Broken rotor bar fault detection in induction motors using Wavelet Transform

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The Fast Fourier Transform (FFT) method is successfully used for the broken rotor bar fault detection purpose in the induction machines. It is based on the common-steady state analysis of the motor. This method is successfully used with Motor Current Signature Analysis (MCSA) technique for last three decades. However, this method is suffered from some serious drawbacks such as; it is applicable only in the constant load condition not for the variable load. The frequency-domain methods which are commonly used need accurate slip estimation for frequency components localization in any spectrum. It is also not suitable at the no-load or light load condition of the motor. At light load condition, it is quite difficult to distinguish between healthy and faulty rotors because the characteristic of broken rotor bar fault frequencies are very close to fundamental component and their amplitude are small in comparison. As a result, detection of the fault and classification of the fault severity under light load is almost impossible. In order to overcome the above problems of the FFT based technique, the Short Time Fourier Transform (STFT) Method was proposed. The excellent feature of this method is that it is capable to diagnose broken rotor bar fault in the transient condition. The STFT method also suffered from the drawback that it shows the constant window for all the frequencies. Therefore, it shows poor frequency resolution. In order to overcome all the problems stated so far, the most recent powerful mathematical tool i.e. Wavelet Transform (WT) has been used in the rotor broken bar fault detection purpose at all loading conditions. It shows variable window size for all the frequencies. Therefore, the WT method does not have resolution problem due to its multiresolution feature. This paper investigates the detection of rotor faults in induction machines by analyzing the starting current using a newly developed quantification technique based on the wavelet transform. The technique applies the wavelet transform to the envelope of the starting current. The envelope extraction is used to remove the strong fundamental component, which overshadows the characteristic differences between a healthy motor and a faulty motor with broken rotor bars.