Superconducting bearings for high load applications

AppIicd Superconductivity

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in Great Britain.

VoI.1, Nos 7-9, pp. 1175 - 1184, 1993 All rights reserved

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56.00 + 0.00 0964-1aoit93 @ 1993 Pcrgamon Press Ltd

SUPERCONDUCTING BEARINGS FOR HIGH LOAD APPLICATIONS Francis C. Moon Czeslaw Golkowski David Kupperman School of Mechanical and Aerospace engineering Cornell University, Ithaca, New York 14853

ABSTRACT Progress in the application of high temperature superconducting ceramic materials to magnetic bearings and the prospects for higher load devices is reviewed. In particular we present experimental evidence from two studies at Cornell. New meas~ments from a hybrid supe~onduct~g bearing shag the possibility of peak magnetic pressures of 6Q?&‘cmz or higher in the temperature range of 20*-40X using a wire wound coil of Nb-Sn wire as the source of magnetic field and a melt-quenched processed YBa2Cu307 bulk superconductor as the other half of the bearing. In a second experiment we demonstrate an 0.85 kg levitated rotor at 28,000 RPM and 5 kJ stored energy. This experiment, conducted at 78X, demonstrates a six-component discrete-element YBCO bearing with a rare earth ring magnet. Stiffness and spin-decay rne~~~nts are reported. These experiments point the way to large rotor levitation of 10-100 kg in the near term and 1000 kg rotors in the not too distant future.

INTRODUCTION Bearings are critical components of machines of the 20th century, be they disc drives, gyros, pumps, generators, or jet engines. There is now sufficient technical progress to predict that magnetic bearings will play an increasing role in the machines of the 21st century. Already active magnetic bearings now lift 50 ton MAGLEV vehicles and suspend 1 ton rotors in large gas pipeline pumps. But, while much progress has been made in active magnetic bearings, the basic disadvantages remain; passive instability, complex sensors and actuators, and the need for a power supply. Some of these problems can and will be solved by passive

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magnetic bearings. These devices were made practically possible by the

development of new high temperature superconducting

materials such as YBa2Cu307

(YBCO). Magnetic bearings of YBCO, until recently, were thought to be mainly limited to low load applicationt) due to both low magnetic pressures l-10 N/cm2 and low magnetic stiffness

102-103 N/m. However, in this work we report on progress in developing higher

pressure (60 N/cm2) and high stiffness magnetic bearings (-105 N/m) using a hybrid bearing of low temperature superconducting wire and a high temperature superconducting

bulk

material. The list of potential applications of superconducting bearings is considerable. Two years ago small cryocoolers were thought to be the prime target for high Te YBCO bearings. With recent advances in materials one can now add to the list; cryoturbo pumps, liquid hydrogen circulators, cryo alternators, turbo molecular pumps, angular momentum wheels, and energy storage flywheels. It is also not unreasonable to imagine jet engine rotors with magnetic bearings. Space applications are, of course, a natural area for superconducting

magnetic

bearings. For example, spacecraft employ several different rotary devices including control moment gyros, reaction wheel assemblies, and stabilizing momentum wheels. Existing designs for momentum wheels have rotors with 6 kg mass, 40 cm diameter, and over 5,ooO RPM. By using su~rcondu~ting magnetic bearings at 50,~ RPM the rotor mass and diameter could be decreased for the same angular momentum. Other space applications involve rocket engine turbo pumps.2) These devices, such as those in the space shuttle main engine, can have a rotor mass of 112ton and run at speeds of 30,~ bearing temperatures can be from 30°K to 83X.

RPM or more. The

Smaller turbopumps (3 kg) can run at

speeds of lOO-200,000 RPM. Active magnetic bearings are now used in centrifugal compressors for natural gas pipelines in North America. These devices produce lift forces over 1.5 ton and rotate at speeds up to 5,000 RPM.3) Existing applications of active magnetic bearings should also become primary targets for superconducting magnetic bearing development. Already higher load su~rconducting

bearings have been reported. Takahata, et a14)

of Kayo-Seiko Japan has reported levitation of a 1 kg rotor at 5000 RPM. Fukuyama, et al.5) of NSK ltd. Japan, and ISTEC Tokyo have reported a prototype rotor-bearing with 2.4 kg rotor and speeds of up to 30,000 REM. D. Rao6) of MT1 Carp, USA has recently reported a 7 kg rotor with 9,000 RPM.

