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Drag torque of high Tc superconductor magnet bearings with multi-piece ring magnets or superconductors
PIWSICA ELSEVIER
Physica C 341-348 (2000) 2615-2616
www,elsevier.nl/locate/physc
Drag Torque of High Tc Superconductor Magnet Bearings with Multi-piece Ring Magnets or Superconductors E.Postrekhin, Hong Ye, Ki Bui Ma and Wei-Kan Chu Texas Center for Superconductivity University of Houston, Houston TX 77204, USA In order for a superconductor magnet levitation bearing to rotate freely, the magnetic field from the magnet must be axisymmetric about the axis of rotation. In contrast, the shape of the superconductor need not be axisymmetric. This allows us to make a large ring of high temperature superconductor out of smaller pieces in the fabrication of superconductor magnet bearings. The problem then arises as to whether the substitution of a single large piece of superconductor by smaller pieces might contribute to additional power loss. We have investigated this by studying the drag torque experienced by multi-piece ring magnets as they turn above multipiece rings of superconductors, and comparing with results from single pieces of similar sizes. We have coordinated the variation of the drag torque during a turn with the relative orientation of the magnet and superconductor pieces. We use the mean value of the drag torque over several turns as a measure of power loss.
1. INTRODUCTION Soon after their discovery, high temperature superconductors (HTS) have been used together with permanent magnets (PM) to construct bearings that are practically frictionless. This makes HTS magnet bearings promising for energy critical applications, such as flywheel kinetic energy storage. The extremely low drag torque observed in HTS magnet bearings depend critically on the extent to which the magnet field from the PM is axisymmetric about the rotation axis. On the other hand, the effect of the HTS not being axisymmetric is small, by comparison. The practical implication of this observation is that the HTS component of an HTS magnet bearing need not be a single continuous circular piece, but a large piece could be composed of several pieces. Here, we have limited ourselves to a first examination of the behavior of the drag torque and an attempt at understanding its relationship to the field asymmetry of the PM, or the geometrical asymmetry of the HTS component.
of four magnetic disks with the same polarity at the corners of a horizontal square rotating above a single HTS disk. Here the four separate magnetic disks represent a PM which is far from being axisymmetric. This makes the drag torque large enough to give a decent signal. The magnetic disks are NdFeB, with diameter 23 mm and height 10 mm. The flux density at the center of a pole face is 3.5 kG. The HTS disk has a diameter of 41 mm and a height of 19 mm. The HTS material is of YBa2Cu307 prepared by seeded directional solidification. The material so prepared has a critical current density on the order of 104 A/cm2 at ,~Y 2
D
wire frame -
s
/laser
~
mirror sample ,
suspended b o x / cooled b o x 3D -table
2. EXPERIMENT We have chosen two HTS magnet bearing configurations for study. Configuration A consists
Figure 1. Setup for torque measurements.
0921-4534/00/$ - see front matter© 2000 ElsevierScience B.E All rights reserved. PII S0921-4534(00)01390-3
2616
E. Postrekhin et al./Physica C 341-348 (2000) 2615-2616
77 K. The drag torque was measured with the setup shown in Fig. 1, with the gap between the PM disks and the HTS disk at 3 mm and the HTS disk rotated at 0.4 revolutions per minute. In the measurement, the HTS was rotated under the PM disks instead. The measurement was performed under both kind of conditions: field cooled (FC) and zero field cooled (ZFC).
0.05 •
,
0.04I E" 0.03
•
,
I
0.02
.
,
.
,
I 4--configuration I ~configuration
•
, B A I
!
== o ol t =o
8 0.0of -0.01 0.04
I 1.....
0t
t J ' 1080 360 720 ANGLE (degree)
14140
~- 0.03
0"00f -0.01
'
Figure 3. Torque as function of rotating angle.
