Motor current
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I have a question regarding current transformers. I have a current transformer and a current transmitter here; when the transmitter is not connected to the circuit, the mA value measured with a multimeter is correct. However, once it is connected to the DCS, the measured current value becomes higher. What’s the problem?
This post was last edited by zyz8438 on 2010-10-12 at 21:57. There are no issues with the circuit; the transmitter has been replaced
Due to interference from a strong magnetic field, the wire shielding of the transmitter is inadequate; moreover, it is placed together with the power cables.
There may be a problem with the transmitter. The main reason why the reading is normal when measured with a multimeter but appears high when measured with a transmitter is likely an increase in the input resistance of the transmitter; as a result, the voltage division across this resistance increases, leading to a higher displayed value.
Has the DCS been checked with standard signals? If the DCS is fine, it’s likely that the output of the current transmitter does not match the impedance of the DCS.
The situation mentioned by the poster is likely a problem with the wiring. Shielded cables should be used for the analog signals from the current transducer to the DCS. Check whether the cable currently in use is shielded Without shielding, 4–20mA analog signals are subject to electromagnetic interference in high-voltage environments, which generates additional induced currents in the wiring; as a result, the current signal that reaches the DCS is greater than the output signal of the transmitter.