Magnet to Gap Ratio.
The original F&P Smartdrive motor has a rotor of 250mm, about 786mm circumference. There are 56 individual magnets, each 9mm wide. Each magnet uses 14mm of the circumference. The ratio of magnet to available space is 9:14, the percentage is 64% magnet, 36% gap.
If the gap is too small the sensors will not be able to switch the coils correctly. To give me greater flexibility in the choice and setout of magnets and coils I have devised a system using 2 sets of Hall Effect sensors offset from each other, and a dual decoder. This will allow much longer magnets and coils to be used, with benefits of better efficiency and slower switching times. The arrangement I am currently testing, (Oct 2012), is set at 11% gap, 89% magnet.