The output of 1C31227G01 can not be connected to the power supply, nor can it be taken off with load in operation, and the switching process should be carried out according to the following procedure. Cyclic cut constant pressure water supply system into operation, when the frequency converter output frequency reaches the upper limit of frequency (frequency converter can be set to 50 Hz), running 60 s pipe network water pressure does not reach the given value, at this time, the pump needs to be switched to industrial frequency operation. Switching process is the first off the pump solenoid valve, and then the frequency converter parking (parking mode is set to free parking), the pump motor inertia, taking into account the residual voltage in the motor, the motor can not be immediately switched to industrial frequency, but delayed for a period of time, to the motor in the residual voltage drops to a smaller value, to ensure that the power supply voltage with different phases caused by the switching current impact is small.

1C31227G01 switching time of 600 ms, connected to the motor frequency circuit of the air switch capacity of 400 A. After closing the valve to stop, the pump motor is basically in no-load operation, to 600 ms when the motor speed does not drop a lot, so that the switching current impact is small. After the switching is completed, the solenoid valve is opened again, and the parked frequency converter starts and runs the other pump. When the frequency converter output detects the lower frequency limit (can be set to 30 Hz), should be cut off the earliest start of the industrial frequency pump, cut off the industrial frequency pump, should also be the first valve, after parking, so that no “water hammer” phenomenon. These operations are automatically completed by the PLC control.