## 4 Experimental Verification
### Comparison of QPR-PI and QPR-SMC Control Strategies in Current Harmonic Suppression Performance
Figure 21 summarizes the data on the suppression effectiveness of PI, QPR-PI, and QPR-SMC control methods on the stator current harmonics and Total Harmonic Distortion (THD) in a PMSM under low-speed (450 r/min), medium-speed (1125 r/min), and high-speed (2250 r/min) operating conditions. The results indicate that the traditional PI control method shows high harmonic content and THD at all speeds. At low speed, the fifth and seventh harmonic components are 24.22% and 5.03%, respectively, with a THD of 25.7%; at medium speed, the fifth and seventh harmonic components are 24.85% and 8.49%, respectively, with a THD of 26.45%; and at high speed, the fifth and seventh harmonic components are 25.11% and 6.64%, respectively, with a THD of 26.67%. Figure 19: Phase current and torque waveforms at low speed (2250 r/min) under the QPR-PI and QPR-SMC control strategies. (**a**) Phase current waveform under the QPR-PI control strategy. (**b**) Torque waveform under the QPR-PI control strategy. (**c**) Phase current waveform under the QPR-SMC control strategy. (**d**) Torque waveform under the QPR-SMC control strategy. Figure 20: Frequency spectrum of the phase current at high speed: (**a**) under the QPR-PI control strategy; (**b**) under the QPR-SMC control strategy. After introducing the QPR-PI control method, the harmonic content and THD significantly decreased. At low speed, the fifth and seventh harmonic components are reduced to 2.98% and 1.16%, respectively, with a THD of 3.95%; at medium speed, the fifth and seventh harmonic components are reduced to 2.19% and 0.88%, respectively, with a THD of 5.41%; and at high speed, the fifth and seventh harmonic components are reduced to 2.18% and 0.84%, respectively, with a THD of 6.16%. The QPR-SMC control method further improves harmonic suppression. At low speed, the fifth and seventh harmonic components are reduced to 1.41% and 1.07%, respectively, with a THD of 3.07%; at medium speed, the fifth and seventh harmonic components are reduced to 1.77% and 1.00%, respectively, with a THD of 3.77%; and at high speed, the fifth and seventh harmonic components are reduced to 1.89% and 0.84%, respectively, with a THD of 4.25%. By comparing the performance of the traditional PI control method, the QPR-PI control method, and the QPR-SMC control method at different speeds, the advantages and disadvantages of each control strategy in suppressing harmonics and reducing total harmonic distortion (THD) can be clearly understood. The following are the detailed comparative results of these three control methods under different speed conditions:  
Before suppression, the fifth and seventh harmonic currents and the THD in the three-phase stator current of the motor were relatively high. After incorporating the proportional resonant controller, the fifth and seventh harmonic components and the THD in the current significantly decreased. Compared to the traditional PI control, the QPR-PI control also showed improvements in reducing harmonic components and THD, but it was not as effective as the QPR-SMC control. The QPR-SMC control strategy resulted in the lowest proportions of fifth and seventh harmonic components and THD under all conditions, indicating that the QPR-SMC control strategy can significantly improve current harmonic issues. Based on these results, the following conclusions can be drawn: When the motor operates at different speeds, the corresponding fundamental electrical frequency and harmonic frequency change accordingly. The introduction of a frequency-adaptive proportional resonant controller allows the controller to adapt to speed variations and adjust the harmonic frequencies in real time. This effectively suppresses specific harmonic frequencies at different speeds, significantly reducing the harmonic components and THD in the stator current and markedly improving the torque ripple of the motor.