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The detection of rotor asymmetries in squirrel-cage induction motors has been an important research topic during the last two decades that has given rise to the development of various diagnostic techniques. Despite these techniques being reliable and providing much information about a machine's state, they still lack the capability to detect a failure when the motor is driving a machine, producing oscillations in the load torque. If this happens, then the motor phase currents are modulated by torque oscillations and the information that they contain about the integrity of the bars and endrings is no longer correct. Currently, the authors are not aware of a simple and reliable method that exists for the industrial diagnosis of rotor asymmetries in working cage motors under arbitrary load conditions. This paper describes a new diagnostic method and a new portable electronic instrument that are based on the measurement of the magnetic flux linked by one stator tooth, which allows for an optimal simple discrimination between the actual presence of rotor asymmetries and the spurious effects that are caused by the oscillations in the load torque of the driven machine.