The air gap, which is also known as the air gap, is used in magnetic circuits to denote the space between the inductor and the ferromagnetic circuit to which it is coupled https://www.mrosupply.com/popular_products/15-hp-56c-motors/ . The flux generated by the magnets must cross the stator to cross their coils. Because of this, the stator winding should be as close as possible to the rotor, thus preventing flow from the north pole to the south pole without crossing the stator winding. Making the stator with grooves is a very efficient way to decrease the air gap size, keeping the winding close enough to the magnets.
Other benefits are the increase of mechanical and electrical stiffness, as well as decrease the thermal resistance which facilitates the dispersion of the heat generated by the Joule losses in the winding. One way to maximize resistance to demagnetization is by determining the air gap. According to values between 0.381 and 0.508 mm will be sufficient to achieve this goal. A minimum distance must also be considered for the determination of the engine air gap in order to ensure that there is no contact between the stator and the rotor during the operation of the motor. In this design a 0.45 mm air gap was used.