Abstract : This paper compares the advantages and disadvantages of two commonly used schemes for multi-pump variable frequency constant pressure water supply. It focuses on the control process of cyclic switching of multi-pump variable frequency constant pressure water supply, and illustrates the feasibility of the cyclic switching scheme with examples. Keywords : Variable frequency drive, cyclic switching, application example 1 Introduction In water supply pumping stations of waterworks, the water supply system generally consists of several pumps with similar heads. The traditional method for regulating water pressure and flow is to manually control the number of pumps operating according to the desired output water pressure and flow. For example, in a waterworks with a water supply capacity of 40,000 to 60,000 tons/day, there are several pump configuration schemes. One feasible scheme is composed of three 160kW and one 90kW pumps. The traditional method for system operation is that if the water supply is large, the flow and network water pressure obviously cannot meet the requirements, and the pumps need to be manually activated. Workers decide whether to activate the 160kW or 90kW pump based on the on-site network water pressure. If the water supply decreases, the network water pressure will increase, and the pumps need to be manually deactivated. When water consumption is low late at night, a 90kW water pump is used to supply water for energy conservation. Due to the large flow rate of the water pump, in order to avoid the "water hammer" effect, when manually switching the pump on and off, the operation procedure should be "start the machine first, then open the valve" and "close the valve first, then stop the machine". For low-power water pumps, ordinary check valves are installed on the outlet side, which can basically automatically ensure the above operating procedures. However, a "water hammer" effect still occurs momentarily before the check valve closes when the pump stops. If a "low-resistance slow-closing check valve" is installed, a water hammer effect will still occur when the pump stops. 2. Control Scheme for Variable Frequency Constant Pressure Water Supply Because the consumption of urban tap water varies with the seasons and different times of day, the most common method to maintain constant water pressure is to use a variable frequency constant pressure water supply system. This involves installing a pressure transmitter on the main pipeline to detect the pipeline pressure signal, and then sending this pressure signal to the analog signal input port of the frequency converter via a PLC. This forms a closed-loop pressure control system, and the constant pipeline pressure depends on the regulation and control of the frequency converter. [b][align=center]For details, please click: Variable Frequency Circulating Switching Constant Pressure Water Supply System for Waterworks[/align][/b]