The two-wire system, three-wire system and four-wire system of electromagnetic flowmeter refer to the differences in the working principle and layout of various transmitters whose output is to imitate DC current signals, rather than just the wiring mode of transmitters.
first look at their definitions.
Two-wire system: two wires and transmission power supply are also called transmission signals, that is, the load and power supply output by the sensor are connected in series, and the power supply is introduced from the outside and connected in series with the load to drive the load.
Three-wire system: The three-wire sensor is the separation of the positive end of the power supply and the positive end of the signal output, but they share a COM terminal.
four-wire system: two power lines, two signal lines. The power supply and the signal work separately.
the difference of different wire system
1. two-wire system
Because to complete the two-wire transmitter, it is necessary to meet the following conditions together:
1.V≤Emin-ImaxRLmax
transmitter is equal to the regular minimum supply voltage minus the voltage drop of the current across the load resistance and the resistance of the transmission conductor.
2. I≤Imin
transmitter must be less than or equal to the output current of the transmitter.
3. P<Imin(Emin-IminRLmax)
transmitter cannot exceed the above formula, generally <90mW.
formula: Emin = minimum power supply voltage, Emin = 24(1-5%)= 22.8V,5% is the negative change allowed by 24V power supply;
Imax = 20mA;Imin = 4mA;RLmax = 250Ω + transmission lead resistance.
If the transmitter is satisfied with the above three conditions in the planning, the two-wire transmission can be completed. The so-called two-wire system means that the power supply and the load are connected in series with a common point, while the signal connection and power supply between the field transmitter and the appearance of the control room use only two wires, which are both power lines and signal lines. The two-wire transmitter supplies a static operating current for the transmitter because the signal starting current is 4mA.DC. together, the external electrical zero point is 4mA.DC, which does not coincide with the mechanical zero point. this "live zero point" is conducive to identifying faults such as power failure and disconnection. And the two-wire system is also easy to use the safety barrier, which is conducive to safety and explosion-proof.
As shown in the figure, the power supply of the two-wire transmitter is 24V.DC, the output signal is 4-20mA.DC, the load resistance is 250 Ω, and the negative line potential of 24V power supply is the lowest, which is the signal common line. As for the intelligent transmitter, the FSK keying signal of HART protocol can also be loaded on the 4-20mA.DC signal.
2. three-wire system
, in order to reduce the volume and component of the transmitter, improve the anti-jamming function and reduce the wiring, some instrument factories have changed the power supply of the transmitter from 220V.AC to low-voltage DC power supply. For example, the power supply is taken from 24V.DC power supply box, because low-voltage power supply has invented conditions for negative line sharing, thus there is a three-wire transmitter product.
Three-wire transmitter is shown in the figure. The so-called three-wire transmitter uses one wire for the positive end of the power supply, one wire for the positive end of the signal output, and one wire for the negative end of the power supply and the negative end of the signal. Most of its power supply is 24V.DC, the output signal is 4-20mA.DC, the load resistance is 250Ω or 0-10mA.DC, and the load resistance is 0-1.5KΩ; Some still have mA and mV signals, but the load resistance or input resistance has different values due to different output circuit modes.
input receives the appearance of the current signal. If the resistor RL is connected in parallel, the voltage signal is received.
can be seen from the above narrative, because the operating principles and layout of various transmitters are different, and then different commodities are presented, the two-wire, three-wire and four-wire wiring modes of transmitters are also decided. As far as users are concerned, the selection should be based on the actual situation of the unit, such as the consistency of the signal system, explosion-proof requirements, the demand for receiving equipment, capital contributions and other questions to sum up the thinking and selection.
, it should be pointed out that the 4-20mA.DC signals output by three-wire and four-wire transmitters are different from those of two-wire transmitters, so can the negative output terminal be connected to the negative line of 24V power supply in operation? Can we share the land? This is to be noted, if necessary, can adopt the barrier method, such as the use of isolators, safety barriers, etc., in order to and other external common electricity, common ground and to prevent the occurrence of additional interference.
3. four-wire system
, because the 4-20mA.DC(1-5V.DC) signal system is widely used and used, in order to facilitate the connection in the application of the control system, the consistency of the signal system is required. Therefore, the appearance of some non-electric unit combinations, such as online analysis, mechanical quantity, electricity quantity and other appearances, can be selected to output the 4-20mA.DC signal system. However, due to the messy conversion circuit and large power consumption, it is difficult to satisfy all the above three conditions, however, if the two-wire system cannot be achieved, the four-wire transmitter with an output of 4-20mA.DC can only be made by using an external power supply.
four-wire transmitter is shown in the figure. Most of its power supply is 220V.AC, and some are 24V.DC. The output signal is 4-20mA.DC, the load resistance is 250 Ω, perhaps 0-10mA.DC, and the load resistance is 0-1.5KΩ; Some have mA and mV signals, but the load resistance or input resistance has different values due to different output circuit modes.