I. Overview

The water supply system is an indispensable and vital component of both national production and daily life. Traditional water supply methods have several drawbacks: they occupy large areas, are prone to water quality contamination, require substantial infrastructure investments, and—most critically—cannot maintain a constant water pressure, which often leads to malfunctioning of certain equipment. With the increasing maturity of frequency conversion technology, variable-frequency constant-pressure water supply systems have fundamentally addressed these issues. This technology boasts advanced features such as stable water pressure, convenient operation, reliable performance, energy savings, and a high degree of automation. As a result, variable-frequency constant-pressure water supply technology has now been widely adopted across various industries.

II. Scope of Application

◆ Residential water supply for newly built residential communities, villas, office buildings, and mixed-use buildings.

◆ Domestic water supply for high-rise buildings, fire-fighting water systems, and upscale hotels and restaurants.

◆ Water booster systems for water treatment plants, agricultural irrigation, wastewater treatment, and artificial fountains

◆ Various boiler cold-water supply systems and boiler hot water systems.

◆ Intermediate booster pump stations at water treatment plants and secondary pressurization of tap water.

◆ Stabilization of water pressure in production and domestic water supply networks for various industrial and mining enterprises.

III. System Composition

The variable-frequency constant-pressure water supply equipment mainly consists of a pump unit, a pressure-measuring and stabilizing tank, a pressure sensor, a variable-frequency control cabinet, and related valve fittings.

The Puchuan PI500 series frequency converters come with a built-in PID controller. In a constant-pressure water supply system, you only need to install one pressure sensor (either a remote pressure gauge or a pressure transmitter) in the user’s pipeline network to establish a closed-loop pressure control system. This system compares the set pressure with the actual pressure feedback from the pipeline network and automatically adjusts the output frequency of the frequency converter, thereby changing the rotational speed of the pump to maintain a constant pipeline pressure.

IV. Product Features

◆ Highly efficient and energy-saving. Compared to traditional water supply methods, variable-frequency constant-pressure water supply systems can save approximately 30% to 60% of energy.

◆ The equipment occupies a small footprint, requires low investment, and boasts high operational efficiency.

◆ The control system can operate in manual or automatic mode.

◆ Online adjustment of PID parameters, with sleep and wake-up functions.

◆ Pump unit and pipeline protection detection, alarm, and signal display, etc.

V. General Principles

The Puchuan PI500 series frequency converters come with a built-in PID controller. In a constant-pressure water supply system, you only need to install one pressure sensor (either a remote pressure gauge or a pressure transmitter) in the user’s piping network to establish a closed-loop pressure control system.

In a variable-frequency drive (VFD)-based constant-pressure water supply system, pressure sensors installed on the water distribution network continuously collect pressure data and convert it into standard electrical signals, which are then transmitted to the VFD. The PLC controller performs logical operations on the received signal (measured value) and the preset pressure value (setpoint), and converts the result of these operations into a frequency-adjustment signal, thereby regulating the rotational speed of the water pump. When water consumption increases and the network pressure drops (i.e., the measured value falls below the setpoint), the PID controller causes the VFD output frequency to rise, accelerating the pump’s rotational speed accordingly. Conversely, when water consumption decreases and the network pressure rises (i.e., the measured value exceeds the setpoint), the PID controller reduces the VFD output frequency, slowing down the pump’s rotational speed. In this way, the network pressure is maintained at a nearly constant level.

6. Installation and Debugging 
According to the instruction manual, connect the frequency converter to components such as the circuit breaker, motor, start-stop switch, and pressure sensor. The frequency converter and the motor must be reliably grounded, and carefully check that all wiring connections are correct. In particular, the input and output lines must never be connected in reverse. Signal lines for devices such as the pressure sensor must use multi-core shielded cables to ensure that the signals remain free from interference during transmission. All power cables and signal cables must be routed separately to prevent signal crosstalk.

