, so wind turbines are more efficient at converting laminar flow into energy than converting rough (turbulent) air flow. Thus, wind turbines are ideally placed in locations characterized by laminar flow.

 

Laminations

 

Electrical circuit core parts, found in motors, generators, alternators and transformers. When core parts are subjected to alternating electrical or magnetic fields, the build-up of Eddy Currents causes physical and electrical power loss. Laminations are made of thin strips of materials are used to reduce power losses from this effect. Each strip is insulated electrically from the next.

 

Leading Edge

 

The edge of a blade that faces toward the direction of rotation.

 

Leeward

 

Away from the direction from which the wind blows.

 

Lenz Effect

 

See also Eddy Currents. From H.F.E Lenz in 1833. Electromotive force is induced with variations in magnetic flux. It can be demonstrated physically in many different ways - for example dragging a strong magnet over an aluminum or copper plate, or shorting the terminals of a PM alternator and rotating the shaft by hand. Laminates are used to reduce power losses from this effect.

 

Lift

 

The component of force generated by the flow of an object through a air (or liquid) that acts perpendicular to the direction of motion, such as the lift generated by an airplane wing or wind turbine rotor when it moves through the air. In a wind turbine, the lift generated by a moving blade increases the rotational speed of the rotors, thereby increasing the amount of electricity generated, so a large amount of lift is desirable.

 

Load

 

Something physical or electrical that absorbs energy. A wind generator that is connected to a battery bank is loaded. A disconnected wind generator is NOT loaded. Unloaded blades are free to spin at very high speed without absorbing any energy from the wind, and there is the danger of destruction from overspeeding.

 

Losses

 

Power that is harvested by a wind generator but is not transferred to a usable form. Losses can be through a number of causes including from friction or electrical resistance.

 

Magnet

 

A body that attracts ferromagnetic materials. Can be a Permanent magnet, Temporary Magnet or Electromagnet.

 

Magnet Wire

 

The kind of wire that is always used in making electromagnets, alternators, generators and motors. Uses very thin enamel insulation to minimise thickness and maximise resistance to heat.

 

Magnetic Circuit

 

The path in which magnetic flux flows from one magnet pole to the other.

 

Magnetic Field

 

Magnetic fields are historically described in terms of their effect on electric charges. A moving electric charge, such as an electron, will accelerate in the presence of a magnetic field, causing it to change velocity and its direction of travel. An electrically charged particle moving in a magnetic field will experience a force (known as the Lorentz force) pushing it in a direction perpendicular to the magnetic field and the direction of motion. Also called magnetic flux.

 

Magnetite

 

A common Iron-containing mineral with ferromagnetic properties.

 

Maximum Energy Product

 

Determines how good a magnet that different materials can make. Technically, the amount of energy that a material can supply to an external magnetic circuit when operating within its demagnetisation curve.

 

MegaGauss Oersted

 

Magnetic force measurement, see Maximum Energy Product above.

 

MGOe

 

See MegaGauss Oersted.

 

Micrositing

 

resource assessment tool used to determine the exact position of one or more wind turbines on a parcel of land to optimize the power production.

 

Moment

 

A force attempting to produce motion around an axis.

 

MW

 

Megawatt, a measurement of power (1,000,000 Watts).

 

Nacelle

 

The nacelle sits on top of the tower and contains the gear box, low- and high-speed shafts, generator, controller and brake. Some nacelles are large enough for a helicopter to land on.

 

NdFeB

 

See Neodymium-Iron-Boron Magnet.

 

Neodymium-Iron-Boron Magnet

 

The composition of the most powerful Permanent Magnets known to man. The materials are mined, processed, and sintered into shape. Then, they are subjected to an extremely strong magnetic field to become Permanent Magnets.

 

Open-Circuit Voltage

 

The voltage that an alternator or generator produces when it is NOT connected to a Load.

 

Permanent Magnet

 

A material that retains its magnetic properties after an external magnetic field is removed.

 

Permanent Magnet Alternator

 

An Alternator that uses moving permanent magnets instead of Electromagnets to induce current in coils of wire.

 

Phase

 

The timing of AC current cycles in different wires. 3-phase alternators produce current that is cyclically timed between 3 different wires and a common wire, while single phase produces it in only 1 wire and a common wire. In a 3-phase alternator, wire #1 receives a voltage peak, then wire #2 receives a peak, then wire #3, and so on.

 

Pillow Blocks

 

Bearings that support a horizontal shaft.

 

Pitch

 

Blades are turned or pitched out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.

 

PM

 

See Permanent Magnet.

 

PMA

 

See Permanent Magnet Alternator.

 

Poles

 

A way of picturing magnetic phenomena. All magnets are considered to be 'dipoles', having both a North pole (which would point North if used in a compass) and a South pole (which would point South if used in a compass. In an alternator, generator or motor the number of Poles is a measure of how many coils, permanent magnets or electromagnets are in the armature or stator.

