DC servo brushed motors generally have two sets of brushes set at right angles, you will find two Pairs PM poles, this together with skewed rotor laminations , increased number of armature poles, and brushes that overlap the commutator bars, create a very smooth rotary motion down to extremely low rpm.
In the case of AC servo's, there are two distinct type that are constructed almost identical, the BLDC Brushless DC and AC Sinusoidal.
Think of a BLDC as a brushed motor turned inside out, the commutation is now electronic, and the PM fields are on the armature.
The (old) hall effect detectors are used to detect the position of the permanent magnet position of the rotor. So the correct stator winding can be energised.
If the BLDC has one pair of poles then there is one electrical revolution/pair of poles for every mechanical revolution. Every pair of poles will increase this and create better resolution, (less cogging).
Nowadays the encoder and the 'Hall effect' tracks are combined, (see Renco site for details).
Although a BLDC and a AC servo are constructed as a 3ph motor, it is called 'Brushless DC' due to only two windings energised at any given time.
With a 3ph sinusoidal, it is a true 3phase motor and will give excellent response down to zero rpm.
The beauty of BLDC and AC servo is the lower inertia and the fact the windings are on the stator, so cooling can be maximized, (smaller motor).
I would consider a BLDC the easiest motor to make, but Magnets are going to be a problem for home built, whatever the type.
Another thing to consider when dismantling a servo motor (although modern ones do not suffer as much) is that you can get PM loss when you remove the armature as it act as a keeper, so you may want to insert a ferrous metal bar in the the armature space while the motor is dismantled.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.