The pressure gauge selected is a YTZ-150 potentiometer-type remote pressure gauge, which is installed on the outlet pipe of the water pump. This pressure gauge is suitable for general environmental conditions and can both directly display the pressure value and output a corresponding electrical signal.

Electrical technical parameters of pressure gauges:

Full-scale resistance: 400Ω (blue wire - red wire); initial resistance value at zero pressure: ≤20Ω (yellow wire - blue wire); maximum resistance value at full-scale pressure: ≤360Ω (yellow wire - red wire). Applied voltage across the terminals: 10V (blue wire - red wire).

The pressure gauge has three lead wires: red, yellow, and blue. Connect the red wire to the VFD’s +10V, the blue wire to the VFD’s GND, and the yellow wire to the VFD’s AI1.

Typically, current-based pressure transmitters are two-wire devices. If you’re using the 24V power supply from a frequency inverter, you’ll need to connect the COM terminal to the GND terminal. Connect the 24V output of the frequency inverter to the positive terminal of the pressure transmitter, and connect the negative terminal of the pressure transmitter to the AI1 terminal of the frequency inverter (i.e., the analog current input terminal). Also, make sure to reposition the AI1 jumper on the control board to the “current” position.

After all the wiring connections have been completed, carefully check and verify them against the relevant instructions to ensure accuracy before proceeding with equipment debugging.

Close the air switch, power on the frequency converter, and run the motor at a lower frequency to check the pump’s rotation direction. If the rotation is reversed, change the motor’s phase sequence.

Variable-frequency constant-pressure operation:

Close the run switch, and the inverter’s operation indicator light will illuminate. The output frequency will increase from 0.0 Hz according to the pipeline pressure until it reaches the corresponding frequency, then automatically adjust based on water usage conditions, ensuring that the outlet pressure remains constant at the set value. Increasing the parameter setting value of E2.01 will raise the outlet pressure, while decreasing the parameter setting value of E2.01 will lower the outlet pressure.

7. Parameter Settings

Note: In general, the wake-up frequency should be greater than or equal to the sleep frequency. If both the wake-up frequency and the sleep frequency are set to 0.00 Hz, the sleep and wake-up functions will be disabled. When the sleep function is enabled and the frequency source uses PID control, whether the PID controller operates during the sleep state is influenced by function code E2.27. In this case, you must select “PID operation upon shutdown” (E2.27=1).

E2.01 Method for Calculating the Keyboard-Provided Signal Value Parameter: E2.01 = Set Pressure / Full-Scale Pressure of Pressure Gauge × 100%. For example: If the pressure gauge’s full-scale range is 1.0 MPa and the required network pressure is to be maintained at a constant 0.5 MPa, then the set value for the E2.01 parameter would be:

50. If you’re unable to calculate the given pressure percentage, you can also use a simple workaround: start the frequency converter and simultaneously monitor the value displayed by d0.16 in the monitoring function group and the reading on the remote pressure gauge. Once the remote pressure gauge reaches the pressure level required by the user, the value shown by d0.16 at that moment will be the value that E2.01 needs to be set to.

8. Multi-pump water supply

The system described above is a single-pump constant-pressure water supply system. However, if the water demand is very high and a single-pump system cannot meet the user’s needs, a multi-pump constant-pressure water supply system should be adopted.

The main circuit diagram for multi-pump constant-pressure water supply is shown in the figure below:

The control circuit for the pump constant-pressure water supply system is shown in the figure below:

Contactors KM2 and KM4 are used for the inverter output, while contactors KM1 and KM3 are used for the power-frequency output, both connected to the water pump.

M1 and M2—using a frequency converter, any pump can be controlled at constant pressure. The air switch (QL) automatically disconnects the motor from the power grid in case of motor overload. The thermal relay (FR) is a protective circuit that operates based on the thermal effect of electric current; it serves as an overload protection device for the motor in the circuit.

