Schneider Altivar VFD Constant-Pressure Pump PID & Sleep Setup Guide

Engineering Approach to Water Supply Automation: Constant Pressure with Schneider Altivar

Maintaining stable pressure in a water supply network is essential for extending the life of hydraulic fittings and minimizing the power consumption of pumping units. Direct-on-line starting of a pump causes severe water hammer and excessive electricity consumption due to continuous operation at the maximum 50 Hz grid frequency. Utilizing Schneider Electric Altivar variable frequency drives (VFDs) enables smooth regulation of motor speed based on feedback from a pressure transducer. At our Kyiv warehouse, we build and supply control cabinets based on ATV12, ATV310, and ATV320 drives daily. Our engineers have accumulated extensive practical experience implementing these solutions for residential deep wells and industrial pressure boosting stations. Below are detailed wiring diagrams, exact parameter codes, and system configuration guidelines.

Wiring a 2-Wire 4-20mA Pressure Transducer

For feedback in the PID control loop, an industrial pressure transducer with a 4-20mA output signal and 24 VDC excitation power is standard. A current loop is highly resistant to electromagnetic interference, allowing the feedback cable to run long distances (up to 150-200 meters) without signal distortion. From our experience, a sensor with a 0-10 bar measuring range is ideal for most residential wells and light commercial systems. Wiring a two-wire transducer varies slightly across different Altivar VFD series:

• Altivar 12 (ATV12): The two-wire sensor is powered by the built-in +24V source. Connect the positive terminal of the sensor (L+) to the +24V terminal on the VFD. Connect the negative signal terminal of the sensor (Signal/L-) to the AI1 analog input. To ensure correct operation, the AI1 analog input must be set to current mode. Under the terminal cover on the control board, locate the analog input DIP switch and move it to the "I" (Current) position. In the programming menu I-O-, set parameter AI1t (Analog Input 1 Type) to 4A (4-20mA signal).
• Altivar 310 (ATV310): The excitation connection is identical — connect the +24V VFD terminal to L+ on the sensor. Connect the return signal wire to the AI1 terminal. Under the HMI screen on the control board, toggle the analog input microswitch to the "I" position. In the configuration parameters, change parameter 502.0 (AI1 Type) to mA.
• Altivar 320 (ATV320): This drive features three analog inputs. The most practical input is AI3, which is hardware-designed as a dedicated current input (0-20mA or 4-20mA) and does not require toggling physical board switches. Connect the positive wire of the transducer to the +24V terminal, and the negative signal wire to the AI3 terminal. If using the AI2 input is necessary, configure the board switch and set parameter AI2t to 4A.

It is mandatory to use shielded cable (with a cross-section of at least 0.5-0.75 mm²) for the signal loop. Connect the cable shield to the PE grounding busbar only on the VFD side. Leave the shield isolated on the sensor side to prevent circulating ground loop currents from distorting the feedback signal.

PID Control Configuration: FUn- / PId- and Group 59 Parameters

The built-in PID regulator of the Altivar drive compares the target pressure (Setpoint) with the actual feedback signal from the pressure transducer and smoothly adjusts the motor's output frequency. The PID loop parameters for the different series are configured in the following menus:

Our practical engineering experience shows that tuning should begin with a proportional gain of rPG = 1.2 and an integral time of rIG = 2.0 seconds. If a severe pressure drop occurs when multiple faucets open, gradually increase the proportional coefficient in 0.1 increments. If the pressure oscillates dynamically (with the VFD accelerating and decelerating under a steady flow rate), it indicates over-regulation. In this case, reduce rPG by 0.2 or increase the integral time rIG to 3.0-4.0 seconds.

Setting up Sleep and Automatic Wake-up Modes

When all water fixtures are closed, system pressure reaches the setpoint, and the PID regulator reduces the motor speed. However, the pump cannot stop completely on its own because even at low speeds (such as 28-30 Hz), it continues to spin, maintaining static pressure while drawing electrical current and overheating the water inside the chamber. Sleep mode is designed to shut down the drive when flow drops to zero, and Wake-up mode restarts the system when pressure drops.

