[Grant-53]

I began building and modify DC permanent magnet motors when first racing slot cars in the 1960's. To stabilize the armature windings at high RPM (+100k ) we would apply epoxy. The field magnets were held in place with spacer clips and the motor shell was used to concentrate the the magnetic field. Air flow was primarily axial. Automotive alternators and most industrial motors I have worked on in robotics follow this practice.

Heat conduction of the stator or field windings passes through the casing to the external heat sinks. Axial air flow may be increased by a fan blade mounted on one end of the armature. This is common on electric drills. Since magnetic field strength follows the inverse square rule great care is given to minimizing the air gap. Dynamically balancing the armature and installing high quality bearings reduces the range of motion of the armature in the air gap.

The inquiry becomes how best to minimize temperature rise in the windings which reduces current flow and shortens motor life through the breakdown of wire insulation. The other question comes in discovering the relative merits of ferrofluids in the air gap. Does the improvement in field strength outweigh the viscous drag as the air gap is reduced?