## 2 Materials and Methods
### Measurements of Drive System  
The values of the output current (I\({}_{\text{O}}\)) and active current (I\({}_{\text{A}}\)) were recorded during the measurements. The output current characterises the energy demand of the motor including the value of losses (magnetisation of the rotor, losses in the core, losses due to heating of the motor components, etc.). In order to verify the quality of the control and to establish its relationship with the energy demand (energy efficiency), it was also necessary to study the speed time courses against the set speed.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Input frequency (f\({}_{\text{I}}\)) & 50 Hz \\ Rated speed (P\({}_{\text{N}}\)) & 1380 rpm \\ Rated power (P\({}_{\text{N}}\)) & 0.37 kW \\ Power factor (cos\(\varphi\)) & 0.7 \\ Thermal class (Th\({}_{\text{CL}}\)) & 155 (F) \\ Nominal supply voltage (V\({}_{\text{N}}\)) & 230 r/400\(\mathrm{\SIUnitSymbola}\) V \\ Rated current (I\({}_{\text{N}}\)) & 1.98 r/1.14\(\mathrm{\SIUnitSymbola}\) A \\ Rated efficiency (eff) & 66.6\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters of the tested electric motor (DRS71S4 BE05/FI/TF/ES7S).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Nominal supply voltage (V\({}_{\text{N}}\)) & 3 \(\times\) 380–500 V \\ Input frequency (f\({}_{\text{N}}\)) & 50–60 Hz \\ Rated current (I\({}_{\text{IN}}\)) & 1.8 A (400 VAC) \\ Output frequency (f\({}_{\text{O}}\)) & 0–180 Hz \\ Maximum output current (I\({}_{\text{O}\_{MAX}}\)) & 2 A \\ Output power (P\({}_{\text{O}}\)) & 0.55 kVA \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of the tested frequency converter (MDX61B0005-5A3-4-00).  
To determine the quality of control, which in the case under consideration is the level of representation of the setpoint speed, the authors propose the following formula:  
\[\mathrm{S_{C}=1-1(n_{A}-n_{S})/(n_{S})\,|} \tag{1}\]  
where:  
Sc--speed convergence;  
\(n_{A}\)--actual speed [rpm]; and  
\(n_{S}\)--setpoint speed [rpm].  
Speed convergence assumes a value from the range 0\(\div\)1.  
In the efficiency analysis, an important parameter is also the level of energy used to power the motor in a given moment, which can be referred to as the maximum energy available in the source of powering signals: the frequency converter. The quantity proportional to the energy is, in the case under consideration, the current intensity, so its values were used by the authors to define the energy saving factor in the following way:  
\[\mathrm{E_{SF}=1-1I_{O}/I_{O\_MAX}\,|} \tag{2}\]  
The energy saving factor takes a value in the range from 0 to 1. The total energy efficiency is the product of the coefficients described by Formulas (1) and (2):  
\[\mathrm{E_{F}=S_{C}\,E_{SF}} \tag{3}\]  
The energy efficiency defined in this way needs to be observed over time in order to assess the dynamics of the drive (variability of its parameters) and the energy resources used to achieve it. Long periods of speed stabilisation or discrepancies, as well as intensive motor current consumption, reduce the average values of the \(\mathrm{E_{F}}\) components. It is therefore possible to use this statistical measure to analyse individual phases of the motion.