Definitions of Two-Wire, Three-Wire, and Four-Wire Electromagnetic Flowmeters
The two-wire, three-wire, and four-wire configurations of electromagnetic flowmeters refer to differences in the operating principle and layout of transmitters that output analog DC current signals, rather than merely the wiring method of the transmitter. Understanding these differences helps with correct selection and installation.
Two-Wire
Two wires carry both power and signal; the load output by the sensor and the power supply are connected in series. Power is introduced from the outside and connected in series with the load to drive the load.
Three-Wire
The positive terminal of the power supply and the positive terminal of the signal output are separated, but they share a common COM terminal. Power supply is mostly 24V.DC, and output signals include 4-20mA.DC or 0-10mA.DC, etc.
Four-Wire
Two wires for power supply and two wires for signal. Power and signal work separately; power supply is mostly 220V.AC or 24V.DC.
Differences and Working Principles of Different Wiring Systems
1. Two-Wire
To implement a two-wire transmitter, the following conditions must be met simultaneously:
- V ≤ Emin - ImaxRLmax: The transmitter output terminal voltage V equals the specified minimum power supply voltage minus the voltage drop across the load resistance and transmission wire resistance.
- I ≤ Imin: The normal operating current I of the transmitter must be less than or equal to the transmitter's output current.
- P < Imin(Emin - IminRLmax): The minimum power consumption P of the transmitter cannot exceed the above formula, generally <90mW.
Where: Emin = minimum power supply voltage; for most instruments, Emin=24(1-5%)=22.8V, where 5% is the allowable negative variation of the 24V power supply; Imax=20mA; Imin=4mA; RLmax=250Ω + transmission wire resistance.
If the transmitter meets the above three conditions in design, two-wire transmission can be achieved. The so-called two-wire system means that the power supply and load are connected in series with a common point, and only two wires are used for signal connection and power supply between the field transmitter and the control room instrument. These two wires serve as both power lines and signal lines.
Because the signal starting current of a two-wire transmitter is 4mA.DC, it provides a static operating current for the transmitter. At the same time, the instrument electrical zero point is 4mA.DC, which does not coincide with the mechanical zero point. This "live zero" helps identify faults such as power failure and wire breakage. In addition, the two-wire system facilitates the use of safety barriers, which is beneficial for intrinsic safety and explosion protection.
As shown in the figure, the two-wire transmitter is powered by 24V.DC, the output signal is 4-20mA.DC, and the load resistance is 250Ω. The negative line of the 24V power supply has the lowest potential and serves as the signal common line. For smart transmitters, FSK keying signals of the HART protocol can also be superimposed on the 4-20mA.DC signal.
2. Three-Wire
Some instrument manufacturers, in order to reduce the volume and weight of the transmitter, improve anti-interference performance, and simplify wiring, changed the transmitter power supply from 220V.AC to low-voltage DC power, such as drawing power from a 24V.DC power box. Because low-voltage power supply creates conditions for a common negative line, three-wire transmitter products came into being.
As shown in the figure, the so-called three-wire system means that the positive terminal of the power supply uses one wire, the positive terminal of the signal output uses one wire, and the negative terminal of the power supply and the negative terminal of the signal share one wire. Its power supply is mostly 24V.DC, with output signals of 4-20mA.DC and a load resistance of 250Ω, or 0-10mA.DC with a load resistance of 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input resistance values differ due to different output circuit modes.
The receiving instrument input is a current signal; if a resistor RL is connected in parallel, then the received signal is a voltage signal.
From the above description, it can be seen that because the operating principles and layouts of various transmitters are different, different products have emerged, which also determines the two-wire, three-wire, and four-wire wiring methods of transmitters. For users, when selecting a model, they should comprehensively consider factors such as consistency of signal system, explosion-proof requirements, requirements of receiving equipment, and investment according to the actual situation of their unit.
It should be pointed out that for the 4-20mA.DC signal output by three-wire and four-wire transmitters, because their output circuit principles and layout are different from those of the two-wire type, attention should be paid in application to whether the output negative terminal can be connected to the negative line of the 24V power supply and whether they can share a common ground. When necessary, isolation measures can be taken, such as using isolators and safety barriers, to facilitate common power supply, common grounding with other instruments, and prevent additional interference.
3. Four-Wire
Due to the popularity and application of the 4-20mA.DC (1-5V.DC) signal system, in control system applications, consistency of the signal system is required for easy connection. For this reason, some instruments not belonging to the electric unit combination series, such as online analysis, mechanical quantity, and electrical quantity instruments, need to adopt a 4-20mA.DC signal output. However, due to complex conversion circuits and high power consumption, it is difficult for them to fully satisfy the above three conditions and achieve a two-wire configuration, so they can only use an external power supply to make a four-wire transmitter with 4-20mA.DC output.
As shown in the figure, the four-wire transmitter is mostly powered by 220V.AC, and some are powered by 24V.DC. Output signals include 4-20mA.DC with a load resistance of 250Ω, or 0-10mA.DC with a load resistance of 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input resistance values differ due to different output circuit modes.
Comparison of Two-Wire, Three-Wire, and Four-Wire
| Wiring System | Power and Signal | Supply Voltage | Output Signal | Load Resistance | Features |
|---|---|---|---|---|---|
| Two-Wire | Share two wires | 24V.DC | 4-20mA.DC | 250Ω | Live zero, facilitates intrinsic safety and explosion protection, must meet three conditions |
| Three-Wire | Power positive and signal positive separated, common negative | 24V.DC | 4-20mA.DC or 0-10mA.DC | 250Ω or 0-1.5KΩ | Small size, strong anti-interference, simplified wiring |
| Four-Wire | Power and signal completely separated | 220V.AC or 24V.DC | 4-20mA.DC or 0-10mA.DC | 250Ω or 0-1.5KΩ | High power consumption, complex circuit, used for special instruments |
Selection Recommendations
When selecting a model, users should comprehensively consider factors such as consistency of signal system, explosion-proof requirements, requirements of receiving equipment, and investment according to the actual situation of their unit. Attention should be paid to whether the output negative terminal of three-wire and four-wire transmitters can be connected to the negative line of the 24V power supply or share a common ground. When necessary, isolation measures such as isolators and safety barriers should be adopted.