Effect of Winding Tension, Support Material and Epoxy Impregnation

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TASC.2017.2655012, IEEE Transactions on Applied Superconductivity

Effect of Winding Tension, Support Material and Epoxy Impregnation on the Strain and Critical Current of YBCO Coil Liyuan Liu, Wei Chen, Haiyang Zhang, Xinsheng Yang, Ye Yang, Yong Zhang, and Yong Zhao

Abstract—The critical current (Ic) of epoxy impregnated YBCO coil is prone to degrade due to the cumulative radial tensile stress. Winding tension has the advantage to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. In this work, the dry and wet coils wound on copper and polytetrafluoroethylene (PTFE, Teflon) bobbin materials with various winding tensions were manufactured and tested. With increasing the winding tension, the winding strain increased but the thermal strain reduced. The low coefficient of thermal expansion (CTE) for bobbin and epoxy impregnation could decrease thermal contraction of the innermost layer of coil. The winding tension effectively reduced the degradation of epoxy impregnated YBCO coil. The bobbin materials and winding tension of 10-50 N had no effect on the degradation of Ic . A proper winding tension of 20 N was successfully used to fabricate an epoxy impregnated YBCO coil of 30 turns without degradation.

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Index Terms—Critical current, YBCO coil, epoxy impregnation, winding tension, strain. I. INTRODUCTION With the advantage of large critical current density and large axial tensile stress higher than 700 MPa [1], YBCO coated conductor is widely used in various HTS devices such as wind power generators [2], superconducting magnetic energy storage system [3], and accelerator magnets [4]. During the fabricating process of YBCO coil, epoxy impregnation for the coil is very important, as it is an effective way to enhance mechanical stability and improve thermal conductivity of the coil. In the This work was supported by the National Magnetic Confinement Fusion Science Program (Grant Nos.2011GB112001, 2013GB110001), the Program of International S&T Cooperation (Grant No. 2013DFA51050), the National Nature Science Foundation of China (grant No. 51271155, 51377138), the Fundamental Research Funds for the Central Universities (SWJTU2682016ZDPY10). Liyuan Liu, Wei Chen, Haiyang Zhang, Xinsheng Yang, and Yong Zhang are with the Key laboratory of Magnetic Levitation Technologies and Maglev Trains (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu 610031, China ( e-mail: [email protected] ). Ye Yang is with Central Academy Dongfang Electric Corporation, Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu, Sichuan, China. Yong Zhao is with Superconductivity and New Energy R&D Center, School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China

cool down process of the impregnated YBCO coil, thermal stress will be generated owing to the different coefficient of thermal expansion (CTE) among various materials of the coil. According to the study in Ref. [5], the epoxy impregnated YBCO coil degraded if the radial tensile stress in the winding locally exceeded the critical transverse stress of the YBCO coated conductor which was as low as approximately 10 MPa [6]. Many methods have been investigated to reduce the degradation of epoxy impregnated YBCO coil, such as covering the YBCO conductor with shrink tube [7] or with polyimide by electro-deposition [8], adopting mixture of epoxy resin and filler [9], and decreasing the radial stress by dividing the coil into many parts in the radial direction [10]. Using a former with less thermal contraction was also an effective way to prevent the degradation [11]. In the existing work [5, 7-11], the epoxy impregnated YBCO coils with respect to various winding tensions were not discussed. Winding tension has the beneficial to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. Characteristics of winding tension on the critical current (Ic) of HTS coil have been studied in Ref. [12, 13], and the proper winding tension for Bi-2223 pancake coil and layer wound YBCO coil has been determined. The influence of epoxy impregnation on the Ic, however, has not been taken into consideration. When the coil was impregnated with epoxy resin, mechanical stability of the coil would be enhanced. The effect of epoxy resin on the thermal strain of the YBCO coils during cool down process has been studied [14], however, the strain during the winding process at room temperature is not reported. In this paper, YBCO single pancake (SP) coils with various winding tensions were fabricated. The investigated topics were as follows: (a) winding strain of dry coil wound on copper bobbin as a function of winding tension, (b) thermal strain for dry and wet coil wound on copper and polytetrafluoroethylene (PTFE, Teflon) bobbin under various winding tensions, and (c) Ic of dry and wet coils wound on copper and PTFE bobbin as a function of winding tension. Another SP coil with 30 turns was fabricated and the radial stress distribution of the coil due to the combination of winding and cool down process was numerically analyzed based on the finite element program.

