Design, Instrumentation and Usage Protocols for Distributed In Situ Thermal Hot Spots Monitoring in Electric Coils using FBG Sensor Multiplexing

Monitoring thermal hot spots within electric coils is critical in the power conduction area as it enables a much better understanding of device health, remaining lifetime, and proximity to design limits. The motor technique enables instituted monitoring of thermal hot spots within electrical coiled structure based on the application of multiplexed electromagnetic immune and power by fiber optic sensing. The advanced FPG sensing performance described in this video is unique and cannot be much like application of conventional sensors such as active thermal couples nor the application of resistance-based thermal estimation techniques.

FBG sensors are inherently responsive to thermal and mechanical excitation and are fragile. Hence, their application for close thermal sensing with an electrical coil structures requires a special procedure that is explained in this protocol. First identify the sensor design and specifications based on your target coil structure and the interrogation system features.

The test coil shown here is a standard IEEE Class H motorette typical of electric machine coils. When you design the sensing screen ensure that the optical sensing fiber remains operative in the thermal and mechanical environments typical of the wound coil sensing applications. Using the standard bend-insensitive polyamide single mode fiber ensures that the sensor is able to operate in temperatures in excess of 200 degrees Celsius and that it has the mechanical properties that allow it to be bent to conform to a desired coil geometry.

In this application, four thermal sensing points are to be installed in four test coil cross-sectional center locations. The individual sensing locations are identified based on their latent thermal monitoring standards for electrical machines. The distance between the sensing heads are based on the coil geometry and the choosing sensing locations.

Next, specify individual FBG heads to be five millimeters in length and graded with different wavelengths spaced in a bandwidth from 1529 to 1560 nanometers to match the used commercial interrogator rating and to prevent shifted wavelengths interference. Here the overall fiber length is specified to 1.5 meters. The initial 1.2 meters is packaged in Teflon and allows connection to the external interrogator device.

The additional length of 3 meters contains the four unpackaged sensing heads. Shown in this video is the specified array sensor, which was commercially manufactured. First, remove the protective cap from the FC/APC connector feral.

Then clean the connector end face by wiping it gently with an optical connector cleaner. Next ensure that the keyway is correctly aligned and plug in the cleaned FBG probe connector to the interrogator channel connector. Turn on the interrogator and run the configuration software.

On the instrument setup tab, observe the reflected wavelength spectrums from the FBG array probe. Four peaks should be observed in the related channel spectrum. In the software, set the sampling frequency to 10 Hertz and set spectrum boundaries between FBG to prevent measurement interference.

Then, in the measurement setting, name the FBG heads as FBG-1, FBG-2, FBG-3 and FBG-4. Choose the wavelengths as a type of quantity to be presented graphically at this stage. Appropriately package the sensing area where the FBG heads are imprinted in the array fiber using a peek capillary.

This will protect the glass fiber and ensure that the sensing head is isolated from mechanical excitation and will yield an exclusively thermal excitation responsive sensor. Cut an adequate length of commercial peek tubing to the length of the target coil structure with a few extra centimeters to allow for fiber insertion and to cover the Teflon to peek capillary joint. Next, take careful measurements of the FBG array and the peek capillary to accurately identify sensing locations on the outer surface of the peek capillary.

This allows for positioning of FBG sensing heads in target locations within the motorette test coil. Then, prepare an appropriately sized shrink tube for later use. Insert the fiber sensing area into the peek capillary and maintain peek and Teflon connection using capton tape.

Calibrate the packaged FBG array sensor by inserting it into the thermal chamber to extract its discrete temperature versus wavelength points. The FBG array sensing area is formed based on the coil geometry. Next, connect the graded optical fiber to the interrogator and launch the pre-configured interrogator software routine.

Operate the oven in a sequence of thermal steady-state points, create a table from the measured reflective wavelengths of each individual FBG in the array. For every constant temperature, emulate it in the chamber. Then, use the recorded shifted wavelength versus temperature measurements to determine the optimal temperature wavelength shift fit curves and their coefficients for each FBG.

Input the calculated coefficients in the relevant settings of the interrogator software to enable online temperature measurements from the FBG array. First, build and instrument the motorette random wound coil. To accomplish this, set the selected Class H enameled copper wire reel in the winder device and wind half of the coil turns at a low speed.

Then, fit the prepared peek capillary in the center of the coil using capton tape. Once properly positioned, wind the rest of the coil turns. Place the finished coil into the motorette frame.

Next, bind the motorette coil and windings. With the FBG array connected to the interrogator, carefully insert the sensing area fiber into the peek capillary until the end openings of Teflon and peek capillaries are in contact. Move the shrink tube to cover the capillary ends and appropriately head it until the desired fit is achieved.

To begin the static test, connect the motorette to a DC power supply and connect the DC power supply to inject the motorette with a DC current. Record measurements until the motorette coil thermal equilibrium is reached. Next, perform a non-uniform thermal condition test.

For this test, first wind the external coil containing 20 turns around a selected test coil section. With the external coil connected to a separate DC power supply, energize the motorette with the same DC current used in the static test. Once the thermal equilibrium is reached, begin recording thermal measurements.

Finally, energize the external coil with a DC current to provide a non-uniform thermal condition by delivering localized thermal excitation on the test coil. Stop recording measurements once thermal equilibrium is reached. During this representative static thermal test, the four internal temperature readings were taken by respective array FBG heads in their corresponding coil locations.

The readings are closely similar with a slight variation between the recorded individual measurements of less than about 1.5 degrees Celsius. Once the external 20 turn coil was excited, to emulate a non-uniform coil condition within the coil structure, a clear change was observed in thermal measurements with redistribution of the coil internal temperature. The sensing point and closest proximity to the external coil, FBG4, measured the highest thermal level and the furthest away sensing point, FBG 2, measured the lowest.

The observed readings clearly relate to variations in the individual sensing head distribution, the examined test coil geometry. This demonstrates the functional capability of the coil-embedded array sensor from monitoring and identifying thermal hot spot distributions in random wound coils. In this video, we have demonstrated how a single optical fiber using FBG techology can enable distributed measurements of thermal hot spots within the structure of an electric coil.

Achieving this will be extremely challenging using conventional sensors. To ensure accurate measurements, take a special care with packaging, installation, calibration procedures. These are needed to mitigate thermal mechanical FBG cross-sensitivity, protect the fiber, and allows for reliable thermal readings to be taken.

The reported technique provides new opportunities for development of dedicated in situ thermal monitoring applications in energy conversion devices where conventional sensors are challenged.