## 5 Results of the Experiment  
To further validate the feasibility of the overall control strategy for the system, an experimental platform was constructed based on the proposed topology and the previously described software and hardware circuit design. The experimental platform established in this study is depicted in Figure 14.  
As shown in Figure 14, a DC power supply was used to replace the rectifier stage and output a DC voltage. The whole intermediate stage was a high-frequency isolated DC-DC stage, which was realized by a high-frequency transformer for electrical isolation and voltage level conversion. The output stage was an NPC three-level inverter circuit, followed by an LC filter to filter the PWM waves of different frequencies. The IGBTs used were model H20R1203, manufactured by Infineon. The isolation stage employed the DA962D4 drivers and the NPC three-level output stage used the 1 EDI60N12 AF gate drivers. The high-frequency isolated DC-DC stage and three-phase inverter stage control circuits used a DSPTMS20F28335 control board to control the IGBT switches. The experimental platform was capable of achieving isolation protection and overvoltage protection. The parameters of the high-frequency isolated NPC three-level inverter frequency control system are shown in Table 3. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Remark** \\ \hline \(U_{\text{dc2}}\) & 20 V & Output DC voltage \\ \(f\) & 2 kHz & Switching frequency \\ \(n\) & 1 & High-frequency transformer ratio \\ C\({}_{\text{2-3}}\) & 450 V/560 uF & Output stage upper and lower capacitance \\ \(LC\) & 2 mH, 4.7 uF & Inductance and capacitance of \(LC\) filters \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of the experimental platform. Figure 13: The three-phase inverter output phase voltage waveform: (**a**) \(f_{1}\) = 40 Hz; (**b**) \(f_{2}\) = 50 Hz; (**c**) \(f_{3}\) = 60 Hz. The DC power supply provided a DC voltage RMS of 20 V to the high-frequency isolated DC-DC stage. Then, the FLUKE oscilloscope and KESIGHT oscilloscope were used to test the output waveforms at all stages, and the experimental results are shown in the figures below. The intermediate stage input voltage \(U_{\text{dc1}}\) was fed into the primary side of the high-frequency isolated DC-DC converter stage, resulting in a high-frequency square wave \(u_{\text{h1}}\) following single-phase inversion. \(u_{\text{h1}}\) was coupled to a high-frequency transformer to obtain high-frequency square wave \(u_{\text{h2}}\). Due to the circuit device power transfer problem, the average value of high-frequency square waves \(u_{\text{h1}}\) and \(u_{\text{h2}}\) on the primary and secondary sides of the high-frequency transformer was 19.9 V, which was slightly less than \(U_{\text{dc1}}\). The waveforms of the input voltage \(U_{\text{dc1}}\) and output voltage \(U_{\text{dc2}}\) at the intermediate stage are presented in Figure 15a. Figure 15b displays the square wave voltage \(u_{\text{h1}}\) on the primary side and the square wave voltage \(u_{\text{h2}}\) on the secondary side of the high-frequency transformer. It can be observed that both the primary-side single-phase inverter square wave \(u_{\text{h1}}\) and the high-frequency transformer-coupled square wave voltage \(u_{\text{h2}}\) exhibited smooth waveforms and demonstrated excellent overall steady-state performance. Figure 14: Experimental platform for high-frequency isolated NPC three-level inverter frequency conversion and speed regulation control system. Figure 15: (a) Intermediate-stage input voltage \(U_{\text{dc1}}\) and output voltage \(U_{\text{dc2}}\) waveforms. (b) Primary side square wave voltage \(u_{\text{h1}}\) and secondary side square wave voltage \(u_{\text{h2}}\). (c) Upper and lower capacitance voltage. The voltage waveform of the upper and lower capacitors in the output stage of the NPC three-level inverter is shown in Figure 15c.