>>The websites documenting the utility scale wind turbines - the megawatt class turbines - seem to be forever unable to say why the blade rotors need to turn at variable rates - slow for low wind speeds and fast for high wind speeds.<<
Lift type blades are most efficient when they are (almost) perpendicular to the airflow and the rotational speed matches the wind speed and TSR. With a constant speed generator this would mean a very narrow range of wind speeds that the turbine would operate with any reasonable efficiency.
The early utitlity scale turbines used syncronous generators and fixed blades and spent most of thier time either not producing power or having to yaw out of the wind to keep from overloading the generator or "stalling" the blades.
Varible pitch on the blades allows for a much wider range of wind speeds on a synchronous generator and in the case of more modern generators to overcome the momentum of the gearbox, generator and the blades themselves by pitching them back for better tourque in a reactive mode until the blades reach a speed where the lift properties can take over. Like an airplane below a certain (forward) speed they will not produce any lift. I even had the chance to experiment a few times with this, with the brake set, pitch the blades up to thier normal operating position, release the brake. In a 20MPH wind it took forever (ok, about 4 or 5 minutes) before the blades began to "fly" where it would be less than a minute if the blades were pitched back 25Deg. until the lift effect could take over.
With varible pitch blades and variable speed generators the Z750 could "cut in" in winds as low as 5 or 6 MPH (10kW) and at about 22 to 25MPH be producing 750kW above that the blades pitch back again and it will produce power up to 70MPH when it will "pause out" by pitching the blades so they are paralel to the wind.
>>The conversion of the power produced to 60 Herz AC power also needs to be clarified. <<
VSCFWRIG (variable speed constant frequency wound rotor induction generator) technology was first used in Zond wind turbines until they were bought out by GE. GE wind turbines of 1.5MW and above also use it now. Vestas and other European turbines manufatured over the past few years use a variation of the same technology.
If you look at how an induction motor or generator works it's like a transformer with a rotating secondary winding. If the rotor is moving slower than syncronous speed it will draw power from the line and tend to accelerate it to synchronous speed. If the rotor is moving faster than Syn. then it will tend to be slowed down by the line and produces power in the process. Many of the early utility scale turbines used synchronous generators.
How the VSCFWRIG handles the wide range of rotor speeds is that it's a wound rotor.
The energized stator doesn't react to the windings in the rotor themselves but to the rotating magnetic field they produce.
In the Zond Z750 the synchronous speed of the Gen. is 1200RPM. Below that speed the rotor is modulated by pulsing currnet into it so that the stator "sees" a rotating magnetic field at the proper speed.
Above 1200RPM the rotor is modulated by drawing power off it at the proper phase and magnitude.
The Z750 will "kick in" at about 850RPM and produce about 10kW at that speed, About 15kW coming off the stator and 5kW going into the rotor. (produced power drops to 0 at about 800RPM, the rotor drawing as much as the stator produces at that point) At 1200RPM the Stator is producing 500kW and the rotor is pretty much idle. At speeds of 1200 to 1320 RPM stator power remains at 500kW and the rotor will produce an additional 250kW.
Tourque management and power factor control are 2 other advantages of VSCFWRIG.
Tourque management more efficiently handles lulls and gusts in the wind. Power factor....
>>So much for the corporate and, indeed, international fights over the ownership of the variable blade rotation rate technology and the questions about the quality of the power produced. Wind energy deserves to make its case better and questions like this one need better explanations.<<
Zond and Xantrex (Trace) developed the first practical varible speed technology. Investors put thier money into an idea so that the Engineers could develop it and bring it to market. This is called Capitalism. Some of the basic research may have been funded by taxpayers but the R&D for this technology at least was private funds.
Quality of power or "Power factor" is a major issue when your dealing with multiple Megawatts of power. Phase and impedance matching are held to within 0.1% even on the obsolete Zonds I was working on. Basicaly anything other than a pure sine wave power output in phase with the grid causes heat, stray currents and can take out transformers and substations in severe mismatches. This is one reason why multiple Megawatt inverters for grid feeding are so expensive.
Another reason the utility companies prefer induction type generators is thier inherent safety. If they lose power from the grid they can't produce power, mechanical safety systems automaticaly kick in the bring the turbine to a stop within 10 seconds.
This is usualy presented as a safty issure for the linemen for small grid connected inverters. But imagine being on the "live" side of a breaker opening at a substation and having 100+Mw of power coming into your small town from the nearby wind farm.....
If only one of the 3 phases is lost there are 3 different systems that will shut the turbine down that all react almost as fast.
I wish the political and legal problems with developing large scale wind and other alternative energy resources were as simple as the technology behind them 