~
,
,
0
360
720
1080
1440
ANGLE (degree)
Figure 2. Torque as function of rotating angle at FC and ZFC conditions for the case of 4 magnets and one superconductor. Configuration B consists of a ring magnet above four HTS disks at the comers of a square with 48 mm sides. The ring magnet is of NdFeB also. It has an outer diameter of 38 mm and an inner diameter of 13 mm. The flux density at the center is 1.2 kG. The HTS disks have a diameter of 28 mm and a height of 15 mm. They were of the same material and have a critical current density of the same order of magnitude as the HTS disk used in configuration A. The drag torque was also measured with the same setup, at the same gap between PM and HTS disks, with the HTS disks rotated under the ring magnet at the same speed. This measurement was performed only under FC condition. Fig. 2 shows the drag torques measured for configuration A, comparing the results under FC and ZFC conditions. Fig. 3 shows the drag torques measured under FC conditions, comparing results from configuration A with B. 3. DISCUSSION The drag torque shows oscillations with an offset average value that gives the power loss at this
rotating speed (0.4 RPM). As expected, the drag torque from four PM on one HTS disk, -0.025 mNm, is much larger than that from one ring magnet on four HTS disks, -0.002 mNm. Even this small residual drag torque can be attributed to small, but visible imperfections in the ring magnet used. More noteworthy, the drag torque from four PM on one HTS disk under ZFC conditions, -0.015 mNm, is smaller than that under FC conditions. This can be understood as a consequence of the interior of the HTS disk being more effectively shielded from the external magnetic field under ZFC conditions. In terms of the pros and cons of ZFC versus FC conditions in bearing design, this consideration favors ZFC. The oscillations of the drag torque can be understood directly as due to uneven trapped field in the HTS disk, reflecting the non-axisymmetric property of the PM. After a few cycles, the transients in the oscillations disappear and the oscillations are reproduced cycle after cycle for many more cycles. Within each cycle, the oscillations show clear correspondence with the number of magnets used in configuration A, but there is no clearly discernible correlation with the number of HTS disks in configuration B, in this case. 4. A C K N O W L E D G E M E N T This work was supported in part by the State of Texas through the Texas Center for Superconductivity at the University of Houston and the United States Air Force under AFOSR grant # F49620-97-0101.
Physica C 341-348 (2000) 2615-2616
www,elsevier.nl/locate/physc
Drag Torque of High Tc Superconductor Magnet Bearings with Multi-piece Ring Magnets or Superconductors E.Postrekhin, Hong Ye, Ki Bui Ma and Wei-Kan Chu Texas Center for Superconductivity University of Houston, Houston TX 77204, USA In order for a superconductor magnet levitation bearing to rotate freely, the magnetic field from the magnet must be axisymmetric about the axis of rotation. In contrast, the shape of the superconductor need not be axisymmetric. This allows us to make a large ring of high temperature superconductor out of smaller pieces in the fabrication of superconductor magnet bearings. The problem then arises as to whether the substitution of a single large piece of superconductor by smaller pieces might contribute to additional power loss. We have investigated this by studying the drag torque experienced by multi-piece ring magnets as they turn above multipiece rings of superconductors, and comparing with results from single pieces of similar sizes. We have coordinated the variation of the drag torque during a turn with the relative orientation of the magnet and superconductor pieces. We use the mean value of the drag torque over several turns as a measure of power loss.
1. INTRODUCTION Soon after their discovery, high temperature superconductors (HTS) have been used together with permanent magnets (PM) to construct bearings that are practically frictionless. This makes HTS magnet bearings promising for energy critical applications, such as flywheel kinetic energy storage. The extremely low drag torque observed in HTS magnet bearings depend critically on the extent to which the magnet field from the PM is axisymmetric about the rotation axis. On the other hand, the effect of the HTS not being axisymmetric is small, by comparison. The practical implication of this observation is that the HTS component of an HTS magnet bearing need not be a single continuous circular piece, but a large piece could be composed of several pieces. Here, we have limited ourselves to a first examination of the behavior of the drag torque and an attempt at understanding its relationship to the field asymmetry of the PM, or the geometrical asymmetry of the HTS component.