 

Power coefficient

 

The ratio of the power extracted by a wind turbine to the power available in the wind stream.

 

Power Curve

 

A graphical depiction that shows the power output of a wind turbine generator as a function of the wind speed.

 

Power Output

 

The amount of power produced by a wind turbine generate for a given wind speed.

 

Propeller

 

The 'spinning thing' that makes an airplane move forward. Often incorrectly used (by Otherpower.com also!) to describe a wind turbine Rotor.

 

PUC

 

Public Utility Commission, a state agency that regulates utilities. In some areas known as Public Service Commission (PSC).

 

Pulley

 

A device for transferring power when using Belts as Gearing. Changing to smaller or larger Pulleys changes the gear ratio, and can be used to make a shaft turn faster or slower.

 

Pulse Width Modulation (abbrev. PWM)

 

A regulation method based on Duty Cycle. At full power, a pulse-width-modulated circuit provides electricity 100 percent of the time. At half power, the PWM is on half the time and off half the time. The speed of this alternation is generally very fast. Used in both solar and wind regulators to efficiently provide regulation.

 

PWM

 

See Pulse Width Modulation.

 

Radial

 

An alternator design in which the armature magnets are attached to the outside circumference of a disc, with the stator coils mounted around the outside.

 

Radius

 

The distance between the centre of a circle and its outside perimeter.

 

Rare-Earth Magnets

 

See Neodymium-Iron-Boron magnets.

 

Rated Power Output

 

Used by wind generator manufacturers to provide a baseline for measuring turbine performance. Rated output may vary by manufacturer. For example, one manufacturer's 1500 watt turbine may produce that amount of power at a 30 mph windspeed, while another brand of 1500 watt turbine may not make 1500 Watts until it gets a 40 mph windspeed! So read manufacturers' ratings statements very carefully.

 

Rated Wind Speed

 

The selected wind speed at which a wind turbine's rated power is calculated.

 

Rayleigh Distribution

 

A statistical distribution of physical values over a spectrum of possible values, commonly used to describe the distribution of wind speeds in nature. The Rayleigh distriubtion is less accurate but simpler to calculate than the related Weibull distribution.

 

Relative Humidity

 

The ratio of the partial pressure of water vapour in a parcel to the vapour pressure of water if that parcel were saturated, expressed as a percent (0-100%). Commonly used as a measure of the density of a volume of air (warm moist air is less dense than cold dry air) and therefore related to the energy that can be generated by a wind turbine.

 

Relay

 

An electromechanical switch that uses a small amount of incoming electricity to charge an electromagnet, which physically pulls down a connecting switch to complete a circuit. This allows a low-power circuit to divert the electricity in a high-power circuit.

 

Root

 

The area of a blade nearest to the hub. Generally the thickest and widest part of the blade.

 

Rotor diameter

 

The diameter of the circle swept by the rotor.

 

Rotor Shaft

 

The rotating shaft to which the rotating blades of a wind turbine are attached. The rotation of the Rotor Shaft as part of a generator assembly is what allows the rotational energy of the blades to be translated into mechanical and then electrical energy.

 

Rotor Speed

 

The rotational speed of a wind turbine rotor, commonly expressed in revolutions per minute.

 

RPM

 

Revolutions Per Minute. The number of times a shaft completes a full revolution in one minute.

 

Savonius

 

A vertical-axis wind turbine design by S.J. Savonius of Finland in the 1920s and 30s. Shaped like a barrel split from end to end and offset along the cut. They are 'drag machines', and thus give very low rpm but lots of torque.

 

Series

 

In DC electrical circuits such as a battery bank or solar panel array, this is a connection where all the negative terminals are connected to the neighboring positive terminals. Voltage increases, but amperage stays the same. In AC circuits such as a wind generator alternator, each coil is connected to the one next to it, and so on, again increasing voltage but leaving amperage the same. Opposite of Parallel. See also Delta.

 

Servo Motor

 

A motor used for motion control in robots, hard disc drives, etc. Generally designed more like an alternator than a standard motor, most Servos need special control circuitry to make them rotate electrically. Some can be used in reverse to generate alternating current.

 

Setting Angle

 

The angle between the blade Chord and the plane of the blade's rotation. Also called Pitch or blade angle. A blade carved with a Twist has a different setting angle at the Tip than at the Root.

 

Shaft

 

The rotating part in the centre of a wind generator or motor that transfers power.

 

Shunt Regulator

 

A bypass device for power not needed for charging batteries. When batteries are full, the regulator shunts all or part of the excess power to a Dump Load to protect the batteries from damage by over-charging.

 

Slip Ring

 

Devices used to transfer electricity to or from rotating parts. Used in wound-field alternators, motors, and in some wind generator yaw assemblies.

 

Star

 

A coil connection scheme for 3 phase alternators and generators in which all 3 coil phases are connected in parallel - they all share a common connection.

 

Start-Up

 

The windspeed at which a wind turbine rotor starts to rotate. It does not necessarily produce any power until it reaches cut-in speed.

