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Superconducting armature for induction motor of axial flux based on YBCO bulks
Physica C 372–376 (2002) 1517–1519 www.elsevier.com/locate/physc
Superconducting armature for induction motor of axial flux based on YBCO bulks lvarez a,*, P. Su A. A arez a, D. C aceres a, X. Granados b, X. Obradors b, R. Bosch c, E. Cordero a, B. Perez a, A. Caballero a, J.A. Blanco b a
Industrial Engineering School, University of Extremadura, Apdo. 382, 06071 Badajoz, Spain b ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Barcelona, Spain c ETSII-Barcelona, UPC, Spain
Abstract Several small-scale SC rotating machines have been developed based on HTS bulk materials. Up to now, the role of the SC has been restricted to the rotor where no coil has been required. Despite the difficulty of building classical coils with ceramic materials, we have developed suitable geometries which can be built by machining melt textured YBCO pellets. In this work, we describe a multiphase armature designed to be made starting HTS ceramic pellets. The field created is designed to interact with a disc-shaped rotor producing both the torque and the forces to maintain it in working conditions. Calculations of the field in the rotor cavity generated by the armature in the working conditions are also reported. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconducting motor; Induction motor; Axial flux
1. Introduction The development of HTSC materials, both tape and bulk, has allowed the design of systems in several areas of electro-mechanical engineering. In particular, YBCO bulks have been used directly to implement hysteresis motors [1] and radial and axial flux motors [2] in which only the rotor is superconductor but the stator is made with conventional coils.
*
Corresponding author. Tel.: +34-924-289600; fax: +34-924289601. lvarez). E-mail address: [email protected] (A. A
Also YBCO bulks have been used to make fault current limiters [3,4] due to the fact that they show a very effective limitation with relative low volume, but in these cases machining of the material is required. In this work we present a superconducting singlewinding coil made starting YBCO bulks for a biphase axial flux motor. As the stator circuits are ceramic superconductors, the material has to be machined as in the case of fault current limiters [4,5]. Since HTSC materials can produce a very high magnetic field, we can remove the ferromagnetic core and design an induction motor with an air core. We therefore suggest a design for an axial flux air core motor made of ceramic coils as stator
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 7 0 - 5
1518
lvarez et al. / Physica C 372–376 (2002) 1517–1519 A. A
windings and a superconducting disk as the rotor, and we show its magnetic field distribution. This kind of motor can be applied for instance to the design of dynamic filters or liquid hydrogen pumps. 2. Details of circuit construction Melt-textured YBCO was used to build the ceramic coils. Pellets of 30 mm diameter were obtained by melting in a cylindrical furnace just above the peritectic temperature. An NdBCO seed was placed on the center of the top face of the pellet in order to grow a single-domain pellet with the c-axis oriented in coincidence with the symmetry axis of the pellet. The small thermal gradient in the radial direction of the furnace helps control the expansion of the fronts growing across the surface of the pellet. An optical recording of the process is shown in Fig. 1. The growing fronts move in the radial direction of the pellet. The pellets were cut into disks of 2 mm in thickness by a diamond saw. After an optical selection to reject unsuitable cracks, the disks were shaped by milling to the geometry of the coil. Fig. 2 shows the coil after milling, except for the ends. Silver pads, painted onto the current injection points of the circuit, form the contacts. The con-
Fig. 1. Growing sequence of a YBCO domain along the surface of a pellet of 30 mm in diameter. The growing fronts are highlighted by dotted lines.
Fig. 2. Coil (single winding) obtained by milling over a substrate of alumina. The circuit will be oxygenated after the pads have been painted on.
tact resistivity of the silver pads is reported in [6]. The coils so constructed are oxygenated conventionally, in an oxygen flow at 450 °C.
3. Suggestion of motor prototype With these ceramic coils we are designing a biphase induction motor with both stator and rotor superconductors. For mechanical stability reasons, the prototype consists of two parallel semistators with the rotor between them. Each semistator has two ceramic circuits p/2 rad out of phase and is arranged as in Fig. 3. The rotor is a superconductor disk. The field generated by the current flowing through the coils interacts with the HTS disk as described in Ref. [2] giving the well-known synchronous and hysteresis regimes. In the synchronous regime, the flux is pinned in the SC disk and does not move through the material. In the hysteresis regime, the energy loss due to the change in magnetization when the applied field moves around the disk generates the torque. As is shown in Ref. [2], there are levitating forces in both regimes. However, we propose to include conventional bearings in this prototype in order to handle lower currents in the SC armature.
lvarez et al. / Physica C 372–376 (2002) 1517–1519 A. A
1519
4. Magnetic field distribution
Fig. 3. Stator arrangement.
