RE: Coreless Servos
That is an AC motor with a built in DC to AC converter.
There are several ways to get a brushless motor to run if you convert the DC to AC. The easiest way is to convert it to 3 phase AC and use a squrrel cage rotor. You then have a synchronous 3 phase motor that you can control the speed by changing the frequency of the converter. Its a very efficient motor if you don't lose too much in the conversion process.
In a permanent magnet motor the magnet fixed. There is no way to get the magnetic flux developed by the direct current to reverse without swiching the current direction. You have to get the magnetic flux to reverse or the motor will only rotate 180 degrees and stop
I repeat:
If you have found a way to eliminate the brushes in a DC motor you have made the greatest discovery since the Wright Brothers.
Run out quickly and get a patent on it.
Lets get back to the original question.
A coreless motor is an inside out motor. The magnet is placed inside the armeture and is fixed to the motor end piece. The armeture is wound in a cup shape and fitted over the magnet and supported by a bearing in the opposite motor end piece so it is held close to the magnet without touching it. The armeture is wound from copper wire held together with epoxi and contains no metal other than the copper winding. The outside of the motor is a metal cylinder fitted over the armeture and the magnet in such a way that the armeture is in air space betweem the magnet and the outside cylinder.
The motor is more efficient because the armeture has less inertia as there is less rotating metal. It is less reliable because the armeture can only be supported at one end. The armeture still carries current that must be switched.