Evaluation and implementation of a system to eliminate arbitrary load effects in current-based monitoring of induction machines

This paper presents a method for removing the load effects from the monitored quantity of the machine. Fault conditions in induction machines cause the magnetic field in the airgap of the machine to be nonuniform. This results in harmonics in the stator current of the motor which can be measured in order to determine the health of the motor. However, variations in the load torque which are not related to the health of the machine typically have exactly the same effect on the load current. Previously presented schemes for current-based condition monitoring ignore the load effect or assume it is known. Therefore, a scheme for determining machine health in the presence of a varying load torque requires some method for separating the two effects. This is accomplished by comparing the actual stator current to a model reference value which includes the load effects. The difference between these two signals provides a filtered quantity, independent of variations of the load, that allows continuous on-line condition monitoring to be conducted without concern for the load condition. Simulation and test results illustrate the effects on the spectrum of the monitored quantity for both constant and eccentric airgaps when in the presence of an oscillating load.