What are the benefits of using a permanent magnets for field excitation on a generator/turbine?

Answer 1

I have a small wind turbine which uses permanent magnets instead of a field coil. The main benefit I can see is in keeping the electronics on the ground. As the wind speed increases, the output voltage (and power) from the generator will increase as well. I have a shunt regulator at the batteries to prevent overcharging. Everything outside on the tower is very simple and less likely to break. Other than mechanical breakdown, repairs to the system can usually be made on the ground level.

On a gasoline powered generator, sometimes permanent magnets are turned inside the AC output coils. This eliminates the need for brushes and the voltage regulator board. AC output coils have been designed to supply 120VAC at 3600RPM. If the engine speed changes, the output voltage will change as well. Output voltage will also change with load variations.

I have more generator information online.
http://members.rennlist.org/warren/genfaq.html
http://members.rennlist.org/warren/generator.html

Answer 2

Low power consumption, no power required to excite field. However, the disadvantage is the extra weight and cost.