Discrimination of Rotor Faults and Low Frequency Load Torque Modulations of Squirrel Cage Induction Machines by means of the Vienna Monitoring Method

Electrical rotor asymmetries such as broken rotor bars give rise to well known side band currents in the vicinity of the fundamental as well as double slip frequency torque modulation. An externally caused low frequency load torque modulation may therefore also cause similar effects such as side band currents and modulation of the electromechanical torque. It is therefore a challenge to clearly distinguish these cases. In this paper rotor fault detection is performed by means of the Vienna monitoring method. This fault detection technique is based on a two model approach. Two mathematical induction machine models are used to calculate certain state variables such as flux and torque. From the difference of the calculated torque values a rotor fault indicator is derived. Measuring results were obtained from a 18.5 kW four pole induction machine with three broken bars. The Vienna monitoring method was applied to the faulty machine with and without load torque oscillations in order to study the ability of the discrimination of externally caused torque modulations and torque modulations due to rotor faults