Novel Method for Direct Measurement of Air Gap Anomalies in Direct-Drive Electrical Motors

This 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 that 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 in either a static or dynamic state. It uses an optical sensory (OS) 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 analog Hall sensory (AHS) 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 and 2 mm. It covers most applications of the direct-drive electrical motors. The new measuring method is validated by using an experimental setup consisting of the presented OS system and validation system consisting of an AHS system integrated on a testing platform (Smart Fortwo car).