With very large electric motors (diameter larger than 10m) it is important for reasons of cost effectiveness and energy efficiency, to know the radial run out of the rotor from the stator inside the motor. These electric motors are intended for use in rock grinding mills for cement production or mining. Due to imbalances during operation, the rotor may touch the stator, which would cause much damage. This is why sensors are used to measure the distance between the stator and rotor and to monitor the so-called rotor gap whilst the motor is in operation. For this application, capacitive sensors with a measuring range of approx. 30mm are used.