The magnetic field in the working plane of the rotor of the proposed motor was calculated from the Biot–Savart law and the superposition principle (since the system is linear). Since it is impossible to integrate the Biot–Savart law analytically [7], we used a numerical integration procedure of the Mathematica program package, assuming that the field lines are parallel to the axis in the plane of the rotor, and neglecting losses in the superconductor. The spatial map of the axial magnetic field was calculated for each instant of time. Fig. 4(a) and (b) show the results at z ¼ 0 for t ¼ 0 and t ¼ 2:5 ms, respectively with 50 Hz AC current. 5. Conclusions We have managed to obtain ceramic superconducting circuits with a shape that is suitable for the construction of a biphasic superconducting stator. Neglecting losses, we evaluated the distribution of the magnetic field created by this configuration. The result is a rotatory bipolar field that has good perspectives for driving a rotating superconducting disk in its interior. Acknowledgements This research is funded in part by the Interministerial Commission of Science and Technology and Government of Extremadura. References
Fig. 4. Axial magnetic field in real space at z ¼ 0. (a) t ¼ 0; (b) t ¼ 2:5 ms.
[1] T. Habisreuther et al., IEEE Trans. Appl. Supercond. 7 (2) (1997) 900–903. [2] I. Marquez et al., IEEE Trans. Appl. Supercond. 9 (2) (1999) 1249–1252. [3] R. Tournier et al., Supercond. Sci. Technol. 13 (2000) 886– 895. [4] P. Tixador et al., IEEE Trans. Appl. Supercond. 11 (1) (2001) 2034–2037. [5] S. Elschner et al., IEEE Trans. Appl. Supercond. 11 (1) (2001) 2507–2510. [6] X. Granados et al., IEEE Trans. Appl. Supercond. 11 (1) (2001) 2406–2409. [7] Y. Chu, IEEE Trans. Magnetics 34 (2) (1998) 502–504.
Superconducting armature for induction motor of axial flux based on YBCO bulks lvarez a,*, P. Su A. A arez a, D. C aceres a, X. Granados b, X. Obradors b, R. Bosch c, E. Cordero a, B. Perez a, A. Caballero a, J.A. Blanco b a
Industrial Engineering School, University of Extremadura, Apdo. 382, 06071 Badajoz, Spain b ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Barcelona, Spain c ETSII-Barcelona, UPC, Spain
Abstract Several small-scale SC rotating machines have been developed based on HTS bulk materials. Up to now, the role of the SC has been restricted to the rotor where no coil has been required. Despite the difficulty of building classical coils with ceramic materials, we have developed suitable geometries which can be built by machining melt textured YBCO pellets. In this work, we describe a multiphase armature designed to be made starting HTS ceramic pellets. The field created is designed to interact with a disc-shaped rotor producing both the torque and the forces to maintain it in working conditions. Calculations of the field in the rotor cavity generated by the armature in the working conditions are also reported. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Superconducting motor; Induction motor; Axial flux
1. Introduction The development of HTSC materials, both tape and bulk, has allowed the design of systems in several areas of electro-mechanical engineering. In particular, YBCO bulks have been used directly to implement hysteresis motors [1] and radial and axial flux motors [2] in which only the rotor is superconductor but the stator is made with conventional coils.