◆ Pumping process

First, M1 operates under variable-frequency control. When water consumption increases and water pressure drops, even after the inverter’s output frequency rises to 50 Hz, the water pressure still falls short. After a brief delay, M1 is switched to mains-frequency operation, while the inverter’s output frequency rapidly drops to 0. Subsequently, M2 is put into variable-frequency operation, thereby maintaining the water pressure at the set value. ()

◆ Pump reduction process

When the water consumption decreases and the water pressure rises, and the constant-pressure water supply controller receives a signal indicating that the lower limit frequency has been reached, after a certain delay, contactor KM1 is de-energized and resets, causing pump M1 to disconnect from the power supply at line frequency and stop running. Meanwhile, the variable-frequency drive continues to power pump M2, keeping the water pressure stable at the set value.

Note: If the motor power is very high, during the transition from variable frequency to line frequency, if the phases of the two systems are not synchronized, the switching current can become extremely large—sometimes exceeding several times the rated current—and may even cause the power grid to trip, resulting in abnormal operation. In such cases, you should consider adopting a synchronous soft-switching approach.

9. Constant Pressure Water Supply Controller 
Terminal Wiring Diagram for VC-3200 Controller

Operation Panel Indication and Parameter Setting Instructions

1. Panel and buttons:

The PV window is the display window for measured values, and the SV window is the display window for setpoint values. The “S” key is the parameter-setting key; the “▲” and “▼” keys are the two numeric increment and decrement keys. In the parameter-setting mode, the “M” key and the …

2. Work status indicator light

The three pump status indicator lights—P1, P2, and P3—represent the three pumps. When an indicator light is green, it means the corresponding pump is operating in variable-frequency mode; when the indicator light is red, it means the corresponding pump is operating in line-frequency mode. When the system is operating at the second pressure setting (fire-fighting pressure), the AL indicator light turns green. When the low-water (shutdown) terminal is activated, the AL indicator light turns red, and all output controls of the controller immediately stop until the low-water (shutdown) condition is resolved, at which point the controller resumes operation. The OUT indicator light is a spare indicator.

3. Parameter Settings

In normal operating mode, press and hold the “S” key for 3 seconds. When the display window shows “-.-.- -.-.-”, release the “S” key to enter the parameter setting mode. At this point, the PV window displays the parameter item P00, and the SV window displays the current value of that parameter item. The “M” key or the spacebar serves as the page-turning key for navigating through different parameter items. Press the “▲” or “▼” key to change the value of the currently selected parameter item; the modified value will be automatically stored in the meter’s memory. Once the parameter settings are complete, press the “S” key again to return the meter to normal operating mode. If P00 = 18 at this time, pressing the “▲” and “▼” keys will directly modify the current pressure setpoint (the setpoint for P01). When the second pressure switch (fire protection)—connected between DI1 and CM2—is closed, the SV window displays the second setpoint pressure. Pressing the “▲” or “▼” keys will directly modify the current second setpoint pressure; alternatively, the second pressure can also be set in P02.

X. Precautions 
◆ The pump must not operate at speeds exceeding its rated speed or falling below a certain minimum speed. Exceeding the rated speed can cause mechanical damage to components such as bearings and piping systems, while operating below the minimum speed will rapidly reduce the pump’s efficiency. Therefore, the frequency converter must be set with appropriate upper and lower limits for the operating frequency.

◆ When switching to constant-pressure water supply and the motor is controlled at power frequency, it is essential to install a suitable thermal relay to protect the motor.

◆ An AC contactor equipped with a mechanical interlock should be used between the power-frequency bypass on the motor side and the inverter output, and logical interlocks should be implemented in the electrical control circuit to prevent short circuits between the power-frequency power supply and the inverter output, thereby protecting the inverter and related equipment from damage.

◆ The phase sequence of the power supply at industrial frequency to which the motor is connected must match the phase sequence of the inverter’s output. To prevent motor reversal accidents, please confirm the phase sequence before operation.

◆ When connecting the control signals for the frequency converter, keep them separate from the power cables and do not route them in the same conduit. Otherwise, false operations may occur. The pressure setpoint signal line and the pressure feedback signal line must use shielded cables.