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Fig. 2. Strain test system. (a) Overall view of the strain test equipment; (b) circuit diagram for strain measurement. TABLE IV SPECIFICATIONS OF THE STRAIN GAUGE

Fig. 1. Ic measurement of 12 m YBCO tape obtained from the HSA system. TABLE I SPECIFICATIONS OF THE TAPE Tape width Tape thickness Copper thickness Ag thickness Substrate thickness Minimum bending diameter Critical bending strain Critical axial tensile stress Critical axial tensile strain Ave. Ic at 77 K, self-field

4 mm 0.1 mm 40 μm 2 μm 50μm 11 mm 0.9% 700 MPa 0.45% 87 A

TABLE II ELASTIC AND THERMAL PROPERTIES OF DIFFERENT MATERIALS E(GPa) Poisson’s CTE Material Ratio (293-77 K)(%) Cu 117 0.35 0.337 YBCO coated conductor 150 0.3 0.211 PTFE 1.42 0.4 2.59 Epoxy 6 0.3 1.02

TABLE III CHARACTERISTICS OF THE COIL Tape length Inner diameter of coil Number of turns Winding tension Support material Winding Impregnation Cure time of coil

0.93 m 50 mm 6 (0), 10, 30, 50 N Copper, PTFE Single pancake Epoxy resin (Stycast 2850) 12 hours

II. EXPERIMENTAL DETAIL A. Sample Preparation The YBCO coated conductor used in the experiment is SCS4050 conductor, purchased from SuperPower. The specifications of the YBCO tape are listed in Table I [1, 15]. Our previous work demonstrated that the Hall sensor array (HSA) method was an efficient method for testing the Ic in long

Parameters Manufacture Model (Adhesive) Gauge factor Gauge width; length (mm) Gauge resistance (Ω) Temperature range Exciting voltage (V)

Values KYOWA KFG-02-120-C1-11 (CC-33A) 2.22 2.4; 3.3 120 77-400 K 5

length HTS tape [16, 17]. Fig. 1 presents the test result of a 12 m YBCO tape. As shown in Fig. 1, the YBCO tape has certain inhomogeneity, and the average Ic is about 87 A, coincident with the Ic parameter in Table I. B. Test Coil Fabrication The SP coils were wound with a winding machine and the number of turns for per coil was 6 turns. The total length of tape in each coil was about 0.93 m, which was cut from the 12 m YBCO tape. YBCO tape was insulated with Kapton wrapping. The winding tensions at room temperature were 10, 30 and 50 N, respectively. Dry and wet coils were compared to study the influence of epoxy resin on the thermal strain and critical current of YBCO coil. “Dry coil” represented that the coil was dry-wound without epoxy; “wet coil” indicated that the coil was wet-wound with epoxy resin (Stycast 2850 FT) for every turn and cured at room temperature for 12 hours. Besides, the PTFE bobbin was used to further study the effect of large CTE on the thermal strain and critical current of YBCO coil. The elastic constants and thermal properties for the coil materials were summarized in Table II [18, 19]. In the winding process, the winding strain was only tested with dry coil wound on copper. During the cool down process, thermal strains of dry and wet coils wound on copper and PTFE bobbins under various winding tensions were discussed. For the critical current test, Ic of dry and wet coils wound on copper and PTFE bobbins under 0, 10, 30 and 50 N was tested and compared with each other. The winding tension of 0 N meant that the winding tension was so small that could be neglected. The main characteristics of YBCO coil were listed in Table III. The critical current of coil was tested using four-probe method.



