Research on Frequency Conversion Switching of Three Phase AC Motors with Phase and No Impact
1 application background
The background of the inverter with the same phase and no impact is proposed. The current mainstream inverter manufacturer has only a simple power conversion scheme that does not consider the grid phase. The disadvantage is that it is switched from the inverter control to the power frequency, because the motor has a strong Back EMF, when the phases of the grid phase and the motor back EMF are inconsistent, although the voltage amplitudes are the same, the voltage difference caused by the phase difference can be close to the grid voltage amplitude. The voltage difference is applied to the stator winding of the motor, directly with the power frequency. The startup situation is similar, so the switching current can reach about 7 times. On the one hand, the large current causes the capacity of the low-voltage circuit breaker and the grid transformer to be amplified, and on the other hand, the rotor's torque jumps, which may affect the production process, and may damage the machinery and cause a decrease in life. For synchronous motors, this problem is extremely serious because the synchronous motor rotor is undamped, and once the switching phase is inconsistent and causes oscillation, it may take a long time or even to be attenuated to a stable state.
Connecting a three-phase reactor at the input end of the motor can improve the impact during non-in-phase switching, but the reactor has a significant voltage drop during heavy load, resulting in insufficient motor supply voltage, affecting motor efficiency, and increasing cost and size. The most ideal solution is to detect the phase of the grid and achieve the same phase switching.
2 traditional direct switching schemes and problems
The direct switching scheme, as shown in Figure 1, has the following control steps:
KM1 is closed, KM2 is closed, KM3 is disconnected, and the inverter runs to the power frequency 50Hz.
The inverter turns off the output, KM2 is disconnected, and KM3 is closed.
Since the motor still has a strong back electromotive force after the inverter is turned off, its phase and the phase of the grid are random. When the phases are completely consistent, there is no impact. When the phase difference reaches 180°, it is added to the motor winding. The voltage reaches about 2 times of the grid voltage. At this time, the inrush current is up to 14 times of the rated current of the motor, which may cause tripping or damage to the transmission bearing.
3 Schemes and problems with reactor buffering
The scheme with reactor buffering, as shown in Fig. 2, is to add a reactor based on the direct switching scheme, and to improve the line inductance, so as to reduce the large current caused by the large voltage difference when the phase of the switching instant is inconsistent. This scheme can significantly reduce the inrush current, but the series reactor will cause a large voltage drop. Therefore, after the switching is completed, the reactor needs to be short-circuited, that is, the KM3 is closed.
