Is it okay for the instrument power to be supplied by the high-voltage cabinet?
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The conditions for one unit are as follows: thermometer, level gauge, and pneumatic valves are controlled by a PLC. However, the drawings provided by the subcontractor show that the 24V power supply for the thermometers and level gauges is located in the high-voltage cabinet, which contains AC/DC transformers; the 220V power supply for the solenoids of the pneumatic valves also comes from this high-voltage cabinet. I have the following questions: 1. Since the 220V power supply for the solenoid valve comes from the high-voltage cabinet, shouldn’t the relay also be placed in that cabinet? Otherwise, how can the PLC control the solenoid valve? 2. Level gauges and thermometers are generally 2-wire systems, with the signal and power sent together; they are connected directly to the PLC. Now they have been connected to the high-voltage cabinet – how can the signal be sent to the PLC? Could it be a 4-wire gauge? Are power supply and signals separated? I work in the manufacturing field and am not very familiar with instruments; I hope everyone can help clarify this for me. Thank you
1. It seems that it doesn’t matter where the relay for the 220V solenoid valve is placed, as long as the wiring is long enough; the PLC’s output is simply used to drive the motor coil of the relay. 220v is connected in series on the field side where the solenoid valve is located. That is, 220v + contact + solenoid valve ； 2. The wiring methods for level gauges and thermometers can be determined based on the model of the instrument or its instruction manual; there should be a wiring diagram available, which will indicate the power supply and signal connections.
Shouldn’t a thermometer have three wires? ? :P
They should all be placed in the PLC cabinet; even after using relays to interface with the solenoid valves, they are still placed inside the PLC cabinet
It is possible as long as measures are taken in the layout of the instrument wires.
If you really want to figure it out, I suggest you upload the drawings so that everyone can give you some advice. The fact that you, as a process engineer, are able to describe control system diagrams in a relatively professional manner shows that you are also a person who is eager to learn and make progress!
What does the high-voltage cabinet here refer to? What is the highest voltage inside?
It is possible as long as measures are taken in the layout of the instrument wires.
I remember that there used to be a dedicated relay cabinet, which contained relays for isolation; the PLC would drive a 24V DC relay, and that 24V DC relay in turn would drive a 220V AC relay. Although it was a bit more complicated, it did provide a certain level of isolation between high-voltage and low-voltage circuits
The original poster is not very familiar with control cabinets. For many small and medium-sized devices, the control cabinet serves both as an instrument panel and a power supply unit. The reason for this is twofold: it saves money, and since there are few instruments to display, it isn’t worthwhile to have a separate cabinet. That’s why many integrators use this approach.