TABLE V THE DATA OF WINDING STRAIN AND THERMAL STRAIN OF TWO STEPS FOR DRY AND WET COIL WOUND ON COPPER. Tension (N)

εw

10 Dry

30

Wet 3065

Dry

50

Wet 3480

Dry

Wet 3890

ε step 2

-9650

-9135

-10210

-9620

-11480

-10435

ε w + ε step 2

-6585

-6070

-6730

-6140

-7590

-6545

ε step1

-6550

-6025

-6750

-6165

-7620

-6575

Relative error

0.53%

0.74%

0.3%

0.41%

0.39%

0.46%

Fig. 3. Winding strain for dry coil wound on copper under various tensions.

Fig. 5. Thermal strains for the dry and wet coil wound on copper and PTFE bobbin under various winding tensions.

III. EXPERIMENTAL RESULTS AND DISCUSSIONS Fig. 4. Thermal strain for the dry and wet coils wound on copper under various winding tensions for the two steps.

C. Strain Measurement The overall view of the strain test equipment is presented in Fig. 2(a). The strain acquisition system was used for the signals collection and transmission of strain measurements. As shown in Fig. 2(b), a low temperature resistance strain gauge was glued on the surface of the first layer of the coil for strain measurement. The strain of the coil was measured using Wheatstone bridge circuit [20, 21]. The specifications of the strain gauge were listed in Table IV. The strain ε was expressed as follows:

ε =

R f − Ri Ri

×

1 K

(1)

where Rf was the final resistance of the strain gauge elongation or compression, Ri was the initial resistance of the strain gauge, and K was the gauge factor.

A. Strain of Winding and Cool Down Process The winding process was performed at room temperature. We only tested the strain of dry coil wound on copper bobbin, as the epoxy and bobbin had negligible influence on the strain of winding process. The strain of first layer was recorded by the strain acquisition system in the winding process. The winding strain of first layer as a function of various winding tensions is shown in Fig. 3. It shows the winding strain of first layer increases with the increase of winding tension. The winding strain is the combination of bending strain and tensile strain. Thus, the winding strain can be expressed as follows:

= ε w ε tension + ε bending ε bending =

t 2r + t

(2) (3)

where ε tension is the tensile strain of coil, ε w is the winding strain of first layer,

ε bending

is the bending strain of first layer, t is the

thickness of YBCO tape, and r is the radius of first layer. The thermal strain of dry and wet coil wound on copper is tested as following two steps. Step1, after the winding strain test was completed, the coil is cooled down to 77 K by liquid



Fig. 8. The critical current of dry and wet coils under 30 N.




nitrogen. The data of winding strain is sustained in the strain acquisition system, which indicates that the strain data in this step is the superposition of winding strain and thermal strain. Step2, after experiment of step1 is finished, the coil is warmed up to room temperature, the data of winding strain is eliminated from the strain acquisition system. Then the coil is cool down to 77 K again. The strain data recorded in this step is only thermal strain. Thus, the strain of step1 can be expressed as follows:

ε st= ε w + ε step 2 ep1

(4)

where ε step1 is strain data of step1 which is the combination of winding and thermal strain of coil, ε w is the winding strain of winding process, and ε step 2 is the strain data of step2 which is thermal strain of coil. Fig. 4 indicates the thermal strain under various tensions for the two steps. All thermal strain of first layer is negative value which means the tape is under axial compressive state. The thermal strains become larger with the increase of winding

tensions, and the strain of ε step1 is lower than that of ε step 2 . Besides, the strain of ε step1 is the real value for the coil after cool down, and the value is (< -8000 micro, i.e., 0.8 %) below the reversible compressive strain -0.95% [22], which illustrates that all thermal strain values are within the thermal contraction strain limit. The thermal strains of wet coil are lower than dry coil, as the epoxy resin fixes the coil tightly and reduces thermal contraction of coil. The data of winding strain and thermal strain of the two steps for dry and wet coil wound on copper bobbin are listed in table V. The relative error between ε step1 and ε w + ε step 2 is in a small range, which demonstrates the equation (4) is reasonable and the strain of winding and cool down process follows the strain superposition principle. The thermal strain of coil wound on different bobbin is tested according to step (2). Fig. 5 presents the thermal strain results of first layer for the dry and wet coils wound on copper and PTFE bobbin under various winding tensions. The thermal



