Permanent Magnet Synchronous Generators

Permanent Magnet Synchronous Generators are becoming the generators of choice in the wind turbine industry.  They are being increasingly used in place of induction generators (induction machines which run above the synchronous speed).  They are advantageous over induction generators as they have a higher efficiency.   Besides, they do not need a magnetizing current which needs to be fed from the grid.
The normal synchronous generator has a wound rotor with poles.  These poles in the rotor are excited by means of a dc current.  In a permanent magnetic Synchronous generator, the wound rotor poles are replaced with permanent magnets.  

The machine is often directly coupled to the wind turbine or through gears.  The AC output of the synchronous generator which has a variable frequency depending on the turbine speed is converted into DC.  This DC is converted into a sinusoidal AC voltage at the system frequency.  This voltage is then fed to the grid.

Permanent Magnet Synchronous Generators (PMSG) are costlier as the permanent magnets are made of rare earth metals.  NeFeB, an alloy of Neodymium, Iron and Boron is used to make these magnets.  The use of permanent magnets in the rotors minimizes the losses in the rotor and improve efficiency.