## 4 Discussion  
### THD of the Stator Current  
The first of the observed sizes is the THD for the stator current--denoted \(\mathrm{THD_{IS}}\) (expressed in %), represented in Figure 4--depending on the k-order and the weight \(\gamma_{\mathrm{Vk}}\) of the voltage harmonics. In the considered specific case, it should be taken into account that the level \(\gamma_{\mathrm{Vk}}\) of a k-order harmonic is defined as the ratio between the RMS value \(\mathrm{Y_{k}}\) of the harmonic and the RMS value \(\mathrm{Y_{1}}\) of the fundamental wave, expressed as a percentage:  
\[\gamma_{\mathrm{Vk}}=\frac{\mathrm{Y_{k}}}{\mathrm{Y_{1}}}\cdot 100,\ \%, \tag{22}\]  
as well as the fact that the deforming residue is defined (in AC, in the absence of a continuous component) by the following expression:  
\[\mathrm{Y_{def}}=\sqrt{\sum_{k=2}^{\mathrm{k_{M}}}\mathrm{Y_{k^{\prime}}^{2}}} \tag{23}\]  
in which the maximum order of harmonics taken into account was noted with \(\mathrm{k_{M}}\). Next, if for the THD of a nonsine wave, the expression given by the ratio expressed as a percentage between the deforming residue and the RMS value of the fundamental is adopted,  
\[\mathrm{THD}=\frac{\mathrm{Y_{def}}}{\mathrm{Y_{1}}}\cdot 100,\ \%, \tag{24}\]  
we have found that, in this case, when the deformed wave contains a single harmonic, the THD of the stator current (24) is equal to the level of the respective harmonic (22), both sizes expressed as a percentage:  
\[\mathrm{THD_{IS}}=\gamma_{\mathrm{ISk}},\ \%. \tag{25}\]  
Graphical representations of \(\mathrm{THD_{IS}(k)}\), with \(\gamma_{\mathrm{Vk}}\) as parameter (Figure 4), obtained by joining the corresponding points through continuous interpolation functions, lead to a second observation--a qualitative one this time: the values of \(\mathrm{THD_{IS}}\) depend only on the order of harmonics and the magnitude of the voltage harmonic level \(\gamma_{\mathrm{Vk}}\) and does not depend on its type of sequence, positive or negative. Figure 4: \(\mathrm{THD_{IS}}\) dependence on the k-order and the weight \(\gamma_{\mathrm{Vk}}\) of the voltage harmonics. The third observation, a quantitative one, is related to the constancy of the ratio between the \(\rm THD_{IS}\) and the level of the voltage harmonic \(\rm\gamma V_{Vk}\), for a constant specific-order k of the voltage harmonic present in the voltage wave. The calculation of this ratio leads to the results presented in Table 4.  
The significant decrease of the current harmonics with the increase of their order can be explained by the significant increase of the inductive reactance of the IM when the frequency of the respective harmonics is growing. The analysis of the numerical data from Table 4 leads to the conclusion that, under the given conditions, the total harmonic distortion of the stator current, at an order k\({}_{2}\), \(\rm(THD_{IS})_{K2}\), can be calculated if the value of the size \(\rm(THD_{IS})_{K1}\) is known, for a known order k\({}_{1}\), with the approximate relation:  
\[\rm(THD_{IS})_{K2}\approx\rm(THD_{IS})_{K1}\frac{k_{1}}{k_{2}}. \tag{26}\]  
An attempt was made to identify a relationship that would allow the calculation of the level of a stator current harmonic of the k-order, denoted \(\rm\gamma_{ISk}\), when the level \(\rm\gamma_{Vk}\) of the voltage harmonics of the same order k, from the voltage applied to the stator, is known. Among the variants obtained, the simplest is the following relation:  
\[\rm\gamma_{ISk}\approx\frac{8\cdot\rm\gamma_{Vk}}{k},\ \%, \tag{27}\]  
based on the observation caused by the graphs from Figure 4, which cuts the lines corresponding to the ordinates with the adopted values \(\rm\gamma_{Vk}\), for a (fictitious) order k \(\approx\) 8. Checked on the data in Table 3, (26) emphasizes that the current harmonics with the order k \(<\) 8 have higher weights in the current wave than the voltage harmonic of the same order. On the other hand, for orders k \(>\) 8, the level of the current harmonics decreases (8/k)-times related to one of the corresponding voltage harmonics.