*
Corresponding author. Tel.: +34-924-289600; fax: +34-924289601. lvarez). E-mail address: [email protected] (A. A
Also YBCO bulks have been used to make fault current limiters [3,4] due to the fact that they show a very effective limitation with relative low volume, but in these cases machining of the material is required. In this work we present a superconducting singlewinding coil made starting YBCO bulks for a biphase axial flux motor. As the stator circuits are ceramic superconductors, the material has to be machined as in the case of fault current limiters [4,5]. Since HTSC materials can produce a very high magnetic field, we can remove the ferromagnetic core and design an induction motor with an air core. We therefore suggest a design for an axial flux air core motor made of ceramic coils as stator
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 7 0 - 5
1518
lvarez et al. / Physica C 372–376 (2002) 1517–1519 A. A
windings and a superconducting disk as the rotor, and we show its magnetic field distribution. This kind of motor can be applied for instance to the design of dynamic filters or liquid hydrogen pumps. 2. Details of circuit construction Melt-textured YBCO was used to build the ceramic coils. Pellets of 30 mm diameter were obtained by melting in a cylindrical furnace just above the peritectic temperature. An NdBCO seed was placed on the center of the top face of the pellet in order to grow a single-domain pellet with the c-axis oriented in coincidence with the symmetry axis of the pellet. The small thermal gradient in the radial direction of the furnace helps control the expansion of the fronts growing across the surface of the pellet. An optical recording of the process is shown in Fig. 1. The growing fronts move in the radial direction of the pellet. The pellets were cut into disks of 2 mm in thickness by a diamond saw. After an optical selection to reject unsuitable cracks, the disks were shaped by milling to the geometry of the coil. Fig. 2 shows the coil after milling, except for the ends. Silver pads, painted onto the current injection points of the circuit, form the contacts. The con-
Fig. 1. Growing sequence of a YBCO domain along the surface of a pellet of 30 mm in diameter. The growing fronts are highlighted by dotted lines.
Fig. 2. Coil (single winding) obtained by milling over a substrate of alumina. The circuit will be oxygenated after the pads have been painted on.
tact resistivity of the silver pads is reported in [6]. The coils so constructed are oxygenated conventionally, in an oxygen flow at 450 °C.
3. Suggestion of motor prototype With these ceramic coils we are designing a biphase induction motor with both stator and rotor superconductors. For mechanical stability reasons, the prototype consists of two parallel semistators with the rotor between them. Each semistator has two ceramic circuits p/2 rad out of phase and is arranged as in Fig. 3. The rotor is a superconductor disk. The field generated by the current flowing through the coils interacts with the HTS disk as described in Ref. [2] giving the well-known synchronous and hysteresis regimes. In the synchronous regime, the flux is pinned in the SC disk and does not move through the material. In the hysteresis regime, the energy loss due to the change in magnetization when the applied field moves around the disk generates the torque. As is shown in Ref. [2], there are levitating forces in both regimes. However, we propose to include conventional bearings in this prototype in order to handle lower currents in the SC armature.
lvarez et al. / Physica C 372–376 (2002) 1517–1519 A. A
1519
4. Magnetic field distribution
Fig. 3. Stator arrangement.
The magnetic field in the working plane of the rotor of the proposed motor was calculated from the Biot–Savart law and the superposition principle (since the system is linear). Since it is impossible to integrate the Biot–Savart law analytically [7], we used a numerical integration procedure of the Mathematica program package, assuming that the field lines are parallel to the axis in the plane of the rotor, and neglecting losses in the superconductor. The spatial map of the axial magnetic field was calculated for each instant of time. Fig. 4(a) and (b) show the results at z ¼ 0 for t ¼ 0 and t ¼ 2:5 ms, respectively with 50 Hz AC current. 5. Conclusions We have managed to obtain ceramic superconducting circuits with a shape that is suitable for the construction of a biphasic superconducting stator. Neglecting losses, we evaluated the distribution of the magnetic field created by this configuration. The result is a rotatory bipolar field that has good perspectives for driving a rotating superconducting disk in its interior. Acknowledgements This research is funded in part by the Interministerial Commission of Science and Technology and Government of Extremadura. References
Fig. 4. Axial magnetic field in real space at z ¼ 0. (a) t ¼ 0; (b) t ¼ 2:5 ms.
[1] T. Habisreuther et al., IEEE Trans. Appl. Supercond. 7 (2) (1997) 900–903. [2] I. Marquez et al., IEEE Trans. Appl. Supercond. 9 (2) (1999) 1249–1252. [3] R. Tournier et al., Supercond. Sci. Technol. 13 (2000) 886– 895. [4] P. Tixador et al., IEEE Trans. Appl. Supercond. 11 (1) (2001) 2034–2037. [5] S. Elschner et al., IEEE Trans. Appl. Supercond. 11 (1) (2001) 2507–2510. [6] X. Granados et al., IEEE Trans. Appl. Supercond. 11 (1) (2001) 2406–2409. [7] Y. Chu, IEEE Trans. Magnetics 34 (2) (1998) 502–504.