In the air gap monitoring, the distance between the outer rotor diameter and the stator inside diameter is measured. The shape and location of the rotor and stator are affected by centrifugal, thermal and magnetic forces. Air gap monitoring can help identify problems of misalignment, magnetic imbalance, short circuit between rotor turns, mechanical pole instability and stator movements that decrease operating efficiency and could lead to damage caused by magnetically induced heating or rotor –stator friction.