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Green Collar Careers - Wind Turbine System Overview

Nacelle is the official wind industry term for the unit containing: Turbine Blade Hub & Bearings Generator/Alternator Gearbox (if equipped) A gear reduction and direct drive (gearless) units are shown to the right.   Currently gear reduction units are selling at lower cost although that will soon change as the direct drive units cost less to produce.   Direct drive units usually include the inverter unit as an integral component of the nacelle.  Most wind turbines require a form of braking or retarding when high wind conditions persist.  Some direct drive systems will use the inverter to retard the turbine speed which is the most efficient way to control overspeed events.

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Gearboxes are generally lubricated with grease to simplify sealing and minimize leakage.  Grease tends to thicken in lower temperatures and adds a significant drag below about 20° F.  Where oil may be used in some gearboxes seals are crucial to maintain.  Grease will tend to last longer between changes where oil will need to be changed periodically. 

Direct drive has no gearbox and the only lubricated parts are the sealed bearings.  These will generally last 20 years.

DC output systems are almost always a brush type generator.  Brushes are a wear component adding to service requirements.

AC output systems use alternators and don't require brushes.  Inductively coupled alternators require a circuit that controls excitation voltage; this circuit may be integrated into an inverter which will be mounted inside the nacelle.  Inductively coupled alternators will require a gearbox with at least a 10:1 ratio. 

Permanent Magnet alternators can operate at direct drive speeds.  Inverters with an integrated control circuit will be installed in the nacelle.
 

As the wind direction changes the nacelle will need to rotate to point the turbine blades into the wind.  Directional changes are known as "yaw".  Most nacelles have a rotary yaw table on the bottom side for mounting to the tower.  Nacelle units equipped with a tail fin position the unit so the wind strikes the blades and the tower is downstream (leading), where units without a fin position the blades downstream of the tower pole (trailing).  Both leading and trailing flows have their benefits. Towers come in a variety of styles and heights. Monopole towers have the least maintenance and highest price Guy Wire poles are the least expensive and the highest maintenance Telescoping poles are the easiest to erect and some allow easy nacelle access Heights range from 20 - 100 feet.