## 5 Results and Discussion  
Figure 7 shows the motor's transient current at 15%, 25% and 100% of the rated supply voltage with two broken bars. The recovered nonstationary signal was very legible at 10% of the supply voltage. Unlike the simulation-based results, these signals contained several higher-order harmonics. The most prominent causes of those harmonics were the non-sinusoidal stator and rotor winding distribution, the stator, the rotor slot openings and the magnetic material's nonlinear behavior. However, those frequency components were of less significance, as the most prominent fault-based pattern remained in the bandwidth of 0-50 Hz. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Number of poles & P & 4 \\ Number of phases & \(\up\) & 3 \\ Connection & - & Delta \\ Stator slots & Ns & 36, non-skewed \\ Rotor bars & Nb & 28, skewed \\ Rated voltage & V & 400 V@50 Hz \\ Rated current & I & 8.8 A \\ Rated power & Pr & 7.5 kW @ 50 Hz \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The machine specifications. Figure 6: (**a**) The rotor with two broken bars. (**b**) The test bench with the loading motor on the right side and the test motor on the left side. The frequency spectrum of the stator current during the steady state interval for healthy and broken rotor bars at no load and rated load conditions is shown in Figure 8. The spectrum in Figure 8b shows the evolution of the left-side band (LSB) and right-side band (RSB) harmonics. These harmonics are the function of slip, and the results are based on the measurements taken under rated load conditions. The sideband frequencies increased in amplitude with the increase in the number of broken bars. Besides that, the sideband fault frequencies tended to shift slightly away from the fundamental component with the rise in broken bars. With the increase in the number of broken bars, the average generated torque decreased with a slight rise in slip. The segregation of these components is very difficult from the rest of the harmonics if the entire spectrum is considered. The severity of this problem increases with the decrease in load. As the load decreased, these harmonics started hiding under the fundamental component, with total disappear  
Figure 8: The frequency spectrum from 47 Hz to 53 Hz under healthy and broken rotor bar cases under steady state and (**a**) no-load and (**b**) rated-load conditions. Figure 7: The motor’s startup currents measured at different voltage levels (**top row**), the corresponding recovered nonstationary signals (**middle row**) and the nonstationary signal at 10% of the rated voltage representing the changing frequency pattern (**bottom**). ance at the no load condition as in Figure 8a. Moreover, the skewness in the rotor and stator slots affected their amplitudes, making them less detectable at the developing stage. The skews tended to attenuate the rotor harmonics for smooth speed torque features. It is also important to mention that these harmonics remain venerable when the inverter feeds the motor. This effect can be in the form of a colossal number of harmonics fed by the inverter, making faulty frequencies even more difficult for segregation. Moreover, the impact of the drive controller cannot be ignored. This impact is worst in DTC-controlled motors, where the controller of the drive tries to eliminate the current harmonics to reduce torque ripples [14]. The leading causes of the speed and torque ripples were the current harmonics, due to non-sinusoidal winding distribution on the stator and rotor side, the supply-based odd multiples of the fundamental component even if the machine was grid-fed, the inherent dynamic eccentricity and the magnetic material's nonlinear behavior. Those harmonics are inevitable, even if the machine is in a healthy condition. Figure 9 shows those harmonics in the stator current spectrum of a grid-fed healthy motor working under 50% of the rated load.