Available online at www.sciencedirect.com
Physics Procedia 36 (2012) 1062 – 1066
Superconductivity Centennial Conference
Influence of high harmonics of magnetic fields on trapped magnetic fluxes in HTS bulk K. Yamagishia*, D. Miyagib, O. Tsukamotoa a
Yokohama National University, Yokohama, 240-8501, Japan b Tohoku University, Sendai, 980-8579, Japan
Abstract Various kinds of HTS bulk motors are proposed and have been developed. Generally, those motors are driven by semiconductor inverters and currents fed to the armature windings contain high harmonics. Therefore, the bulks are exposed to high harmonics magnetic fields and AC losses are produced in the bulks. The AC losses deteriorate the efficiency of the motors and cause temperature rise of the bulks which decrease the trapped magnetic fluxes of the bulks. Usually, electro-magnetic shielding devices are inserted between the bulks and armature windings. However, the shielding devices degrade compactness of the motors. Therefore, it is important to have knowledge of the influence of the high harmonics magnetic fields on the AC losses and trapped magnetic fluxes of the bulk for optimum design of the shielding devices. In this work, the authors experimentally study the influence of high harmonics magnetic fields.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. Peter Kes. Open access under CC BY-NC-ND license. Keywords: HTS Bulk, AC Loss, Trapped magnetic flux, Motor, Harmonics;
1. Introduction HTS bulks can trap higher magnetic flux than permanent magnets. Therefore, more compact and higher-performance electric motors and actuators can be realized by using HTS bulks. The HTS bulks in these electric machines are exposed to AC magnetic field perturbations, which causes AC losses in the bulks. The AC losses affect the efficiency of the machines and, moreover, decrease the trapped magnetic fluxes of the bulks by raising the temperature [1]-[3]. Therefore, it is important to know the AC loss
* Corresponding author. Tel.: +81-45-339-4109, Fax.: +81-45-338-1157 E-mail address:
[email protected]
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. Open access under CC BY-NC-ND license. doi:10.1016/j.phpro.2012.06.106
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characteristics to design the machines. And there is almost nothing that is examining the AC loss of the bulk according to a harmonics magnetic field in these AC loss characteristics. In the AC loss measurement, electric method has high sensitivity and resolution. However, we cannot measure the AC loss due to the distorted waveform electrically. In previous works, we measured the AC losses by a calorimetric method using a calibration heater [4]. Calorimetric method is less sensitive and has lower resolution but give actual losses regardless of the electro-magnetic environment around the sample, provided that the calibration is done correctly. This time, we measured AC loss combining the calibration heater and the highly sensitive thermometry system. We experimented in order to compare the influence of a magnetic field including a fundamental wave and harmonics. 2. Experiment Specifications of a disk shaped HTS bulk used in the experiment are shown in Table 1. Figure 1 (a) and (b) shows the distribution pattern of the trapped magnetic flux of the bulk at 77.3K. The peak of the trapped magnetic flux is 1.17T. Figure 1 (b) is distribution of the trapped magnetic flux along the line through the center of the bulk. The straight broken line is estimation of Jc in the peripheral area of the bulk. The peak of a broken line is a penetration magnetic field. The experimental set-up is illustrated in Figure 2 (a) and (b). Fundamental and distorted external magnetic fields are applied to the HTS bulk parallel to the bulk axis by the AC magnet immersed in liquid Table 1 Specifications of bulk used in experiment. Material
GdBaCuO
Size Dia.34.4 mm)×10.4 mmt Trapped magnetic flux 1.17 T (at 77 K) Field Cool Jc 6.838×107 A/m2 * n value 20 * Estimated from experiment of trapped magnetic field of bulk.
1.6
Trapped magnetic flux [T]
1.478T Peak 1.17T
1.2
0.8
0.4
0 -20
-15
-10
-5
0
5
10
15
20
Position x [mm]
(a) (b) Figure 1 Distribution of Trapped magnetic flux of the HTS bulk. (a) Patterns of the trapped magnetic flux of the HTS bulk. (b) Distribution of the trapped magnetic flux along the line through the center of the bulk.