Configure the sleep logic for Altivar drives as follows:

1. Sleep Frequency Threshold (Low Speed Limit): Configured using the low speed limit parameter LSP (located in the SEt- menu for ATV12/320 or 512.0 for ATV310). This value must equal the frequency at which the pump operates against a closed valve without producing useful flow. For surface centrifugal pumps, this threshold is typically 30.0-32.0 Hz. For submersible borehole pumps (due to the high vertical water column backpressure and heavy impellers), the sleep threshold is higher, around 34.0-37.0 Hz.
2. Sleep Delay Time (Low Speed Operating Time): Set via parameter tLS (in the SEt- or Stt- menu for ATV12/320, or 512.1 for ATV310). Configure this to 5-10 seconds. This is the duration the VFD must run continuously at the minimum speed LSP before shutting down output power to the motor. This delay prevents rapid cycling during brief water use.
3. Wake-up Level: Configured using parameter rSL in the PID menu (or 59.19 / 59.20 for ATV310). This parameter defines the pressure drop below the setpoint required to wake up the VFD. It is entered as a percentage of the pressure sensor's full scale. We recommend setting this between 10.0% and 15.0%. For example, with a target pressure of 3.0 bar on a 10 bar sensor, a 10% wake-up threshold will restart the VFD when system pressure falls to 2.7 bar.

Dry-Run Protection Configuration via LI3/LI4 Digital Inputs

Operating a pump without water causes rapid friction overheating of the diffusers, wearing out shaft seals and seizing the hydraulic stages. Schneider Altivar VFDs provide reliable dry-run protection by mapping digital inputs to the External Fault function. A float switch inside a storage tank or an electromechanical dry-run pressure switch (whose contacts open when pressure drops below 0.2-0.5 bar) can be used as the dry-run sensor.

For a failsafe wiring scheme resistant to cable breaks, a normally closed (NC) sensor contact is recommended. Connect the sensor contacts between the digital input power terminal +24V and the digital input LI3 (or LI4) of the VFD. When water is present, the contact is closed, sending a continuous +24V signal to the LI3 input. If a dry-run condition occurs, the contact opens, the +24V signal disappears, and the VFD instantly stops the motor, displaying an external fault code.

Parameter configuration for dry-run protection:

• ATV12: Navigate to COnF -> FULL -> FLt-. Locate parameter EtF (External Fault Assignment) and set it to L3L (LI3 Input, Active Low level).
• ATV310: In the configuration parameters, set parameter 603.0 (External Fault Assignment) to 3 (Digital Input LI3). The logic is configured so that the fault is triggered when the circuit opens.
• ATV320: Go to CONF -> FULL -> FLT- -> ETF-. Set parameter ETF (External Fault Assignment) to LI3. Then, set parameter EPL (External Fault Level) to LO (Low level — active low, meaning the fault triggers when the +24V signal disappears).

When the protection is triggered, the VFD transitions to a fault state, displaying code EPF1 (External Fault 1). To restart the system after the well recovers, press the reset button on the VFD keypad or configure automatic fault reset within the protection parameters group.

Membrane Expansion Tank Volume and Pre-charge Calculations

A variable frequency drive dynamically regulates pump speed, but the physics of liquids demands a membrane expansion tank in the system. Water is virtually incompressible. When all valves close and demand drops to zero, system pressure rises instantly. Without an expansion tank, even a microscopic water leak (such as a weeping fitting or check valve leakage) causes an immediate pressure drop down to the wake-up threshold. The VFD will wake up, run for 2-3 seconds to restore pressure, go back to sleep, and repeat this cycle every few seconds. This rapid cycling, known as "tacturing," quickly burns out the VFD's power transistors and damages motor windings.

When designing VFD-controlled water systems, adhere to these rules:

1. Tank Volume: VFD-controlled systems do not require massive 80-150 liter expansion tanks standard in classic on/off pressure switch setups. A small membrane tank between 8 and 24 liters is sufficient. Excessively large tanks add hydraulic lag, slowing the PID regulator's response when faucets are opened.
2. Pre-charge Air Pressure: This is a critical parameter. Measuring pre-charge air pressure must be done exclusively when the tank is empty of water (with system pressure at zero). The pre-charge air pressure must be exactly 60-70% of the operating setpoint pressure. For example, if the VFD is set to maintain a constant 3.0 bar, the empty expansion tank must be pre-charged with an air compressor to 1.8-2.1 bar.
3. Maintenance: Check the pre-charge pressure with a pressure gauge at least once every six months. A loss of air pre-charge will lead to rapid pump cycling and pressure surges under low flow conditions.

Conclusions and Practical Recommendations

Implementing variable frequency speed control using Schneider Electric Altivar drives ensures high-precision pressure stability, smooth starting with no inrush currents, and substantial energy savings. At our Kyiv warehouse, we maintain a robust inventory of ATV12, ATV310, and ATV320 drives. You can purchase original Schneider Electric frequency inverters with an official manufacturer warranty from our catalog. For borehole and booster pumps, head to the pump frequency converters section to explore dedicated models. If your equipment operates on a single-phase 220V household power supply, browse our single-phase frequency converters. Chastotnik.ua engineers are ready to provide complete technical support, design your electrical diagrams, and pre-configure the VFD parameters for your specific application requirements.