Magnetoelectric Speed Sensor T-03S: Dealing with Signal Drops Caused by Oil Film Adhesion

In thermal power plants, the T-03S speed sensor for the feedwater pump turbine is a core component for ensuring safe equipment operation. It monitors the rotational frequency of the rotor gears, providing real-time speed data to the control system, ensuring stable operation within the design range. However, when operators notice a sudden drop in the T-03S speed sensor‘s signal, while the sensor’s resistance readings appear normal, they often need to investigate further for issues such as oil film adhesion on the gear surface. This article will analyze the physical mechanism by which oil film causes gap failure and explore how to standardize cleaning procedures to maintain speed sensor reliability.

I. The Physical Mechanism of Oil Film Adhesion: From Magnetic Field Interference to Signal Attenuation

The T-03S magnetoelectric speed sensor operates based on electromagnetic induction. The constant magnetic field generated by its internal magnet forms a closed magnetic circuit with the gear tooth tips. As the gear rotates, the gap between the tooth tips and the magnet periodically changes, causing magnetic flux fluctuations that induce a pulsed electrical signal in the sensor coil. The amplitude and frequency of this signal directly reflect the rotor speed.

When a 0.1mm thick oil film adheres to a gear surface, even if it’s barely noticeable to the naked eye, it can cause the following physical effects:

Magnetic flux attenuation: As a non-magnetic material, the oil film impedes the transmission of magnetic flux lines, reducing the strength of the magnetic field. The effective magnetic flux between the gear tooth tips and the magnet decreases, causing the amplitude of the signal sensed by the sensor coil to decrease.

Equivalent gap increase: When the oil film covers the gear tooth tips, the magnetic circuit originally formed by the metal tooth surface is partially replaced by the oil film medium, increasing the equivalent magnetic path length and further weakening the magnetic field coupling efficiency.

Signal frequency distortion: The viscous properties of the oil film can cause a slight hysteresis in gear rotation, causing the tooth tip to pass the sensor at a time that deviates from the theoretical value, thereby interfering with the signal frequency accuracy.

For example, in a power plant feedwater pump turbine failure case, the signal from the speed sensor T-03S dropped sharply from a normal value of 12V to 2V, but a resistance test showed that the sensor was intact. Disassembly and inspection revealed a 0.1mm thick oil film on the gear surface. This film formed a localized magnetic resistance jump at the interface with the metal tooth surface, causing signal attenuation. This phenomenon demonstrates that even if the sensor itself is intact, interference from an external oil film can still cause serious signal anomalies.

II. The Root Cause of Oil Film Adhesion: The Synergistic Effect of the Lubrication System and the Operating Environment

The formation of oil film is often closely related to lubrication system management. In the gearbox of a feedwater pump turbine, lubricating oil can oxidize or emulsify under high temperature and high pressure, forming a viscous oil film deposit. These deposits adhere to the tooth surfaces as the gears rotate, gradually accumulating to a thickness of 0.1mm. Furthermore, leaks in the cooling water system or seal oil system can allow oil to seep into the gear area through gaps, exacerbating the formation of oil film.

Humidity and temperature fluctuations in the operating environment can also affect the stability of the oil film. For example, high temperatures increase the fluidity of the oil film, allowing it to penetrate into the gear tooth gaps through capillary action. Low temperatures, on the other hand, harden the oil film, increasing its adhesion strength. These factors combine to make oil film a potential cause of abnormal signal from the T-03S speed sensor.

III. Standardized Cleaning Process: From Prevention to Advanced Maintenance

To address oil film adhesion issues, a systematic cleaning process is required to restore the performance of the T-03S speed sensor and extend its service life. The following standard operating procedures are based on industry practices:

1. Preventive Inspection and Early Intervention

Regularly Monitor Signal Fluctuations: Set a threshold alarm for the speed sensor signal in the DCS system. When the signal amplitude falls below the set value, an alert is triggered and an inspection is scheduled.

Gear Condition Assessment: Use infrared thermal imaging or vibration analysis to determine whether there is abnormal temperature rise or vibration on the gear surface, indirectly identifying the possibility of oil film adhesion.

Oil Quality Control: Regularly sample and analyze the lubricating oil for acidity, moisture, and particle size. If oil deterioration is detected, replace or replenish the oil promptly.

2. Physical Methods for Oil Film Removal

Ultrasonic Cleaning: Immerse the gear in a specialized cleaning fluid and use the high-frequency vibrations of ultrasonic waves to remove the oil film. This method is suitable for oil films thinner than 0.1 mm and does not damage the metal surface.

Steam Purging: Briefly heat the gearbox with steam to soften the oil film, then purge it with compressed air. Care must be taken to control the steam temperature during this method to prevent thermal deformation of the gear material.

Mechanical Scraping: Manually remove the oil film from the gear surface using a non-metallic scraper or cloth wiping tool. Wear protective gloves during this operation to avoid scratching the tooth surface.

3. Chemical Cleaning and Protection

Neutral Detergent Treatment: Use a neutral detergent compatible with the lubricant (such as a hydrocarbon-based detergent) to avoid damaging the gear coating with corrosive solvents. After cleaning, rinse thoroughly and dry to prevent residual liquid from contaminating the new oil.

Surface Passivation: After removing the oil film, apply an anti-oil film coating (such as Teflon) to reduce the oil’s affinity with the metal surface and delay reattachment.

4. Sensor Calibration and Retesting

Signal Verification: After cleaning, reinstall the T-03S speed sensor and verify that the signal amplitude and frequency have returned to their designed values using a standard speed test (such as low-speed rotation). Resistance Retest: Use a multimeter to measure the sensor’s output resistance to ensure it matches the factory specifications and to rule out internal damage caused by improper operation.

The T-03S speed sensor serves as the “eyes” of the feedwater pump turbine, and its signal quality directly impacts equipment safety and efficiency. While oil film adhesion is not a sensor fault, it can trigger a chain reaction, leading to control failure or misoperation of protective devices. By understanding the physical mechanisms of oil film and strictly implementing cleaning procedures, power plants can minimize the risk of signal anomalies and extend sensor life.

When looking for high-quality, reliable speed sensors, 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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