Disk-shaped superconductor rotor for an axial flux induction motor

Disk-shaped superconductor rotor for an axial
flux induction motor
A. Álvarez, Member, IEEE, P. Suárez, D. Cáceres, X. Obradors, X. Granados, R. Bosch, B. Pérez, J.
M. Ceballos and M. Torres
1
Abstract-- Until now, most works about superconductor
electric motors have been done only with superconducting
materials in the rotor due to the difficulty in machining the
material into the conventional coil shape. The authors of this
paper have designed a superconducting induction motor with
superconductors in both rotor and stator. Ceramic coils were
made from YBCO bulks to construct an easy-to-build modular
stator for a biphasic axial flux motor. In this paper the
behaviour of YBCO disks working as the rotor in this type of
motor is studied and the torque-speed characteristic is
calculated.
Index Terms-- Superconducting induction motor, axial flux,
YBCO bulks.
I.
INTRODUCTION
Y
BCO bulks have been used directly to implement
hysteresis motors [1] and radial and axial flux motors [2]
in which only the rotor is superconducting while the
stator is made with conventional coils.
Since HTSC materials can produce a very high magnetic
field, we can remove the ferromagnetic core and design an
Manuscript received August 6, 2002. This research is funded in part by
the Inter-ministerial Commission of Science and Technology of Spain and
Government of Extremadura.
A. Álvarez is with the Electrical Engineering Department, University of
Extremadura. Apdo. 382. 06071 Badajoz. SPAIN. He is IEEE Member
(corresponding author to provide phone: 0034-924-289646; fax: 0034-924289601; e-mail: [email protected]).
P. Suárez is with the Electrical Engineering Department, University of
Extremadura. Apdo. 382. 06071 Badajoz. SPAIN (e-mail: psuarez@
unex.es).
D. Cáceres is with the Applied Mathematic Department, University of
Extremadura. Apdo. 382. 06071 Badajoz. SPAIN (e-mail: dcaceres@
unex.es).
X. Obradors is with the Institute of Material Science ICMAB (CSIC).
Barcelona. SPAIN
X. Granados is with the Institute of Material Science ICMAB (CSIC).
Barcelona. SPAIN
R. Bosch is with the Electrical Engineering Department. UPC. Barcelona.
SPAIN.
B. Pérez is with the Electrical Engineering Department, University of
Extremadura. Apdo. 382. 06071 Badajoz. SPAIN (e-mail: belenpc@
unex.es).
J. M. Ceballos is with the Electrical Engineering Department, University
of Extremadura. Apdo. 382. 06071 Badajoz. SPAIN
(e-mail:
[email protected]).
M. Torres is with the Electrical Engineering Department, University of
Extremadura. Apdo. 382. 06071 Badajoz. SPAIN
(e-mail:
[email protected]).
induction motor with an air core. In [3] we showed the
superconducting stator windings for an axial flux air core
motor. These windings are made of ceramic coils from
machined YBCO bulks. We propose a design with the rotor
of this motor being a superconducting disk between two
stators like those in [3] and study the behaviour of a YBCO
disk working as the rotor in this type of motor, calculating its
torque-speed characteristic.
II. DESIGN OF THE SYSTEM AND EXPERIMENTAL PROCEDURE
The disk was made from YBCO bulks with 15 mm of
radius and 3 mm of thickness. To study the behavior of the
disk in similar conditions to those in the motor, we
constructed a metallic structure to hold a small cryostat where
the disk is immersed in L-N2 at 77 K. Figure 1 shows this
system and Figure 2 shows the detailed working of the same.
In Figure 2, the disk (1) is at the end of an axle (2) that was
machined from an aluminum cylinder. The other end of the
axle is fitted into a pulley used to apply a torque on the rotor
and to measure the torque-speed characteristic of the rotor.
Two sets of bearings were suitably placed near the pulley,
separately by about 2 cm. A bipolar rotating magnetic field
(3) is located under the disk, outside the cryostat. This field
comes from a permanent magnet linked to the axle of a
single-phase induction motor. An inverter allows its rotation
speed to be graduated between 0 and 1500 rpm. Placed over
the disk, a ferromagnetic washer (4) guarantees the axial
distribution of the magnetic flux in the disk.
III. MEASUREMENTS OF THE MAGNETIC FIELD
For the measurement of the magnetic field in the zone
where the superconductor rotor works we placed a Hall probe
(THS-118). The bias point of the sensor was obtained with a
5 mA current source based on an operational amplifier.
The Hall voltage is cleaned and amplified in an
instrumentation amplifier and is read by a data acquisition
board (NI DAQ 16-E10).
An application based on Labview software manages the
process. It calibrates the Hall probe at the working
temperature, adjusts the offset voltage, and displays the
magnetic field and its waveform on screen. Figure 3 shows
the scheme of this process and Figure 4 the results. In this
figure one sees that the superconducting disk reduces the field
as measured either on the top or the bottom of the disk.
resisting torque. An optical tachometer gave the rotation
speed of the disk, n, for each torque measurement, M.
Fig. 3. Scheme to measure the magnetic field.
Fig. 4. Magnetic field just above and below the disk in normal and in
superconductor states.
Fig. 1. System to study the behaviour of the superconductor disk under a
rotating magnetic field.
Tests in FC and ZFC conditions were carried out to compare
the two situations. The rotation speed of the magnetic field
(synchronism speed) was about 1430 rpm.
Figure 6 shows the torque-speed characteristic in FC and
ZFC conditions. Hysteresis motor behaviour can be observed
near the synchronism speed. However, there is a trend as one
moves further away from this speed that was found to be well
described by a parabola.
Fig. 2. Detailed working of the superconductor disk.
IV. TORQUE-SPEED CHARACTERISTIC
The torque-speed characteristic was obtained using two
dynamometers and two auxiliary pulleys as shown in Figure
5. The difference in the readings of the dynamometers, D1
and D2, multiplied by the main pulley radius gives the
Fig. 5. Scheme to measure the torque-speed characteristic.
that rotates due to a rotating bipolar magnetic field. This field
was measured on the top and the bottom of the disk and was
found to be reduced as expected in superconducting
conditions. Nevertheless, the field was not zero which is
strongly indicative of trapping in the disk.. The torque-speed
characteristic in ZFC and FC conditions was determined and
showed the expected hysteresis motor behaviour near the
synchronism speed.
ACKNOWLEDGMENT
We wish to thank J. M. Moya for machining of the
superconducting disk axle.
REFERENCES
Fig. 6. Torque-speed characteristic in FC and ZFC conditions.
[1]
V. SUMMARY AND CONCLUSION
[2]
We have described a superconducting rotor for an axial
flux induction motor. This rotor is a disk made from a YBCO
bulk
[3]
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L.K. Kovalev, IEEE Trans. Appl. Supercond. 7 (2) (1997) 900-903.
I. Márquez, X. Granados, X. Obradors, J. Pallarés and R. Bosch, IEEE
Trans. Appl. Supercond. 9 (2) (1999) 1249-1252.
A. Álvarez, P. Suárez, D. Cáceres, X. Granados, X. Obradors, R.
Bosch, E. Cordero, B. Pérez, A. Caballero and J.A. Blanco. Physica C
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