## 4 Analysis of the Diagnostic Signals--Experimental Results
### Results of Stator and Rotor Fault Extraction
Summarizing, the FFT analysis of the instantaneous positive-sequence symmetrical component \(i_{1}\), in particular the changes in the slip frequency amplitudes calculated according to Equation (15) for \(k=1\): \(f_{\mathrm{s}}-2\)\(\mathit{sf}_{\mathrm{s}}\) and \(f_{\mathrm{s}}+2\)\(\mathit{sf}_{\mathrm{s}}\), can be used to diagnose the rotor windings of the squirrel cage motor. However, it should be noted that they are in close proximity to the fundamental frequency component and for low motor loads will be difficult to isolate. Figure 6 shows the FFT analysis of instantaneous negative-sequence symmetrical component \(i_{2}\) for the motor loaded with rated torque \(T_{L}=T_{LN}\), the frequency of the supply voltage \(f_{\mathrm{s}}=50\) Hz and various defects of the motor windings. For the stator winding faults (Figure 6a), significant changes are especially visible in the magnitude changes of the fundamental frequency harmonics \(f_{\mathrm{s}}\). With the increase in stator damage, the amplitude value of this harmonics increases strongly (almost 3-fold for \(N_{sh}=5\) compared to the healthy motor). As can be seen in Figure 6b, the failure of the rotor results in significant changes of the magnitudes calculated according to Equation (16) for \(k/p_{b}=5\): \(5\)\(f_{\mathrm{s}}-2\)\(\mathit{sf}_{\mathrm{s}}\) and \(5\)\(f_{\mathrm{s}}-4\)\(\mathit{sf}_{\mathrm{s}}\) harmonics. In addition, the amplitude of the \(5\)\(f_{\mathrm{s}}\) harmonic is a multiple of the fundamental frequency harmonics \(f_{\mathrm{s}}\), which allows for the easy separation of slip components \(2\)\(\mathit{sf}_{\mathrm{s}}\) and \(4\)\(\mathit{sf}_{\mathrm{s}}\) around it, even at low motor loads. It can therefore be concluded that the changes in the characteristic amplitudes of the harmonics specific for the stator and rotor winding faults, contained in the instantaneous negative-sequence symmetrical component \(i_{2}\) of the stator current, should be taken into account in the diagnostics of the electrical failures of the squirrel-cage motor. In addition, the considered damage causes changes in well-separated harmonics, thanks to which it is possible to conduct parallel diagnostics of the stator and rotor winding faults. In Figure 7a detailed analysis of changes in characteristic frequency amplitudes for various degrees of damage to the stator winding (Figure 7a) and the rotor cage (Figure 7b,c) at different load torque and frequency changes of the supply voltage are presented. The obtained experimental results show the impact of the motor load torque and the frequency of the supply voltage on the process of fault symptoms extracting and fault diagnostics using symmetrical component \(i_{2}\). It can be observed that the selected magnitudes of characteristic frequencies practically do not react to changes in the load torque. The influence of frequency changes in the supply voltage is visible for the fundamental component \(f_{\mathrm{s}}\) (its decrease causes a reduction in the intensity of changes under fault conditions, Figure 7a); however, it is still possible to detect the initial phase of failure even  
Figure 6: FFT analysis of the negative-sequence instantaneous symmetrical component \(i_{2}\) of the stator current for nominal load torque and \(f_{\mathrm{s}}=50\) Hz with (**a**) different number of shorted turns \(N_{sh}\); (**b**) different number of broken bars \(N_{bb}\). for relatively low supplying frequency \(f_{\rm s}=20\)Hz. In the case of damage to the rotor bars, the impact of the supplying frequency is very small. The results of the above analysis of the stator current signal made it possible to prepare datasets for Kohonen network training that were then used to detect and classify failures of the stator and rotor windings.