 

Stationary

 

With wind generator towers, this is a tower that does not tilt up or down. The tower must be climbed or accessed with a crane to install or service equipment at the top.

 

Stator

 

The part of a motor, generator or alternator that does not rotate. In permanent magnet alternators it holds the coils and laminates.

 

SuperGlue®

 

Cyanoacrylate adhesive. Fast bonding glue, easy to find in different viscosities. Sets on its own, and sets instantly when sprayed with an accelerator chemical. Hard, but somewhat brittle. Does not react adversely with Fiberglass® resin or epoxy.

 

Swept Area

 

The area of air that is swept by the rotors of a wind turbine, commonly expressed in square meters or square feet. The power output of a wind turbine is directly related to the area swept by its blades, so longer blades lead to greater power generated.

 

Tail

 

See Vane. The proper term is actually Vane, but Tail is commonly used.

 

Tail Boom

 

A strut that holds the tail (Vane) to the wind generator frame.

 

Tape Drive Motor

 

A type of permanent magnet DC motor often used as a generator in small wind generator systems.

 

Taper

 

The change in wind turbine blade width (chord) along the length.

 

Temporary Magnet

 

A material that shows magnetic properties only while exposed to an external magnetic field.

 

Thrust

 

In a wind generator, wind forces pushing back against the rotor. Wind generator bearings must be designed to handle thrust or they will fail.

 

Thrust Bearing

 

A bearing that is designed to handle axial forces along the centreline of the shaft in a wind generator. This is the force of the wind pushing back against the blades.

 

Tilt-Up

 

A tower that is hinged at the base and tilted up into position using a gin pole and winch or vehicle. Wind turbines on tilt-up towers can be serviced on the ground, with no climbing required.

 

Tip

 

The end of a wind generator blade farthest from the hub.

 

Tip-speed ratio

 

The speed at the tip of the rotor blade as it moves through the air divided by the wind velocity. This is typically a design requirement for the turbine.

 

Torque

 

Turning force, equal to force times radius. See also Moment.

 

Tower

 

A structure that supports a wind generator, usually high in the air.

 

Trailing Edge

 

The edge of a blade that faces away from the direction of rotation.

 

Turbulent Flow

 

The 'rough' and uneven flow of air (or fluids), characterised by irregular variations in wind speed and wind direction. Turbulent air flow is inconsistent and chaotic, so wind turbines are less efficient at converting turbulent air flow into energy than they are at converting smooth (laminar) air flow. Consequently, it is common practice to try to place wind turbines as far away from sources of turbulence as possible.

 

Turn

 

In winding stator coils, this is one loop of wire around a form. A coil will often be referred to by how many turns of a certain gauge wire are in each coil.

 

Twist

 

In a wind generator blade, the difference in Pitch between the blade root and the blade tip. Generally, the twist allows more Pitch at the blade root for easier Startup, and less Pitch at the tip for better high-speed performance.

 

Upwind

 

On the same side as the direction from which the wind is blowing—windward.

 

Vane

 

A large, flat piece of material used to align a wind turbine rotor correctly into the wind. Usually mounted vertically on the tail boom. Sometimes called a Tail.

 

Variable Pitch

 

A type of wind turbine rotor where the attack angle of the blades can be adjusted either automatically or manually.

 

Vertical Axis Wind Turbine (VAWT)

 

A wind turbine design in which the rotor shaft is perpendicular to the ground.

 

Weibull Distribution

 

A statistical distribution of physical values over a spectrum of possible values, commonly used to describe the distribution of wind speeds in nature. The Weibull distribution is more accurate but harder to calculate than the related Rayleigh distribution.

 

Wild AC

 

Alternating Current that varies in Frequency.

 

Wind Direction

 

In meteorological convention, the compass direction that winds are blowing from. Thus a North East wind is blowing from the North East.

 

Wind Direction

 

This is an 'upwind' turbine, so-called because it operates facing into the wind. Other turbines are designed to run 'downwind,' facing away from the wind.

 

Wind Farm

 

A group of wind turbines, often owned and maintained by one company. Also known as a wind power plant.

 

Wind Generator

 

A device that captures the force of the wind to provide rotational motion to produce power using an alternator or generator.

 

Wind Turbine

 

A machine that captures the force of the wind. Called a Wind Generator when used to produce electricity. Called a Windmill when used to crush grain or pump water.

 

Wind vane

 

Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

 

Windmill

 

A device that uses wind power to mill grain into flour. But informally sometimes used as a synonym for wind generator or wind turbine, and to describe machines that pump water with wind power.

 

Windward

 

Toward the direction from which the wind blows.

 

Yaw

 

Rotation parallel to the ground. A wind generator Yaws to face winds coming from different directions.

 

Yaw Axis

 

Vertical axis through the centre of gravity.

 

Yaw drive

 

Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don’t require a yaw drive, as the wind blows the rotor downwind.