## 6 Case Study  
As a complement to the previous examples, in which synthetic signals based on examples appearing in the IEC Standard were used, an experimental study has been carried out. An induction motor was tested by being fed from two inverters with different harmonic contents--Allen Bradley's PowerFlex 40 (sinusoidal PWM modulation type) and Telemecanique's Altivar 66 (random carrier frequency modulation); thus, obtaining a greater variety of harmonic contents. The motor used was a Siemens 0.75 kW, star connected, with rated values of 50 Hz, 400 V, and 1.86 A. A magnetic powder brake was used as a load, adjusted so that part of the tests were carried out with low load (motor slip close to 0.3%), and others with high load (slip around 4%). The acquisition system was based on a National Instruments PCI6250 card, plus an external interface module with LEM Hall Effect sensors used as transducers. The acquired data was processed using Matlab R2015 software. The induction motor used mixed eccentricity, which produces great interharmonic content in the low part of the current spectrum, which, together with the abundant harmonic and interharmonic content in the high part of the spectrum present in the output of the inverters, allowed us analyzing situations with a multitude of nearby tones and similar amplitudes. The motor operated at steady state during 60 s long tests, and the sampling frequency was 80 KS/s. All measurements were made at the output of the inverters, with fundamental frequencies and harmonic contents different from those of the mains signal, so it was necessary to adapt the analysis system, based on IEC standards, and orient it to the measurement of the utility network; to the specific characteristics of the signals. Figure 9 shows the laboratory test bench. Figure 10a shows the low part of the frequency response of the output current of the induction motor fed by a PowerFlex inverter with slip s \(\approx\) 4% and a fundamental frequency of 45 Hz. Figure 10b shows the low part of the frequency response of the output voltage of the Altivar inverter working at 55 Hz and with low loaded motor (s \(\approx\) 0.3%). The frequency responses were obtained based on the IEC standard, (using DFT, with spectral bars separated by 5 Hz), not yet grouped by frequency. It can be seen that the interharmonic content was much higher for the current signal (Figure 10a) than for the voltage signal (Figure 10b). Therefore, high values of the groupings and current rates containing those frequencies were to be expected, and a greater interaction between tones, which motivates an increase in the aggregation time necessary to obtain stable values, as explained before. On the other hand, the tone interaction was predicted to be lower in the case of Figure 10b, since the voltage measured in that zone presents practically only harmonics (which do not interact with each other, as they do not produce spectral leakage). Figure 9: Laboratory test bench. Figure 11 presents the time evolution, using 0.2 s long windows, of the harmonic distortion rates for low frequencies (THD (Total Harmonic Distortion) of current (Figure 11a) and voltage (Figure 11b)), measured at the output of the inverters of the same tests of Figure 10. RW was used in the graphs on the left and HW in the graphs on the right. Blue indicates the values without time aggregation or corresponding to each analysis window. In red, the values aggregated until each instant are shown so that it is possible to observe the value of any size of aggregation up to 60 s (duration of the test). The aggregated values for 3 s and 60 s, as well as the window number corresponding to each aggregation time, are highlighted in all graphs.