I am making a little dc generator. The generator is arranged in an axial flux configuration, similar to the ones you find in homemade windmills. Similar to this photo, **broken link removed**. I am going to have the coils wrap around iron cores. I was wondering, for the permanent magnets on the rotor, should they only be as large as the iron cores? I am assuming that the coils will only produce voltage from flux traveling through the center of the coils (the core). If the magnet was larger than just the core some of that flux would be traveling through the copper instead of through the center. Would this just be an inefficiency?