Fig. 10. The cumulative radial stress distribution in the coil due to cool down under different winding tensions.

stain of coil wound on PTFE is larger than that of coil wound on copper due to the PTFE bobbin’s large CTE value as seen in Table II. It indicates that the thermal contraction of coil increases with the increase of CTE of bobbin. To decrease thermal contraction and increase the fixing condition of coil, the bobbin with a thermal contraction lower than the coated conductor tape, or bobbin which expands with cool down process as mentioned in Ref. 13 should be used. In this experiment, the strain generated by Lorentz force during measurement is neglected, as the coil has only a few turns and the strain is so small. B. Critical Current To confirm the effect of the winding tension, bobbin material and epoxy on the Ic degradation of coil, the Ic of dry and wet coils wound on copper and PTFE bobbin under various winding tensions are tested. The winding tensions are 0, 10, 30 and 50 N, respectively. The Ic for each coil is determined at the coil voltage with 92 μV, corresponding to the electric field of 1 μV/cm. The critical currents of coils wound with 0 N are shown in Fig. 6. The Ic values are 70 and 58 A for dry and wet coils wound on copper bobbin, respectively. For coil wound on PTFE bobbin, the Ic values are 68 and 56 A for dry and wet coils, respectively. The Ic values of the wet coils wound on both copper and PTFE bobbin are much lower than those of dry coils, which could be attributed to the fact that the stress concentration on the coil edge causes degradation of Ic for the epoxy impregnated YBCO coil [19, 23]. Fig. 7 shows the experimental results of coils with 10 N. The Ic values of dry and wet coils are 69 and 68 A for copper bobbin, 70 and 69 A for PTFE bobbin. Fig. 8 is the obtained results of coils with 30 N and the Ic values of dry and wet coils wound on copper and PTFE are 68, 69, 69 and 72 A, respectively. The Ic values of dry and wet coils are almost the same, which means the degradation of wet coils under both 10 N and 30 N is not occurred. The little difference of Ic among these coils may be caused by inhomogeneity of the HTS tape as depicted in Fig. 1, which has been mentioned in Ref. 12. Fig. 9 is the obtained

Fig. 11. The critical current of dry and wet coil with 20 N. The inset shows the picture of the epoxy impregnated coil.

results of coils with 50 N. The Ic values of dry and wet coils for copper and PTFE are 69, 71, 68 and 69 A, respectively. The n-values of dry and wet coils for copper and PTFE are 13.4, 23.9, 23.2, 23.6, respectively. Even if the Ic of each coil is not decreased, the n-value of dry copper coil is as low as 13.4. As n-value can be a clear sign of tape’s damage [12], it is possible to determine a certain mechanical damages in the dry copper coil. However, the damage may not be caused by winding tension of 50 N, it is assumed to be influenced by the inhomogeneity of Ic of YBCO tape. If the coil is wet wound without winding tension, it may easily degrade due to the thermal contraction of epoxy resin. The degradation of the impregnated YBCO coil can be restrained by controlling the winding tension, as winding tension has the beneficial to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. The change of winding tensions from 10 N to 50 N does not affect the Ic degradation of the coils, due to the fact that the tensile strain and stress for YBCO coils is within 0.45% and 700 MPa as shown in Table I. However, much higher winding tension may cause micro-cracking in the YBCO coated conductor, which results in the degradation of coil [12]. Therefore, winding tension which is below 50 N is suitable for the YBCO pancake coil in our experiment. The Ic of all coils with various tensions is almost the same even if the bobbin materials are different. It means that the bobbin materials do not affect the Ic of the coil, which is similar with the result reported in Ref. 14. IV. FABRICATION OF NON-DEGRADATION IMPREGNATED COIL We manufacture another SP coil to further demonstrate whether the winding tension is effective or not in restraining the degradation of the epoxy impregnated YBCO coil. The SP coil is 50 mm in inner diameter and the number of turns is 30. The cumulative radial stress distribution of YBCO coil due to the combination of winding tension and cool down is numerical analyzed based on the finite element analysis program ANSYS. Due to the mirror symmetry in coil geometry and loading, only



