Wiring Diagram for Connecting a Two-Wire 4-20mA Pressure Sensor to a Veichi VFD
To connect a standard two-wire pressure sensor with a 4-20 mA output signal to a Veichi variable frequency drive, you must use the +24V power terminal and the AI2 analog input, having previously switched it to current measurement mode using the hardware switch. The physical connection is made using a current loop circuit: the positive power supply from the +24V terminal is connected to the positive terminal of the sensor, and the sensor's signal output is returned to the AI2 analog input of the frequency inverter. In this setup, the common GND/COM wire acts as the reference point for the internal control board.
The two-wire scheme is a classic solution for industrial systems because it uses the same two wires for both powering the sensor and transmitting the information signal. This minimizes cabling costs and reduces the impact of electromagnetic interference over long distances. Before starting the connection, make sure that the signal type switch for the AI2 input on the Veichi VFD control board (usually labeled as AI2 or CCI/CVI) is set to the current (I) position rather than voltage (V). This connects an internal 250 Ohm shunt resistor in parallel with the input, converting the 4-20 mA input current into a 1-5 V voltage that the microcontroller's analog-to-digital converter can recognize.
To build a reliable water supply system, it is important to select the right equipment, where a specific category of sensors will ensure accurate measurement of physical parameters. Below is a step-by-step sequence for making the electrical connections:
- Completely de-energize the variable frequency drive and wait for the DC link capacitors to discharge (the Charge indicator must go out).
- Locate the AI2 analog input DIP switch on the control board and set it to the current signal (I) position.
- Connect a wire from the +24V terminal of the frequency inverter to the (+) terminal of the pressure sensor.
- Connect a wire from the (-) terminal of the pressure sensor to the AI2 analog input terminal of the frequency inverter.
- Check the tightness of the screw terminals and ensure there are no short circuits between adjacent terminals. It is recommended to ground the cable shield on one side to the PE terminal of the inverter to protect against high-frequency interference from the motor power cable.
Analog Input Parameter Configuration and PID Controller Activation
Recommended Parameters for System Configuration
To activate automatic pressure maintenance, the variable frequency drive must be switched to PID control mode, the feedback source must be specified, and the operating setpoint must be set. Without proper programming of the internal registers, the device will ignore signals on the analog inputs and operate according to the standard frequency reference curve.
In Veichi frequency inverters (for example, AC10 or AC310 series), process control is implemented through parameters of group F13 (PID settings) and group F05 (linear processing of analog inputs). The table below shows the basic set of parameters that must be written to the non-volatile memory of the device to work with a current pressure sensor.
| Parameter Code | Parameter Name | Factory Value | Recommended Value | Setting Description |
|---|---|---|---|---|
| F13.00 | PID Controller Reference Source | 0 | 0 | Setting from the control panel (keypad) via parameter F13.01 |
| F13.01 | PID Digital Setpoint (Keypad) | 50.0% | Depending on requirements | Target pressure as a percentage of the full sensor range (for example, 50% for a 10 bar sensor equals 5 bar) |
| F13.03 | PID Feedback Source | 2 | 3 | Sets analog input AI2 as the feedback channel from the pressure sensor |
| F05.55 | AI Input 2 Lower Limit | 0.0% | 20.0% | Corresponds to the lower limit of the 4 mA current signal (in current mode the AI2 input is digitized over the 0-20 mA = 0-100% range, so 4 mA = 20%) |
| F05.56 | AI2 Lower Limit Setting | 0.00% | 0.00% | Lower measurement limit of the sensor (0 bar) |
| F05.57 | AI2 Upper Limit | 100.0% | 100.0% | Corresponds to the upper limit of the 20 mA current signal |
| F05.58 | AI Input 2 Upper Limit Setting | 100.00% | 100.00% | Upper measurement limit of the sensor (e.g., 100% of scale) |
The Reverse Regulation Problem: Why Does Frequency Increase with Pressure?
If, as the system pressure increases, the pump motor speed begins to rise instead of falling, you must immediately change the direction of the PID controller or invert the analog input characteristic in parameter group F05. This system behavior is a critical configuration error that leads to accidental overpressurization of the pipeline, as the VFD attempts to reach the setpoint by increasing speed when the feedback signal rises.
