## 5 Experimental Results and Discussion
Compared with traditional MTPA control, Figure 10 shows that the proposed system efficiency improvement control can decrease both the motor loss and inverter loss. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Motor Torque** & **System Input Power** & **Motor Input Power** & **Mechanical Power** & **Inverter** & **Motor** & **System** \\
**(Nm)** & **(kW)** & **(kW)** & **(kW)** & **Efficiency** & **Efficiency** & **Efficiency** \\ \hline
668 & 24.15 & 22.81 & 21 & 94.45 & 92.07 & 86.96 \\
600 & 21.71 & 20.44 & 18.85 & 94.15 & 92.22 & 86.82 \\
500 & 18.17 & 17.03 & 15.71 & 93.73 & 92.24 & 86.46 \\
400 & 14.65 & 13.66 & 12.57 & 93.24 & 92.02 & 85.80 \\
300 & 11.18 & 10.33 & 9.42 & 92.04 & 91.17 & 84.26 \\
200 & 7.79 & 7.07 & 6.28 & 90.76 & 88.83 & 80.62 \\
100 & 4.46 & 3.86 & 3.14 & 86.55 & 81.35 & 70.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Experimental results in the rated speed by the system efficiency improvement control. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Motor Torque** & **System Input Power** & **Motor Input Power** & **Mechanical Power** & **Inverter** & **Motor** & **System** \\
**(Nm)** & **(kW)** & **(kW)** & **(kW)** & **Efficiency** & **Efficiency** \\ \hline
668 & 24.17 & 22.83 & 21 & 94.44 & 91.98 & 86.87 \\
600 & 21.73 & 20.47 & 18.85 & 94.18 & 92.09 & 86.73 \\
500 & 18.19 & 17.06 & 15.71 & 93.80 & 92.08 & 86.37 \\
400 & 14.68 & 13.70 & 12.57 & 93.31 & 91.74 & 85.60 \\
300 & 11.22 & 10.38 & 9.42 & 92.50 & 90.75 Figure 10 shows that the system efficiency improvement control strategy can decrease the motor loss and increase the 0.94% motor efficiency at rated speed. Furthermore, the proposed control strategy can also reduce the losses of power converter, by which the inverter efficiency can be added at 10.85% at rated torque. Figure 11 shows the system efficiency by the traditional MTPA control method and the proposed control strategy. It can be seen that the system efficiency improvement control method can obtain higher system efficiency than the traditional MTPA control over the whole operation condition range of the PMSM direct drive system. The proposed control strategy can increase the system efficiency by 0.54% at rated speed and improve the efficiency of PMSM drive system by 8.38% at rated torque. To verify the efficiency optimization performance of the proposed system efficiency improvement respect to conventional MTPA control, this paper also designs a SiC-MOSFETs-based power converter which is shown in Figure 12.  
The SiC-MOSFETs based power converter utilizes SiC-MOSFET modules (CAS300M12 BM2) as the power devices which can obtain lower switching losses than Si-IGBT. The fitted coefficients of CAS300M12 BM2 are shown in Table 7.  
Figure 11: Efficiency optimization performance on the system efficiency by the proposed system efficiency improvement control. (**a**) System efficiency at rated speed; (**b**) system efficiency at rated torque. Figure 10: Efficiency optimization performance on the motor efficiency and inverter efficiency by the proposed system efficiency improvement control. (**a**) Motor efficiency at rated speed; (**b**) inverter efficiency at rated torque. The experimental results of the PMSM system drivne by the SiC-MOSFETs based power converter are shown in Tables 8-11. Tables 8 and 9 show the performance of the PMSM drive system under the traditional MTPA control. Table 8 shows the experimental results of the PMSM drive system at rated torque, and Table 9 shows the performance of the PMSM drive system at rated speed.