Online rotor bar fault detection in induction motor under load oscillation

The most common method for detecting broken rotor bar faults is to observe the corresponding fundamental sidebands harmonics through motor current signature analysis (MCSA). If a motor is subjected to low frequency load oscillation, then the oscillation related sidebands exhibit similar behaviors as broken rotor bar failure. In practice, the presence of load oscillation near to twice slip frequency has the same effect on a stator current of induction motor as a broken rotor bar fault. In such cases the stator current spectrum analysis can be misleading and therefore the detection of broken rotor bar failure becomes difficult. In this thesis, three different methods are proposed to separate broken rotor bar failure from low frequency load torque oscillation in three-phase induction motor. To discern broken rotor bar fault and load torque oscillations, firstly, the Analytical Signal Angular Fluctuation (ASAF) method, which is a combination of Hilbert Transform (HT) and the Space Vector Angular Fluctuation (SVAF) method, is used. In this newly proposed method, space vector angular fluctuations are calculated by sampling the analytical signal obtained from Hilbert transform (HT) with a predefined sampling frequency. The separation process is achieved from obtained ASAF signal spectrum. Proposed ASAF method can be applied to either line fed or V/f scalar controller induction motors. Another method to separate broken rotor bar failure from low frequency load torque oscillation is to analyze torque and motor phase voltage waveforms for defined separation harmonics. In this method, induction motor should be star connected to measure the phase voltage accurately. In order to define and justify the separation harmonics, 2-D Time Stepping Finite Element Method (TSFEM) is used. Induction motor drivers are widely run in direct torque control (DTC) mode to control its torque and speed. However, discerning of broken rotor bar failure from low frequency load oscillation for DTC drives remains unexplored. In this thesis, finally, a separation method is proposed to discern broken rotor bar failures from low frequency load torque oscillation in (DTC) induction motor drives using fault indexes that are defined from stationary vqs voltage and iqs current components spectrums. Computer simulations and experimental results are presented to prove the proposed methods for separation broken rotor bar faults from load torque oscillation in three-phase induction motor.