This problem occurs due to a mismatch between the controller's operating logic and the process type. Water supply systems require a positive feedback characteristic: when pressure drops, the frequency must increase, and when pressure reaches the target, the frequency must decrease to the minimum or the VFD must enter sleep mode. This problem can be solved in two independent ways:
- Software inversion of the analog input characteristic in group F05. This is the most elegant method, which does not require changing the logic of the PID controller itself. To do this, you need to change the scaling points of the AI2 input. Set parameter F05.56 (AI2 lower limit setting) to 100.00%, and parameter F05.58 (AI input 2 upper limit setting) to 0.00%. Thus, at a minimum current of 4 mA, the system will see the maximum demand for regulation, and at a maximum current of 20 mA, the feedback will be perceived as full saturation, forcing the frequency to decrease.
- Changing the PID feedback characteristic via parameter F13.07. The LED"0" digit of parameter F13.07 sets the feedback characteristic. Constant-pressure water supply needs the positive characteristic (LED"0" = 0): when pressure exceeds the setpoint, frequency drops. If the negative characteristic (LED"0" = 1) is set there by mistake, the controller behaves the opposite way. However, the first method via group F05 is more universal because it directly corrects the physical signal perception curve of the control board.
Diagnosing the E.Pid Error (Loss of Feedback)
The appearance of the E.Pid error code on the Veichi frequency inverter display indicates a break in the communication line with the pressure sensor or a drop in the signal level below a critical threshold, requiring an immediate check of cable integrity, power supply voltage, and the correctness of the fault detection parameters. This function protects the system from dry running and pipe bursts, as without feedback, the PID controller would run continuously at maximum frequency.
To resolve the E.Pid error and prevent its false triggering, perform the following diagnostic steps:
- Measure the voltage at the sensor terminals with a multimeter. It should be within 12-24 VDC. If there is no voltage, check the +24V output on the VFD or the presence of an external power supply.
- Switch the multimeter to current measurement mode (mA) and connect it in series into the break of the AI2 signal wire. At atmospheric pressure, a working sensor should output exactly 4.0 mA. If the current is 0 mA, this clearly indicates a cable break or a failure of the sensor's sensing element.
- Check the feedback loss detection parameters in group F13. Parameter F13.26 defines the filtering and delay time before the protection trips when the signal is lost (1.0 s by default). Parameters F13.27 (upper limit) and F13.28 (lower limit) set the signal thresholds; the feedback going beyond them for the F13.26 time is treated as a sensor break, while the response type itself (stop with an E.Pid trip, or keep running with an A.Pid warning) is selected in F13.25. If the sensor operates under heavy pulsation conditions, increase the delay time in F13.26 to 5.0-10.0 seconds to filter out short-term pressure drops.
Comparative Analysis of Signal Inversion Methods
When choosing a method to correct the wrong control direction, engineers often hesitate between software configuration and physical wiring changes. Software inversion via group F05 parameters has significant advantages, as it preserves measurement linearity and allows flexible system tuning without interfering with the electrical circuit of the control cabinet.
Below is a comparison table of the two main approaches to solving the reverse regulation problem in Veichi frequency inverters.
| Comparison Criterion | Software Inversion (Group F05: F05.56=100, F05.58=0) | PID Characteristic Change (F13.07, LED"0") |
|---|---|---|
| Implementation Complexity | Low. Requires changing only two parameters in the VFD menu. | Very low. Changing one digit in parameter F13.07. |
| Impact on Pressure Display | Inverts the physical perception of the signal. Requires attention when monitoring actual pressure on the display. | Preserves direct pressure display on the screen, changing only the logic of the PID error calculation. |
| Tuning Flexibility | High. Allows building non-linear cutoff curves and limiting control ranges. | Low. Only two-position characteristic switching (positive/negative). |
| Compatibility with Break Protection | Fully compatible. The E.Pid error detection algorithm continues to work correctly. | Requires additional verification of signal comparison polarity for dry-run protection. |
Thus, for most standard applications in pumping stations, it is recommended to use the PID feedback characteristic change via parameter F13.07, as this preserves the correct display of the current pressure on the frequency inverter screen. However, if the sensor has a non-standard characteristic or operates in a reverse process where maximum pressure corresponds to minimum cooling demand, inversion via group F05 is the only alternative and the most reliable tool for fine-tuning the analog path.
