The Brushless DC Motor is very efficient. It uses a combination of permanent magnets and electromagnets. Other motors use extra power to create magnetic fields, but their switching is simpler. They use the AC switching to create alternating fields.

The BLDC uses alternating polarity pairs of magnets. Two of these pairs, or 4 magnets, are associated with 3 coils, or electromagnets. The coils are supplied with power through a 3 phase bridge.

The BLDC uses alternating polarity pairs of magnets. Two of these pairs, or 4 magnets, are associated with 3 coils, or electromagnets. The coils are supplied with power through a 3 phase bridge.

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.

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.

The magnet length (direction of rotation) and width should also match the length and width of the coil. The magnets I am using are 25mm wide ferrites, and the coils are 9mm wide. The magnetic field created is quite strong at a distance, but the strength reduces as the gap diminishes, to a point where the polarity reverses. They may need to be changed to 10mm Rare Earth type as used for the levitation.

The ratio of magnets to coils, or electromagnets, is always 4:3 or multiples of these numbers. The F & P motor I am using for coils has 56 magnets and 42 coils. If you divide each number by 14 then you have the 4:3 ratio.