Radio-frequency-based detection of electrical discharge machining bearing currents

The possibility of a bearing damage caused by inverter-induced bearing currents in modern variable-speed drive systems is well recognised at present. In this study a non-intrusive method to detect and monitor the frequency of occurrence of these currents is presented. A technique of this kind makes it possible to analyse a certain drive system with respect to the appearance of inverter-induced bearing currents and to estimate the risk they might pose to the bearings. The proposed method is based on measurement of the radio frequency signal emitted by the discharge and transmitted by the machine itself. The theory behind the technique and its realisation is discussed. Exemplary measurements are presented to demonstrate the feasibility of the theory by studying the relationship between the motor shaft speed and the bearing current discharge activity for a 15 kW induction motor.