A new electronic instrument for the early detection of broken rotor bars in asynchronous motors working under arbitrary load conditions

Different techniques have been developed for the early detection of rotor asymmetries in squirrel cage asynchronous motors over the last two decades. Although their reliability and the amount of information they provide about the a machine’s state is indubitable, they still have a serious limitation: failure detection when the motor is driving a machine capable of producing oscillations in the load torque. In this case, the motor phase currents are modulated by torque oscillations and the information they contain about the integrity of bars and endrings is hidden or altered. In fact, it can be affirmed that almost all diagnosis methods based on spectral analysis of external electrical variables are affected by this phenomenon. In order to overcome this drawback, attempts have been made to find new procedures leading to new indicators to discriminate between the influence of the load and the rotor failure. Although certain complex systems applied to specific motors for on-line diagnosis have been successfully designed, as far as the authors know, no simple and reliable method exists for industrial diagnosis of rotor asymmetries in working cage motors under arbitrary load conditions. This paper describes a new diagnosis method and a new electronic instrument, based on the measurement of the magnetic flux linked by one stator tooth, which allows perfect, simple discrimination between the actual presence of rotor asymmetries and the spurious effects caused by the oscillations in the load torque of the driven machine.