When an induction motor is used as a generator, an AC current is required for producing the magnetic field therein.  The windmill's electrical power does not --- repeat, does not --- provide that current.  (It can't because it's out of phase by 90 degrees.)  Therefore, the utility that is required to buy the expensive power from the windmill is also required to provide it (at no cost) with the current needed to excite the magnetic field.

At the north end of Hawaii's main island, when the wind blows well, the windmills crank up fully, and it becomes necessary for the utility to run an auxiliary diesel generator for that purpose.

UPDATE 8/16/01  Better generators.

By electronically controlling the magnetic field in a turbine, it is possible to produce 60-Hz electricity regardless of the rotation rate of the turbine.  There is nothing particularly new here, but it has been found by utilities that the additional cost of using the technique provides a benefit in performance, hence revenues.

The Lake Benton project in Minnesota uses Enron type Z-750 turbines with a 750-kW nameplate rating, for a 107 MW capacity.  A total of 143 units are expected to produce 327,000 MWh per year.  This amounts to a 37% capacity factor.

The tip speed is typically 50 to 70 meters per second (110 to 150 miles per hour).