Abstract: In industrial applications where a single frequency converter controls the soft starting of multiple high-power asynchronous motors, the switching process from frequency conversion to mains frequency is inevitable. During this process, the motor being switched may experience excessive stator winding voltage, resulting in excessive switching inrush current, which can cause destructive damage to the motor's electrical and mechanical characteristics. This paper provides a detailed analysis of the switching process of asynchronous motors from frequency conversion to mains frequency and proposes solutions to limit the switching inrush current. Keywords: AC asynchronous motor, frequency conversion to mains frequency switching 1. Problem Statement In the process of soft starting multiple high-power asynchronous motors controlled by a single frequency converter, after the supply voltage of the first motor starts gradually increases from 0Hz/0V to the mains voltage and frequency (50Hz/380V), there is little difference between the frequency converter power supply and the mains frequency power supply. At this point, the motor can completely switch from the frequency converter power supply to the mains frequency power supply, freeing the frequency converter to control the soft starting of the next high-power asynchronous motor. During the rapid switching process from frequency converter to mains frequency in the aforementioned motor, it is crucial to ensure that the switching current is not excessive, especially for large motors. As described earlier, the switching from frequency converter to mains frequency generally occurs when the frequency and magnitude of the inverter output voltage and the grid voltage are equal. Superficially, it seems that the equal magnitude and frequency of the two power supply output voltages allow for a smooth switch without any impact on the motor. However, this is not the case. A crucial, yet overlooked, issue is phase—that is, whether the changes in the two power supply voltages are synchronized. 2. Problem Analysis—The Impact of Phase Inconsistency on the Frequency Converter/Mains Frequency Switching Process At the instant of switching from frequency converter to mains frequency, due to the randomness of the initial phase of the inverter output voltage, the phase of the three-phase power supply it outputs may be completely inconsistent with the phase of the mains frequency power supply. The impact of this situation on the switching process can be illustrated using the phasor diagram of any phase of a three-phase asynchronous motor (Figure 1). According to the motor principle, when a three-phase motor is running normally, the main magnetic field rotating at synchronous speed will induce symmetrical three-phase electromotive forces in the stator three-phase windings. The induced electromotive force generated by each phase stator coil of a three-phase asynchronous motor and the power supply voltage applied to each phase stator are the same in frequency but different in amplitude and phase, which is represented by a certain angle between them on the phasor diagram. For high-power motors, if the power is disconnected, although the main magnetic field disappears, the rotor core that was once magnetized by the main magnetic field still has residual magnetism. At the same time, due to the inertial rotor still rotating at high speed, the induced electromotive force generated in the stator coil will not disappear in a very short time, but will only be attenuated. Since the conversion from frequency converter to power frequency is extremely short, the induced electromotive force generated in the stator coil still exists, so the impact on the switching process must be fully considered. [align=center] Figure 1 Phasor diagram of a three-phase asynchronous motor from frequency converter to power frequency[/align] When the water pump motor is running in frequency converter mode, the initial phase of the inverter output voltage is random, only ensuring that the voltage phase difference between the two phases is 120°[sup]0°[/sup]. When its output frequency rises to 50Hz, we switch from variable frequency to common frequency. Assume that at this time, one phase of the inverter's three-phase voltage is , the induced electromotive force generated by the corresponding phase stator coil of the motor is , and the corresponding phase of the industrial power grid's power frequency voltage is . It has a phase difference φ with , as shown in Figure 1. After the switch, the voltage applied to the motor stator winding will be superimposed on the induced electromotive force remaining in the motor stator winding itself, resulting in a total voltage U = + for each phase of the motor stator winding. Careful observation of Figure 4-8 shows that if the phase difference between and increases from φ to φ between 0° and 180°, the total voltage across each phase of the motor stator winding will increase from to . When the phase difference between the variable frequency drive (VFD) and the power grid increases from 0° to 180° and then becomes synchronous, this superposition is most intense. The total voltage across each phase of the motor stator winding directly becomes U² + E¹, which far exceeds the motor's rated voltage. This will cause serious problems such as excessive motor current and severe insulation damage. Therefore, whether the phases of the VFD and the industrial power grid are consistent during the switching process of a high-power motor is a key factor in the success of the switching. 3. Solution Approach Based on the above analysis, the key to the successful switching of a high-power three-phase asynchronous motor from a VFD to the industrial power grid lies in the timing of the switching. As shown in Figure 1, if we take the instant when the phase of the variable frequency power supply and the mains power supply are in phase as the switching moment, then the phase of the variable frequency power supply and the mains power supply are opposite. At this moment, the total voltage across the stator winding, |U| = |+| = U[sub]2[/sub] - E[sub]1[/sub], will be at its minimum. Switching at this instant will achieve the optimal switching from frequency converter to mains frequency. However, this optimal switching is difficult to achieve, mainly due to the difficulty in detection. Since the optimal switching is not easy to achieve, we can take a less ideal approach. If we take the instant when the phase of the variable frequency power supply and the mains power supply are in phase as the switching moment, we can observe from the phasor diagram in Figure 1 that the total voltage across the stator winding, U=+, although not as small as the total voltage |U| = |+| = U[sub]2[/sub] - E[sub]1[/sub] at the optimal switching moment, will still be very small. Moreover, detecting the phase is much easier than detecting the phase of the variable frequency power supply and the mains power supply. If we use a frequency and phase detector to detect the phase of the variable frequency power supply and the mains power supply, and issue a switching command when the phases of the two are detected to be consistent, then the smooth switching of the high-power asynchronous motor can be achieved.