Regarding the issue with the control valve signal
Thread Content
Today, while debugging a temperature control valve, we encountered a strange issue: when measuring the input voltage of this valve module, it was 24V, but during on-site testing, no voltage was detected. We have already tested and confirmed that the connection is intact. Additionally, we tried using other channels as well, but it’s not possible to control it; even by changing the surge protection device, it doesn’t work. What’s going on? Could that expert give an answer?
4-20mA current signal; the voltage is not that high
At the site, remove the two wires of the positioner connected to the control valve and check whether there is voltage. If there is no voltage, it indicates an error in the wiring; if voltage is present but the positioner does not function, check the impedance matching of the entire circuit to see if the high impedance caused by a safety barrier or surge protector is preventing the positioner from operating.
There is no need to measure voltage; it is sufficient to check whether a current of 4–20 mA is being output
The input signal of the locator is a current signal, so it is necessary to check whether the current is normal.
It’s not that high; the voltage at the site is 10V, right? No matter what is done on site, there is no voltage nor current. The wiring is fine; I have checked for continuity. What’s the problem? Why is the voltage at the site so low?
It’s not that high; the voltage at the site is 10V, right? No matter what is done on site, there is no voltage nor current. The wiring is fine; I have checked for continuity. What is this problem?
The voltage at the site should be around 1~5V; you can apply a 4~20mA signal there to check whether the valve moves. If it can be controlled properly, it indicates that there is no problem with the locator. The problem lies in the channel; I suspect that the channel might be incorrect. Check the wiring in the junction box to see if it is connected to another channel.
In most cases like yours, it’s due to a wiring error; one channel has 4 terminals, for power distribution and non-power distribution, and the wiring methods are different. Check the DCS wiring diagram again
Was a 4–20mA signal not provided? In other words, no output signal was generated
Measure it – for the signal line relative to ground, is it grounded there?
Your answer is correct. . Add a point
It’s obvious that there is a grounding issue here.
1. First of all, my friend, you need to know what signal is used to control the valve. 2. You can check step by step to determine whether the problem lies with the valve itself, the wiring, or the signal. If it’s not clear which one is the issue, then it means you’re facing a strange problem; and in future, you’ll encounter countless such odd issues at work. I’ve been working for N years now, and I’ve never heard of any instrument technician measuring the voltage of the control signal when repairing control valves. The operating voltage for electric valves is DC24V, and the control signal is 4–20MA. If it’s an electric valve and no DC24V voltage is detected, it might be because there’s no load connected; if there is a load, then it could be that the load is too heavy, which indicates that there might be a problem with the voltage supply. If still no voltage is detected, then there might be a grounding issue in the wiring. One more thing regarding your system design: if there’s a voltage drop, there should be a circuit breaker to control each output voltage, so that protection can be activated and the circuit breaker trips. In my opinion, no further action is needed. Thank you for understanding! This post was last edited by Ah Hai on 2009-4-9 at 21:34
Well, I’ll tell you: it’s because your cable is broken. When there’s a large open circuit in the AO output channel, the voltage is indeed 24V; this situation only occurs when there’s a break in the cable. So I think either your cable is broken, or your valve positioner is damaged, or there’s an error in the wiring