In the power monitoring and control system of the power plant, the AC current transmitter DBS/Q-111 is a key device that is responsible for converting large current signals into standardized 4-20mA current signals. The stability of its output signal is directly related to the safety of equipment operation and the accuracy of system control. When the output signal of DBS/Q-111 is constant at 4mA, it usually indicates that there is an abnormality in the measurement circuit, which may be caused by sensor damage, power failure or wiring error. In response to this phenomenon, it is necessary to use a systematic troubleshooting method, combined with equipment characteristics and actual working conditions, to gradually locate the root cause of the fault.

I. Fault phenomenon and preliminary judgment

The 4mA output of the current transducer DBS/Q-111 is its minimum range value, which usually corresponds to the zero current input state. If in actual operation, the output of the device remains at 4mA for a long time and cannot change with the load, external factors should be eliminated first, such as abnormality of the measured current source or system setting errors. However, after eliminating external interference, if the problem persists, it is necessary to further analyze the possibility of internal faults.

From a technical point of view, the output signal of the transmitter DBS/Q-111 is formed by the sensor detection module, signal conditioning circuit and power supply system. The sensor is responsible for converting the measured current into a processable electrical signal, the signal conditioning circuit filters, amplifies and linearizes the signal, and the power supply system provides a stable working voltage for the entire circuit. Therefore, when the output signal is constant at 4mA, the following three types of faults may be involved: sensor damage leads to signal input failure, power supply abnormality or wiring error causes signal transmission interruption.

1. Characteristics and judgment of sensor damage

The sensor is the core component of the current transmitter DBS/Q-111, and its performance directly affects the accuracy of the output signal. If the sensor is damaged, such as internal magnetic circuit abnormality, coil open circuit or insulation failure, the signal input may be completely lost. At this time, even if the power supply and wiring are normal, the device cannot obtain valid measured current data, and the output signal will remain at the default value of 4mA.

To determine whether the sensor is damaged, it is necessary to verify it through the following steps:

• Bypass test method: temporarily short-circuit the input end of the current transmitter DBS/Q-111 to simulate the zero current state. If the output signal is still 4mA, it indicates that the sensor is not damaged; if the output signal is abnormal (such as offset or fluctuation), it may reflect that there is a local fault in the sensor.
• Replacement verification method: Use a spare sensor of the same model to replace the original device and observe whether the output signal returns to normal. If the signal can change with the load after replacement, it can be confirmed that the original sensor is damaged.
• Signal input detection: Measure the voltage signal at the output end of the sensor through an oscilloscope or a high-precision multimeter. If there is no signal or the signal amplitude is much lower than expected, it indicates that the sensor may have failed.

It is worth noting that sensor damage is usually accompanied by obvious physical characteristics, such as burnt coils, cracked packages, or decreased insulation resistance. Regularly checking the appearance and insulation performance of the sensor can help detect potential problems in advance.

2. Identification and troubleshooting of power supply failures

The power supply system provides a stable DC voltage (usually 24V DC) for the current transducer DBS/Q-111, and its stability directly affects the normal operation of the circuit. If the power supply voltage is insufficient or the power supply is interrupted, the device may enter a low-power protection mode, causing the output signal to be locked at 4mA.

Typical manifestations of power failure include:

• Abnormal voltage: Measure the voltage at the power input end. If it is lower than the minimum value required by the device (such as 12V DC), insufficient power supply may be caused by aging of the power module, excessive load or line loss.
• Reverse polarity: If the positive and negative poles of the power supply are reversed, the internal circuit may be damaged and the device may not work. At this time, the transmitter output signal may be constant at 4mA, and the device indicator light may be abnormal (such as off or flashing).
• Momentary power failure: After the power supply is disconnected momentarily, the device may output a fixed value due to reset failure.

When troubleshooting power failure, it is necessary to first check the connection status of the power supply line to ensure that the wiring is firm and there is no short circuit. Use a digital multimeter to measure the power supply voltage and compare it with the parameters marked on the device nameplate. If the voltage is normal but there is still a fault, it is necessary to further check the output capacity of the power module or replace the power supply for testing.

3. Characteristics and verification of wiring errors

Wiring errors are one of the common causes of abnormal output of transmitter DBS/Q-111, which may occur at the input, output or power supply end. If the signal line has poor contact, reverse polarity or short circuit, the signal transmission may be interrupted, forcing the output signal to remain at 4mA.

Typical manifestations of wiring errors include:

• Loose signal line: Long-term vibration or environmental corrosion may cause the signal line connector to loosen, resulting in intermittent disconnection. At this time, the output signal may fluctuate around 4mA, but it cannot stably reflect the actual load.
• Reverse polarity: The positive and negative poles of the signal line or power line are reversed, which may damage the input circuit of the transmitter and cause the signal to be unable to process.
• Short circuit or ground interference: The signal line is laid in parallel with the ground line or high-voltage line, which may introduce noise or short circuit and interfere with signal transmission.

Methods for verifying wiring errors include:

• Visual inspection: Check whether all wiring terminals are tightened and whether the cables are worn or the insulation layer is damaged.
• Continuity test: Use the on-off position of the multimeter to measure the continuity of the signal line and the power line to confirm that there is no open circuit or short circuit.
• Shield layer detection: For signal lines transmitted over long distances, it is necessary to check whether the shield layer is properly grounded to reduce the impact of electromagnetic interference.

In actual operation, the wiring specifications should be strictly followed to avoid parallel laying of signal lines and power cables, and ensure that the grounding resistance meets the standard requirements.

II. Comprehensive analysis and maintenance recommendations

When distinguishing sensor damage, power failure and wiring errors, it is necessary to combine the equipment operation log, historical data and on-site environment for comprehensive analysis. For example, if the sensor has been replaced or the power supply has been adjusted before the failure, it may point to human operation errors; if the failure is accompanied by abnormalities in other equipment, it may reflect systemic problems (such as power supply fluctuations or poor grounding).

In addition, the maintenance of the current transmitter DBS/Q-111 should focus on preventive measures:

• Regular calibration: Calibrate the sensor and output signal according to the manufacturer’s recommended cycle to ensure measurement accuracy.
• Environmental monitoring: Monitor the temperature, humidity and electromagnetic interference level of the environment in which the equipment is located to avoid exceeding the design range.
• Redundant design: Use dual-channel transmitters or redundant power supplies in key circuits to improve system reliability.

When the output signal is constant at 4mA, technicians need to gradually troubleshoot the fault with a systematic mindset to avoid blindly replacing components. Through scientific diagnostic methods and rigorous operating procedures, not only can the equipment function be restored quickly, but also the stable operation of the power plant can be guaranteed.

When looking for high-quality, reliable current transmitters, YOYIK is undoubtedly a choice worth considering. The company specializes in providing a variety of power equipment including steam turbine accessories, and has won wide acclaim for its high-quality products and services. For more information or inquiries, please contact the customer service below:
E-mail: sales@yoyik.com
Tel: +86-838-2226655
Whatsapp: +86-13618105229

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