## 1 Introduction  
Optimal and reliable operation of transport infrastructure is determined by the state of serviceability of the electric traction drives of a rolling stock [1]. Operation of this type of drive is characterized by the impossibility of performing operational diagnostics and controlling the current status of the electrical equipment [2]. Asynchronous motors with short-circuited rotors have been widely used as traction motors in railway vehicles [3, 4]. Vector control system [5, 6, 7] and direct torque control (DTC) system [8, 9, 10] are the most common systems used for the control of the traction drives with asynchronous motors. When designing a traction drive system, the advantages and disadvantages of these control schemes should be considered, taking into account the operating factors of the electric rolling stock, in order to choose one or the other control scheme. The strengths and weaknesses of each type of control system could be analyzed for different operating modes of the traction drive. Given that the stator voltage frequency is equal to zero before the moment of activation of the speed setting device, the operation of the asynchronous motor is similar to the operation of the electric alternating current motor. This factor leads to power losses and subsequently to a general decrease in the energy efficiency of the traction drive compared to the DTC system [11; 12]. The weakness of the DTC system compared to the vector control system lies in the presence of higher torque pulsation in the former at the same sampling rates under the pulse-width modulation (PWM) of the autonomous voltage inverter [13; 14]. Torque pulsation can be reduced in the DTC system by using two methods: the application of multilevel inverters [15; 16; 17] or increasing the PWM sampling rate. The application of multilevel inverters would cause a decrease in efficiency and an unacceptable increase in the physical dimensions of the traction drive system in rolling stock [18]. An increase in the sampling rate leads to an increase in the temperature of the IGBT module elements of the voltage inverter [19; 20; 21]. Increased temperature is detrimental to the overall efficiency of the drive and can cause thermal failure [22; 23; 24]. The operation of electric rolling stock is characterized by a varying nature in the process of operation [25]. For example, starting off, master controller position switching, and wheel slippage lead to the development of short-term transient processes in the traction drive system. The transient processes cause the asymmetry of the phase current systems of the traction motor and the asymmetry of the phase voltage system at the inverter output, thereby causing an increase in the torque pulsations on the motor shaft. In addition, the process of variation of the catenary system voltage is non-stationary, non-deterministic, and non-Gaussian in the process of operation of the electric rolling stock [25]. Furthermore, the change in temperature of the IGBT modules of the autonomous voltage inverter alters the level of thermal noise in the traction drive system [26; 27]. The factors listed above suggest that the process of variation of the phase voltages of the asynchronous motor stator is non-stationary, non-deterministic, and non-Gaussian. A single autonomous inverter of the electric rolling stock may feed several traction motors. A wheel slippage attempt of one of them leads to the non-uniformity of the current voltage, which, in turn, leads to the asymmetry of the traction current system [28; 29]. Moreover, the dynamic load changes continuously in the process of movement of the electric locomotive within the rolling stock due to the dynamic oscillations of the wagons that take place during the variation of the nature of operation of the electric locomotive [30; 31; 32]. The above factors lead to the non-stationary and non-deterministic character of the load of traction motors causing the asymmetry of the traction current system.