The feedback signal for electric valves has been changed from four wires to three wires
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There was an electric valve that previously used a four-wire system for position feedback, with two wires for opening and two wires for closing; 24VDC was supplied between these wires. After replacing it with a new valve, this one has only three contacts, one of which is a common contact. Is it possible to combine the two wires that were previously at zero potential for use together? And if a safety barrier is needed for isolation, how should that be done?
There are two types: new and old.
It seems the original poster really comes from the instrumentation field.
No problem; a two-channel safety barrier or another single-channel safety barrier will do.
Is your family very rich? Why is a safety barrier needed for several relays?
The negative terminals of the DI signals from the DCS and PLCs we use are shorted together by the manufacturer; they function as a common terminal. All that is needed is to short together the field common terminal and the control system’s common terminal.
It’s easier to handle things when there’s a safety barrier. It’s because there was no such barrier that in the past, the connections were made directly to the input cards of the DCS system; that’s why I’m asking whether they can be combined together
In our system, the card is inserted directly into it, and the 24V DC voltage on the feedback line is also provided by the instrument; there is no electrical power available in that circuit to begin with. The 50V DC voltage is merely induced electricity, so it’s not suitable to use relays for isolation. Here we use two types of relays for isolation: those with 24V DC and those without 24V DC. I’m not sure how you handle this issue over there; could you tell me?
Hehe. . . . . . . .
The feedback signal should be a dry contact signal; I’m not sure why your valve has 24V
Let’s talk about the issue of induced electricity first. There are essentially two possibilities. The first is electrostatic interference; to overcome this, the cable needs to have a shielding layer and be grounded at one end ； The second is electromagnetic interference; in addition to a shielding layer, the cable also needs to be designed as a twisted pair. Furthermore, during cable laying, it is necessary to make allocations based on voltage level or noise level, which can basically overcome interference. For digital signals, the petrochemical industry generally does not use safety barriers, but instead feeds them into the DCS via isolation relays.