## 1 Introduction
Similarly, as in the case of stator winding failures, the consequences obviously encompass significant losses in production and large costs. This is why the diagnosis of this kind of damage makes sense only at the initial stage [4]. When a fault is detected early, it is possible to prevent it from spreading to other coils of stator winding or rotor bars. Therefore, both stator and rotor winding faults have attracted much attention over the last decades. For ITSC faults (in low-voltage motors) intensive tests were carried out using various variables, methods and techniques, in open- [13; 14] and closed-loop drives [15; 16]. The symptoms of damage to stator windings occur in many signals available directly by measurements or indirectly--by calculated (estimated) quantities [13; 14; 15; 16; 17]. Most of the significant methods and related techniques are based on monitoring one or more signals and the use of advanced signal processing methods. An overview of these methods is presented in detail in [18; 19; 20; 21; 22; 23; 24; 25; 26; 27], where the ITSCs are detected in the stator phase current [18], in the phase shift between phase currents, phase shift between current and stator voltage [14; 19], axial flux [2; 20], instantaneous active and reactive power [21] or mechanical vibrations [22]. The most popular signal processing methods are based on motor current signature analysis (MCSA) [1; 2; 7], symmetrical components analysis (SCA) [23; 24; 25; 26; 27; 28; 29] and the stator current Park's vector approach analysis [29; 30]. The detection of ITSC using SCA is based on studying the variation of the amplitude of the negative-sequence component of the stator current or the motor impedance [13; 14; 23]. When choosing these methods, their sensitivity to the asymmetry of supply voltages is an important issue [24]. In the case of an IM supplied from frequency converter the ITSC can be detected using instantaneous symmetrical components as presented in [25; 26]. A spectral analysis of the instantaneous negative-sequence component of the stator current makes it possible to find characteristic frequencies whose amplitude strongly depends on the degree of damage to the stator windings. The diagnostic information contained in the zero sequence symmetrical component of the stator voltage [27] can also be used to detect shorted turns in the stator winding. This component also shows symptoms of damage to the cage rotor [28]. In the case of the asymmetry of the stator and rotor windings in the zero voltage waveforms, there are deformations with specific characteristic frequencies that can be distinguished in the Fast Fourier Transform (FFT) analysis. However, in order to diagnose the motor windings by monitoring the zero voltage component, a neutral point of the stator windings must be available. Thus, it should be noted that the measurement of the zero voltage component in industrial conditions is difficult to implement due to safety reasons. The problem of detecting damage to cage rotors also has a rich bibliography. The discussion on most of the methods used is included, among others, in [29; 30]. To detect damage in cage rotors, the most commonly used methods are the Motor Current Signal Analysis (MCSA) and the Extended Park's Vector Analysis (EPVA) [29; 30; 31; 32]. They are effective and feasible methods based on non-invasive measurement of the stator current. The processing of the current signal or the spatial vector module of the stator current by means of the FFT, allows us to obtain the information on the frequency spectrum characteristic to the cage rotor failures (so-called slip frequencies). On the basis of the evaluation of their amplitude value, the technical condition of the rotor is evaluated. The accuracy of calculations is greatly influenced by the frequency resolution, noise and the leakage of the fundamental frequency harmonic as well as the motor load level and hence this method is not recommended in nonstationary processes.