1/2 two dimensions (2D) axial symmetry model has been modeled, and the coupled field element (PLANE13) is employed for the structural and thermal analysis. In order to obtain accurate result of winding pre-tension, each layer of coil is assumed to be a rectangle strip. An element birth as well as death technique and multi-load steps method are adopted to simulate the mechanical behavior of winding and cool down process. The cumulative radial stress distribution due to the combination of winding and cool down is numerical analyzed to confirm a proper winding tension, as shown in Fig. 10. The cumulative radial stress here is the sum of the tensile radial stress due to winding process and the compressive radial stress due to cool down. When the winding tension is 0 N, the radial stress shows a tensile peak of 4.26 MPa at the distance of almost 28 mm from the coil center. This result is close to the delamination strength of 10 MPa for YBCO coated conductor under transverse tensile stress [6], and the radial tensile stress easily causes the degradation of YBCO coil according to the study in Ref. 5. After the coil is wound with winding tension of 10, 20, 30 and 50 N, respectively, the cumulative radial stress is transferred from the radial tensile stress to compressive state after cool down. The radial stress is -0.47 MPa, -0.94 MPa, -1.41 MPa and -2.35 MPa, respectively. The radial compressive stress increases with the increase of winding tension. It demonstrates that the radial tensile stress can be reduced by controlling winding tension, and the delamination may not be occurred in the coil after cool down. According the numerical analysis result, we fabricate another dry and wet coil with the winding tension of 20 N. The coil is charged in liquid nitrogen until exceeding the coil terminal voltage of 510 μV, corresponding to an electric field of 1 μV/cm. The critical current of the dry and wet winding is shown in Fig. 11. The inset shows the picture of the epoxy impregnated coil. The Ic for dry and wet coil are 50 and 51 A, respectively. The Ic of coil with 30 turns is smaller than that of 6 turns, as the magnetic field applied to the tape increases with the number of turns and the IC decreases with the increase of field [14]. The Ic of wet coil with 30 turns coincides very well with that of dry coil. Thus, degradation of the epoxy impregnated YBCO coil is not occurred by wound a tension of 20 N. V. CONCLUSION To study the effect of winding tension and epoxy resin on the strain and critical current of YBCO coil, dry and wet coil wound on copper and PTFE bobbin under different winding tensions are fabricated and tested under liquid nitrogen temperature. When increasing the winding tension, the winding strain increases but the thermal strain of cool down reduces. The thermal strain of coil reduces with the increasing of bobbin’s thermal contraction. Besides, the thermal strains of wet coil are larger than of dry coil. Therefore, both the low thermal contraction bobbin and epoxy impregnation can decrease thermal contraction of coil. The epoxy impregnated YBCO coil is easily degraded without winding tension due to the thermal contraction of epoxy resin. It can be restrained by controlling the winding

tension, as winding tension has the beneficial to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. The change of winding tension from 10 N to 50 N does not affect the degradation of Ic. The Ic values of all coils are almost the same even if the bobbin materials are different, i.e. the bobbin materials also do not affect the Ic of the coil. Finally, a proper winding tension of 20 N is successfully used to fabricate an epoxy impregnated YBCO coil of 30 turns without degradation.

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