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(a) (b) Figure 2 Illustration of the experimental set-up. (a) Arrangement of the experimental set-up, (b) Sample arrangement for calorimetric method.
nitrogen bath. The AC magnet generates 0.1T peak magnetic field. The sample set-up was placed in a bore of the AC magnet that applied AC magnetic field to the bulk parallel to the axis of the disk-shaped bulk. The DC magnet for trapping of the magnetic flux to HTS bulk is arranged by the outside of the liquid-nitrogen cryostat. The bulk was magnetized by the Zero field cooling method applying DC external magnetic field parallel to the bulk axis. The thermo-couple was attached to the bulk surface is for the temperature measurement of the bulk and a hall sensor is for the trapped magnetic flux measurement. A calibration heater of a manganin wire was wound around the side of bulk and a thermocouple at the bottom center for the calorimetric measurement. The bulk sample was thermally insulated by polystyrene-form. The cooling condition of bulk is controlled by the regulation of thermal insulating materials. It is an apply magnetic field waveform for Figure 3 to generate the AC loss of HTS bulk. The Eq. (1) is an equation of a command signal waveform. A waveform composes the 3rd harmonics or the 5th harmonics to a fundamental wave. The AC magnet was controlled by the waveform, and the magnetic field waveform which made it generate was shown in Figure 3. We measured the AC loss of bulk by the method explained in the reference [4] using these magnetic field. F (t )
1 1 ½ A®sin(Zt ) sin(3Zt T 3 ) sin(5Zt T 5 )¾ B C ¯ ¿
Қf =60Hz
3. Results and discussion AC losses of the bulk were measured in the cases of with and without the trapped magnetic fluxes and shown in Figure 4. AC losses are not dependent on frequency in the both cases, which means the losses are hysteresis. AC losses in the case of with trapped magnetic flux are higher than those in the case of without the trapped magnetic flux. A reason for the difference is that the Jc and Bmp of the bulk are decreased by the trapped magnetic flux [5]. Bmp is the full penetration magnetic field. The solid lines in Figure 4 (a) were calculated from the Bean model. The AC losses calculated from the Bean model well agree with measured data. The AC losses in the HTS bulk without trapped magnetic flux of a fundamental wave magnetic field is well in agreement with the calculation result of a Bean model. However, the AC losses by a magnetic field including harmonics differ a little. As seen in Figure 4 (a),
(1)
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(a)
(c)
(b)
Figure 3 The magnetic field waveform of AC magnet. (a) Fundamental waveform. (b) + 3rd harmonic waveform. (c) +5th harmonics waveform. 0
0
10
10
Q1: Fundamental wave
Bean model Q : Fundamental wave
Q+3: +3rd harmonics
1
Q : +3rd harmonics
-1
Q+5: +5th harmonics
-2
10
-3
-2
10
-3
10
10
-4
10
Q+5: +5rd harmonics
10
Qm [W]
Qm [W]
-1
+3
10
-4
-2
10
-1
B [T] m
(a)
10
10
-2
10
-1
B [T]
10
m
(b)
Figure 4 Measured values of Bm vs. Qm. (a) AC loss in the bulk without trapped magnetic flux. (b) AC loss in the bulk with trapped magnetic flux.
the averaged error between Q+5 and Q+3 is 6% of Q+5, while that between Q+5 and Q1 is 10% of Q+5. As seen in Figure 4 (b), the measurement results of the AC loss of bulk with trapped magnetic flux by a fundamental wave and a harmonics magnetic field became the almost same value. 4. Conclusion We studied the influence of harmonics magnetic field on trapped magnetic flux in HTS bulk. The AC loss by the harmonics magnetic field of HTS bulk was able to measure by the calorimetric method. It is
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found that AC loss of the HTS bulk with trapped magnetic flux is hardly influenced by harmonics magnetic field. Acknowledgements This work was supported in part by MEXT Grant-in-Aid for Scientific Research (C) (21560292). References [1] K. Yamagishi, J. Ogawa, O. Tsukamoto, M. Murakami and M. Tomita, Physica C , 2003; 392-396: pp659-663. [2] J. Ogawa, M. Iwamoto, K. Yamagishi, O. Tsukamoto, M. Murakami, M. Tomita, Physica C, 2003; 386: pp26-30. [3] Y.Zushi, I.Asaba, J.Ogawa, K.Yamagishi, O.Tsukamoto, M.Murakami and M.Tomita, Physica C; 2004; 412-414: pp708-713. [4] K. Yamagishi, S. Sekizawa, O. Tsukamoto and J. Ogawa, J. Physics: Conference Series, 2006; 43: pp295-598. [5] J. Ogawa, M. Iwamoto, O. Tsukamoto, M. Murakami, M. Tomita, Physica C, 2002; 386-376: pp1754-1757.