of four magnetic disks with the same polarity at the corners of a horizontal square rotating above a single HTS disk. Here the four separate magnetic disks represent a PM which is far from being axisymmetric. This makes the drag torque large enough to give a decent signal. The magnetic disks are NdFeB, with diameter 23 mm and height 10 mm. The flux density at the center of a pole face is 3.5 kG. The HTS disk has a diameter of 41 mm and a height of 19 mm. The HTS material is of YBa2Cu307 prepared by seeded directional solidification. The material so prepared has a critical current density on the order of 104 A/cm2 at ,~Y 2
D
wire frame -
s
/laser
~
mirror sample ,
suspended b o x / cooled b o x 3D -table
2. EXPERIMENT We have chosen two HTS magnet bearing configurations for study. Configuration A consists
Figure 1. Setup for torque measurements.
0921-4534/00/$ - see front matter© 2000 ElsevierScience B.E All rights reserved. PII S0921-4534(00)01390-3
2616
E. Postrekhin et al./Physica C 341-348 (2000) 2615-2616
77 K. The drag torque was measured with the setup shown in Fig. 1, with the gap between the PM disks and the HTS disk at 3 mm and the HTS disk rotated at 0.4 revolutions per minute. In the measurement, the HTS was rotated under the PM disks instead. The measurement was performed under both kind of conditions: field cooled (FC) and zero field cooled (ZFC).
0.05 •
,
0.04I E" 0.03
•
,
I
0.02
.
,
.
,
I 4--configuration I ~configuration
•
, B A I
!
== o ol t =o
8 0.0of -0.01 0.04
I 1.....
0t
t J ' 1080 360 720 ANGLE (degree)
14140
~- 0.03
0"00f -0.01
'
Figure 3. Torque as function of rotating angle.
~
,
,
0
360
720
1080
1440
ANGLE (degree)
Figure 2. Torque as function of rotating angle at FC and ZFC conditions for the case of 4 magnets and one superconductor. Configuration B consists of a ring magnet above four HTS disks at the comers of a square with 48 mm sides. The ring magnet is of NdFeB also. It has an outer diameter of 38 mm and an inner diameter of 13 mm. The flux density at the center is 1.2 kG. The HTS disks have a diameter of 28 mm and a height of 15 mm. They were of the same material and have a critical current density of the same order of magnitude as the HTS disk used in configuration A. The drag torque was also measured with the same setup, at the same gap between PM and HTS disks, with the HTS disks rotated under the ring magnet at the same speed. This measurement was performed only under FC condition. Fig. 2 shows the drag torques measured for configuration A, comparing the results under FC and ZFC conditions. Fig. 3 shows the drag torques measured under FC conditions, comparing results from configuration A with B. 3. DISCUSSION The drag torque shows oscillations with an offset average value that gives the power loss at this
rotating speed (0.4 RPM). As expected, the drag torque from four PM on one HTS disk, -0.025 mNm, is much larger than that from one ring magnet on four HTS disks, -0.002 mNm. Even this small residual drag torque can be attributed to small, but visible imperfections in the ring magnet used. More noteworthy, the drag torque from four PM on one HTS disk under ZFC conditions, -0.015 mNm, is smaller than that under FC conditions. This can be understood as a consequence of the interior of the HTS disk being more effectively shielded from the external magnetic field under ZFC conditions. In terms of the pros and cons of ZFC versus FC conditions in bearing design, this consideration favors ZFC. The oscillations of the drag torque can be understood directly as due to uneven trapped field in the HTS disk, reflecting the non-axisymmetric property of the PM. After a few cycles, the transients in the oscillations disappear and the oscillations are reproduced cycle after cycle for many more cycles. Within each cycle, the oscillations show clear correspondence with the number of magnets used in configuration A, but there is no clearly discernible correlation with the number of HTS disks in configuration B, in this case. 4. A C K N O W L E D G E M E N T This work was supported in part by the State of Texas through the Texas Center for Superconductivity at the University of Houston and the United States Air Force under AFOSR grant # F49620-97-0101.