## 1 Introduction  
Nowadays variable speed drives (VSD) with induction motors (IM) are widely used in many industrial processes and plants. During the operation of IM-VSD, different types of damage occur in the electrical circuits of the stator and rotor, in the mechanical system of the motor and the loading machine, in coupling elements as well as in the power converters, control systems and measuring sensors [1]. Statistical surveys of electrical machine failures were conducted in previous years by various independent centres and the published results were slightly different from one another [2; 3; 4; 5; 6]. The quantitative share of individual types of damage of induction motors is reported as follows:  
* bearings--40%,
* stator faults--38%,
* rotor faults--10%,
* others--12%. The pulse frequencies of pulse width modulation (PWM) converters suppling modern PSDs were increased to 20 kHz due to the rapid progress in the switching devices. A very short rise time of each modulated signal wave front causes a highly non-linear voltage distribution among various turns in the winding and often results in high voltage stresses [6; 7; 8]. These stresses are several times higher than those caused by sinusoidal waveforms and negatively affect the stator winding insulation. This in turn causes the degradation of the winding insulation over time and can lead to an increase in stator winding faults and, as a consequence, to an outage of the whole drive system [6; 8]. Modern inverter-fed VSDs introduce additional problems to the failure detection and extraction of fault symptoms [8; 9; 10]. It results from variable (regulated) power frequency, variable load conditions, the influence of PWM modulation of high frequency on diagnostic signals, current and voltage distortion, voltage surge, the occurrence of voltage and current zero-sequence components. Therefore, for the diagnosis of VSD systems, it is necessary to use appropriate measuring and processing equipment. As seen above, electrical winding faults, namely stator short-circuits and rotor bar damage, together constitute around 50% of all faults of the induction motors (IMs) applied in VSDs. The most common electrical faults of IMs are failures of the stator windings, especially inter-turn insulation. Damage to the main and interphase insulation is less frequent. Stator faults usually start as inter-turn short circuits (ITSC). When an ITSC occurs, extremely high currents flowing in the short-circuited coil cause the propagation of destruction [4; 5; 6; 7; 8]. Hence, early and accurate incipient faults detection in this machine is crucial. If incipient stator faults are not diagnosed, it may result in severe phase to phase or phase to ground short circuits. This in turn may lead to a catastrophic damage of machine core and windings. Thus ITSCs of the stator windings are recognized as among the most difficult failures to detect, because detection makes sense only at the initial stage of the damage. For low-voltage motors, incipient stator fault detection procedures are yet to be standardized [10; 11; 12]. Damages to IM rotors (breaks of bars or a squirrel-cage ring or connections of bars with rings) and the resulting asymmetries are a relatively frequent cause of IM failures. They occur mainly in motors that work in difficult conditions or are subject to frequent and long starts. Damage to the cage (degeneration of welds between the cage bars and the shortening ring, ring section cracks, bar cracks) starts with a fault to the individual segments of rings or several bars. The damage leads to unfavourable phenomena in the motor: vibrations caused by alternating electromagnetic torque components, uneven magnetic tension, increased motor slip, imbalance and eccentricity of the rotor. As a result, during further exploitation the machine damage expands (e.g., mechanical damage to the stator insulation system occurs), and consequently an emergency stop of the industrial process and the necessity of replacing or repairing the motor may follow.