Novel method for direct measurement of air gap anomalies in direct-drive electrical motors

The paper presents a novel measuring system for detecting air gap anomalies in direct-drive electrical motors. The air gap anomalies can be of different types of eccentricity which may in extreme cases result in a mechanical rub between the stator and rotor, potentially leading to a motor breakdown. A novel measuring method is proposed using a sensory system integrated in a motor air gap to measure its width directly either in a static or dynamic state. It uses an optical sensory system to measure reflection of the infrared radiation between the rotor and stator. The method is validated by using a parallel measuring system employing an analogue Hall sensory system that measures the change in the magnetic-flux density. The two measuring systems are calibrated by comparing them to a reference laboratory measuring system consisting of a camera and lenses. The calibration is performed in a laboratory in a motor static state within one mechanical rotation cycle of the rotor. Using different calibration techniques, the optimal system accuracy, with a maximum permissible error of 0.15 mm is obtained, in the measuring range between 0 mm and 2 mm. It covers most applications of the direct-drive electrical motors. The new measuring method is validated by using an experimental set-up consisting of the presented optical sensory system and validation system an analogue Hall integrated on a testing platform (Smart Fortwo car).