Unambiguous Detection of Broken Bars in Asynchronous Motors by Means of a Flux Measurement-Based Procedure

Different techniques have been developed to detect rotor asymmetries in asynchronous motors. Although the reliability and the amount of information these techniques provide about a machine's state are indubitable, they still have a serious limitation: failure detection when the motor is driving a variable 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 end rings is altered. This paper reports a new diagnostic method, 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 off the driven machine even when it is part of an adjustable speed drive and the motor is operating under a variable supply frequency.