Winding type and carbon brush placement principle
   

3-pole rotor/commutator winding graph (drawing 13)           5-pole rotor/commutator winding graph(drawing 14)
a. Angel between slots of rotor and                                   a. Angle between slots of rotor and
   slots of commutator is 60°                                                slots of the commutator is 0°
b. Coil C is in the process of commutating. It is being         b. Coil B is in the process of commutating. It is being
shortcut by brush at negative terminal.                            shortcut by brush at positive terminal.
c. Brush locates at the centre line of the magnetic poles.   c. Brush locates at the centre line of the magnetic poles.

● Principle of carbon brush placement
 a.  Try to get maximum effective conductors. In other words, make the direction of current the same in as many
conductors under the same pole as possible. In some cases (such as 12-pole rotor), we will sacrifice the number
of effective conductors to improve commutation. Such cases will not be discussed here.
b.   Minimize the electric potential of the commutating coil (the one that is shortcut). Typically the sides of that coil
are placed at the edges of the magnetic poles or between the magnetic poles. So the carbon brushes are
usually placed in the middle of the commutator bars that are connected to the coil.
c.  Electric angle between positive and negative brushes is 180°.
Conclusion: There is not a sole way to connect coil and commutator segments.