## 4 Verification of Simulations and Experiments
### Experimental Results  
To verify the proposed optimal design process, a prototype of the optimal PMSM was fabricated. The prototype PMSM motor assembly, stator, and rotor, are shown in Figure 14, and the experimental setup is illustrated in Figure 15.  
The results of the cogging torque experiment are compared and shown in Figure 16. The optimum model yielded a cogging torque of 0.2153 Nm which is reduced by 37.1% compared to 0.3369 Nm generated in the initial model. It is expected that the cogging torque characteristics have been improved based on rotor shape optimization. The results of the cogging torque experiment are compared and shown in Figure 16. The optimum model yielded a cogging torque of 0.2153 Nm, which was reduced by 39.2% compared to 0.3369 Nm of the initial model. It is expected that the cogging torque characteristics have been improved based on rotor shape optimization. However, there are several reasons for the existence of pulsation in the cogging torque waveform that are attributed to the harmonic component of the pole number. It is known that the number of pole harmonics is mainly caused by the shape error of the stator generated during manufacturing. This is also a factor that adversely affects the cogging torque that cannot be easily considered at the design stage. Thus, its complementary method needs to be studied further.  
The experimental results of motor dynamo testing for the initial and optimal models are presented in Figure 17. The measurement targets include the rated efficiency in response to the rated load.  
Figure 16: Comparison of the experimental cogging torque characteristics.  
Figure 14: PMSM prototype for an electric compressor: (**a**) Stator; (**b**) rotor; (**c**) motor assembly.  
Figure 15: Experimental setup for motor dynamo testing.  
The input direct current voltage was 380 V. To compare the operating characteristics, the PMSM motor was started and had an initial speed of 6000 rpm, while the load torque was continually increased until it reached the rated load of 4.775 Nm during the experiment. The experimental results showed that the rated efficiency of the initial model was 91.4%, and that of the optimal model was 92.9%, at the rated operating conditions (rotational speed of 6000 rpm and output power of 3 kW). The rated efficiency test characteristics were also found to be approximately 1.5% lower than the analysis. This is expected to be attributed to the difference between the physical properties considered in the design, the analysis used for the same material, and the physical properties applied to the prototype, as well as the component tolerances of the magnetic materials generated during the manufacturing of the prototypes, assembly tolerances, and errors attributed to test equipment settings. Table 4 summarizes the simulation and experimental results from each model.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Items** & **Initial** & **Optimal** & **Unit** \\ \hline \multirow{2}{*}{Simulation} & Rated efficiency & 92.0 & 93.5 & \% \\  & Cogging torque & 0.3479 & 0.2778 & Nm \\ \hline \multirow{2}{*}{Experiment} & Rated efficiency & 91.4 & 92.9 & \% \\  & Cogging torque & 0.2778 & 0.2153 & Nm \\ \hline \hline \end{tabular}
\end{table}
Table 4: Simulation and experimental results.  
Figure 17: Comparison of experimental motor dynamometer test results: (**a**) Initial model; (**b**) optimal model.