## 4 Experimental Verification
However, under self-fault-tolerant control, the increase in these harmonic amplitudes is relatively small (approximately 5 dB for the 2 nd harmonic and 5.6 dB for the 4 th harmonic). Under self-fault-tolerant control conditions, the spectral variation characteristics of the electric drive system closely align with the simulation predictions. The increased amplitude of the 2 nd harmonic in the gear system's vibration spectrum is primarily caused by the increase in the 2 nd harmonic component of the electromagnetic torque, which is transmitted to the gear side through electromechanical coupling. The increased amplitude of the 2 nd harmonic in the electromagnetic torque is primarily caused by the increase 3 rd and 5 th harmonic components in the phase current. During coordinate transformation, the 3 rd and 5 th harmonic components in the phase current appear as a 2 nd harmonic component in the q-axis current. This q-axis current harmonic subsequently propagates into the electromagnetic torque as its 2 nd harmonic component. The amplitude increase at the 4 th harmonic occurs because the fourth harmonic frequency (399.6 Hz) of the electromagnetic excitation from the motor is very close to the fundamental meshing frequency (405 Hz) of the gear system, thereby exciting the vibration response of the gear system near this meshing frequency. Figure 16: Experimental dynamic characteristic spectra diagram of the electric drive transmission systems at rated conditions before and after self-fault-tolerant control: (**a**) phase A current; (**b**) electromagnetic torque; (**c**) planetary gearbox vibration signal. Figure 18 compares the dynamic responses of the electric drive transmission system before and after applying self-fault-tolerant control under light load conditions. As shown in Figure 18a,b, after implementing the self-fault-tolerant control strategy, the electromagnetic torque exhibited a brief amplitude fluctuation but rapidly recovered and stabilized at the reference value. However, the amplitude of the second harmonic component in the electromagnetic torque spectrum increased significantly (approximately 3 dB). This transient recovery process aligns closely with simulation predictions, effectively demonstrating the model's accuracy in predicting both transient recovery and steady-state harmonic response following the application of self-fault-tolerant control under light-load conditions. Additionally, the spectral analysis in Figure 18c indicates that under self-fault-tolerant control, the amplitudes of the 2 nd and 4 th harmonic components in the planetary gearbox vibration spectrum exhibit an increasing trend. Specifically, the 2 nd harmonic amplitude increases by approximately 3 dB, and the 4 th harmonic amplitude by approximately 1.6 dB. This trend mirrors the dynamic response characteristics observed in the planetary gearbox under rated operating conditions after applying self-fault-tolerant control, confirming these characteristics during self-fault-tolerant operation. Figure 17: Experimental spectral characteristics of the electric drive transmission systems at rated operating conditions before and after self-fault-tolerant control implementation: (**a**) phase A current; (**b**) electromagnetic torque; (**c**) planetary gearbox vibration signal.