## 4 Experimental Verification
### Comparison of QPR-PI and QPR-SMC Control Strategies in Current Harmonic Suppression Performance  
To compare the effectiveness of the QPR-PI and QPR-SMC control strategies in suppressing current harmonics, this experiment was conducted under low-speed, medium-speed, and high-speed operating conditions. The harmonic components of the current and the torque ripple were analyzed for each condition. Figure 15 shows the experimental results for the PMSM operating at a low speed (set speed of 450 r/min, motor current fundamental frequency of 30 Hz). As shown in Figure 15a, when using the QPR-PI and QPR-SMC control methods, the fifth harmonic component of the stator current accounts for 2.98% and 1.41%, respectively, while the seventh harmonic component accounts for 1.16% and 1.07%, respectively. The total harmonic distortion (THD) is 3.95% for QPR-PI and 3.07% for QPR-SMC. Figure 15b indicates that the maximum torque ripple is 0.45% for QPR-PI and 0.34% for QPR-SMC. As illustrated in Figure 16, the frequency spectrum of the phase current at low speed is presented for the QPR-PI control strategy and the QPR-SMC control strategy. Figure 14: Frequency spectrum of the phase current at high speed. Figure 13: Phase current and load torque waveforms at low speed (2250 r/min). (**a**) Phase current waveform. (**b**) Load torque waveform. Figure 17 shows the experimental results for the PMSM operating at a medium speed (set speed of 1125 r/min, motor current fundamental frequency of 75 Hz). As illustrated in Figure 17a, when using the QPR-PI and QPR-SMC control methods, the fifth harmonic component of the stator current accounts for 2.19% and 1.77%, respectively, while the seventh harmonic component accounts for 0.88% and 0.37%, respectively. The total harmonic distortion (THD) is 5.41% for QPR-PI and 3.77% for QPR-SMC. Figure 17b indicates that the maximum torque ripple is 0.47% for QPR-PI and 0.37% for QPR-SMC. As illustrated in Figure 18, the frequency spectrum of the phase current at medium speed is presented for the QPR-PI control strategy and the QPR-SMC control strategy. Figure 16: Frequency spectrum of the phase current at low speed: (a) under the QPR-PI control strategy; (b) under the QPR-SMC control strategy. Figure 15: Phase current and torque waveforms at low speed (450 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 19 shows the experimental results for the PMSM operating at a high speed (set speed of 2250 r/min, motor current fundamental frequency of 150 Hz). As illustrated in Figure 19a, when using the QPR-PI and QPR-SMC control methods, the fifth harmonic component of the stator current accounts for 3.06% and 1.89%, respectively, while the seventh harmonic component accounts for 2.18% and 0.84%, respectively. The total harmonic distortion (THD) is 6.16% for QPR-PI and 4.25% for QPR-SMC. Figure 19b indicates that the maximum torque ripple is 0.55% for QPR-PI and 0.47% for QPR-SMC. As illustrated in Figure 20, the frequency spectrum of the phase current at high speed is presented for the QPR-PI control strategy and the QPR-SMC control strategy. Figure 17: Phase current and torque waveforms at low speed (1125 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 18: Frequency spectrum of the phase current at medium speed: (**a**) under the QPR-PI control strategy; (**b**) under the QPR-SMC control strategy.