Issues with the 4-20mA signal after the motor current transmitter
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This post was last edited by zyz8438 on 2010-10-14 at 10:21. I have a centrifuge current transmitter here, and I’ll first explain the wiring to everyone. I installed a current transformer with a ratio of 300/5 on one of the phases, and then used a current transmitter. The transmitter is three-wire type, and it outputs a 4-20mA signal that is sent to the DCS. Problems arise at this point: when I connect it to the DCS, the displayed current differs from the actual current by 60 mA, meaning the displayed value is higher than the actual one. When I open the circuit and measure the mA value, it is the same as normal. However, once the circuit is reconnected, the mA value increases by 5 mA. When I measure the circuit separately, with the transmitter disconnected, there are no issues with the wiring. Today, I adjusted the two adjustable knobs on the transmitter; the measurement values remained the same at that time. But as the power level decreases, the value displayed by the DCS also decreases. Yet, the lower the value, the greater the discrepancy between the actual value and the measured value. Why is this?
This post was last edited by zyz8438 on 2010-10-14 09:40. According to the original poster’s description, the mA signal not only did not attenuate but actually increased! It is estimated that the presence of some constant interference source results in the superposition of mA signals; the lower the power, the stronger the interference signal relative to it. Recommendation: 1. Use shielded cables for 4-20mA signals ； 2. Install active signal isolators on both sides of the cable (or at least on one side)!
First, on the motor side, measure the current to determine the magnitude of the error. As for the instrument circuit, the cable should have a shielding layer covering more than 70% of its length, and it must be properly grounded. Additionally, cables used in the current circuit should serve only for transmitting current; their length should not exceed 200 meters. If this limit is exceeded, a current amplifier is required. It’s also advisable to use isolators between the two sections – there shouldn’t be any problems with that.
This post was last edited by zhaohh3211 on 2011-6-29 21:45. This situation occurs because the appropriate AI interface is not selected when the milliampere signal is connected to the DCS. There are two types of current AI interfaces: one that directly accepts external milliampere signals (such as 4-20), and the other that outputs 24VDC for external use, with the milliampere signal connected in series afterward. The former is referred to as active signal access, while the latter is called passive signal access. If the output milliampere signal is connected directly to the second type, an error will appear in that milliampere signal, and it will be nonlinear.
Reply to 4# Mingri Dongsheng: “The former is referred to as active signal access, and the latter is also called active signal access.” ”Is there a mistake? The latter should have an active signal output, right!
Is the DCS connection point a standard analog card? After connecting, the current increases; it seems that the impedance on the DCS side is too low?
This post was last edited by zhaohh3211 on 2010-10-26 at 10:33. It needs to be confirmed that the range of the current transmitter is the same as that of the DCS input signal； The mA value of the DCS analog input and the displayed value are properly adjusted ； The output of the electro-rheological relaxer and the displayed values are properly adjusted. If all of the above has been done, then it can be said that the fault is caused by external interference, and shielding measures must be taken; the shielding requirement is single-ended grounding.
Reply to 1# lizhongliang: 1. If the transmitter is three-wire, the DCS must be connected in a power distribution configuration; otherwise, the current signal will not be detected. 2. There is also no possibility of interference signals, as it is a 4-20mA signal; it belongs to a constant current source, and interference signals generally have little impact on it. 3. I think the possible issue is related to the load capacity of the current transformer, which may be due to a mismatch with the input impedance requirements of the current transmitter.
Thank you all. I’ve resolved this issue by downloading the necessary software; the problem was that the interference was too strong. I moved the current transmitter outside, and that solved the problem. Thank you, Comrade Dianlaozi. It was the constant interference that caused the signal to become distorted