We call the gap between the rotor and the stator. The magnetic fields in the air gap are of paramount importance for the operation of alternating current motors, since the energy conversion and voltage induction processes are entirely related to these fields. https://www.mrosupply.com/popular_products/bx-section-cogged-classical-v-belts/ The main characteristics of the lines of a magnetic field are that they are always closed lines, they leave and return to the same point and never intersect; are off the magnet, the lines leave the North Pole and head towards the South Pole; inside the magnet, the lines are oriented from the south pole to the north pole; they exit and enter the direction perpendicular to the surfaces of the poles; in poles the concentration of lines is greater, that is, the greater the concentration of lines, the more intense the magnetic field at a certain point.
The magnetic fields in the other parts of the machine are usually weaker than the air gap and are neglected when we analyze the behavior of the electric motors. The air gap fields are traditionally made based on the concept of magnetomotive force (fmm) and are directly related to the magnetic induction (flux density) in the air gap.