An idea occured to me after a visit to the National Windpower internet
site. I got there searching on 'induction generators' - a topic I
have had an interest in.  One of the articles which met my search
criteria explained that direct drive permanant magnet generators were
proposed to replace current technology which utilizes induction generators
- powered through a speedup drive which causes mechanical losses.
 
The induction generators, the article explained, typically are 1800 rpm
motors connected to the grid, and so require  6:1 speed up gearing to match 
a typical the windmill prop speed.  Another article at the same site 
discussed the benefits of variable speed units to optimise energy 
extraction.
 
My thought was: The frequency at which induction generators operate is
set by the excitation frequency. In the example, that is the 60hz grid.
But variable frequency AC drives to power motors are readily available.
  
So..
  
Why not use a variable frequency drive to excite an induction generator?
Motors w/ nominal 60hz rpm of 900 are a standard offering and variable
frequency drives can run down to 20% of line frequency.  This would
allow prop speeds down to 180rpm's.  A tach on the prop shaft to send
a control signal to adjust the excitation frequency of the VSD could
control the system for maximum efficiency.
 
Some, maybe most, VSD's advertise that breaking energy is returned to
the power supply system. (Braking energy here would be the windmill's
output.)
 
So this is a variable speed direct drive and the best of both ideas
without the need for an expensive special generator, no?
 
JMcA
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