Air gap is the distance between the rotor outside and the stator inside diameter. Air gap monitoring of hydro generators, motors and gearless mill drives is important because the stator and the rotor geometry can be quite flexible, and their shape and location are significantly influenced by operating conditions (eg. centrifugal and magnetic forces, thermal effects and structure stiffness failures).

The fact that generated energy is transferred from rotor to stator through air gap, makes reliable identification of conditions in the air gap one of most important tasks in machine behaviour control.