If I understand correctly, a slotless motor is like a hacker, some kontronic motors and some of the latest brushless motors from HiMaxx. The slotless motors use no iron that the copper is wrapped around. The copper windings are simply wound in a special manner to generate the magnetic fields. They can be more efficient that a generic slotted motor, but require a super high frequency speed control. Because the motor cannot "hold" a magnetic force as long as energized iron, the spacing between the pulses must be less. 21 kHz I think is about the average minimum, slotted brushless motors only need about 8 Khz. My guess as to the biggest advantage is no cogging effect which can hurt gearboxes, and reduced weight. I hope I got this right!
