An investigation of shaft current in a large sleeve bearing induction machine

Irregularities in the magnetic circuits of machines may result in spurious voltages that lead to shaft currents through the shaft, bearings, bearing supports and closing through the machine framework. The IEEE Standard Test Procedure for Polyphase Induction Motors and Generators discusses the shaft current and presents a measurement method for recording either the voltage across the ends of the shaft or the current. This thesis discusses an alternative measurement approach and its application to the identification of shaft current in a large induction machine.; Possible causes of shaft current are discussed. The magnetomotive force-permeance approach is used in the determination of the possible causes.; An eight pole 2611 kW oil ring lubricated induction motor which exhibited signs of bearing damage due to shaft current was used in a series of experiments conducted to evaluate the cause(s) of the bearing damage. The new method of shaft current measurement, known as the Rogowski Coil Method, was applied to this machine. The experiments included oil analysis and microscopic analysis of the bearing. The experiments showed that the main cause of the shaft current is the presence of joints between stator lamination segments. Saturation accentuates the magnitude of shaft current. The Rogowski coil method is shown to be reliable; it does not disturb the shaft current conducting circuit. Furthermore, it requires little maintenance, unlike the ammeter/brush method.; Oil ring delivers oil to the bearing. An oil film will develop between the shaft journal and the bearing. The oil film acts as an insulator against shaft current. It is thus important that sufficient quantity of oil is delivered to the bearing and an oil film is developed as quickly as possible to minimize the shaft current. Hence, the performance of the oil ring is studied. The Rogowski method is used to study the performance of the oil ring. The oil ring is determined to slip early with respect to the shaft. As a result, the amount of oil delivered to the bearing is determined to be less than calculated. The proposed method is simple, reliable and less labor intensive than other methods. Patent application for this method is being filed.; A method of minimizing shaft current is described following the experimental work to determine the cause(s) of the shaft current. Patent application for this method is being filed.; The present on-line monitoring system utilizes the spectral analysis of the stator current to determine stator and rotor faults in alternating current machines. The Rogowski coil method can be used as an alternative on-line monitoring system for such faults.