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LOW TEMPERATURE AND HIGH FIELD MAGNETIC BEARINGS Inanearlier report7) our laboratory has presented magnetic force data for a hybrid NbTi wound coil and a YRCO bulk bearing at 4.2%

These resufts showed that peak magnetic

pressures of 60 N/cm2 and average values of 32 N/cm2 could be obtained for YBCO at 4.2’K. Also magnetic stiffness in the range of 105 N/m could be obtained by increasing the magnetic field to 2 Tesla. Low temperature measurements of levitation forces on YBCO have been measured in our laboratory (see e.g., Ch~g8)).

These experiments showed that dramatic increases in

magnetic force could be achieved by going to lower temperatures. These experiments were conducted with small rare-earth magnets whose maximum field was around 0.4 Tesla. Thus, the limiting magnetic pressures were of the order of lo-15 N/cm2. Last year we reported new experiments at 4.2-K using a 2.4 cm diameter superconducting

coil of

niobium-ti~ium wire, and a bulk YBCO sample processed by the melt-powder-melt-awn method at IS’IEC by Dr. M. Murakami. At a field of 2 T, the 3.5 cm diameter sample produced 300 N of force, capable of lifting 30 kg of mass. The force-stiffness relation is shown in Figure 1 and shows a vertical magnetic stiffness of over 16 N/m.

F t*

I

o6

1

o5

z= *-

ii5

I o* Force [N] Figure1. Mess&

andcalculated magnetic stiffness. (From Ref. [rJ)

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In recently completed experiments we have developed a hybrid Nb3Sn wire coil and YBCO bulk bearing and have obtained force measurements in the range 4.2’K - 30’K. This range is especially irn~~~t in place of the Nb$n

for cryofluid pumps and could eventually employ BSSCO wire

coil.

These experiments were carried out with the same YBCO sample from ISTEC as reported in the previous study, however, the diameter of the low-temperatum superconducting coil was much larger due to the special fabrication problems of Nb3Sn wire. In order to obtain a variation in temperatures in the YBCO sample, a copper thermal shield was placed between the YBCO sample and the Nb3Sn coil. Using heating elements on the copper, the temperature of the YBCO sample, as measured by a thermocouple, could be varied while the P&&n coiI was still in the liquid helium bath. The higher critical temperature of the Nb$n insured that the coil would remain superconducting even if the coil temperature rose above 10’K. The experiment is shown in Figure 2. The results from these experiments are shown in Figures 3 and 4. One can see that there is a small drop-off in levitation force with temperature from 4.2-K - 30’K especially at small gap (Figure 3). The force-distance Relation shows a characteristic exponential decay with gap. Finally, we show the force versus current (Figure 4a,b). For linear magnetic material, one would expect an @ dependence of force on current.9) However, apparently, as current was increased, some flux that was effectively screened at low field penetrated the YBCO superconducting at higher fields, as shown in Figure 2, resulting in a linear F - I relation. At a larger gap, and hence lower fields, the F - @ behavior seems to hold.

Superconductingcoil Figure 2. Event set-up and the magnetic flux lines behveen the MySn coiI and the bulk YBCO superconductor.

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Force versus Temp. at dist. 3 mm 250 A

0

5

10 15 Temperature

Figure 3. Measured force between the Werent tempemmms.

Nb3Sn

20 f’K]

25

30

coil and YBCO superconductor

at

HIGH-SPEED STORED ENERGY ROTOR In a separate study, our laboratory has built and tested a levitated stored energy rotor of a size that might be used for spacecraft gyros or a momentum wheel device. The 0.85 kg mtor achieved speeds of 28,000 RPM with a stored kinetic energy of around 5 W. A sketch of this device is shown in Figure 5, and a list of properties and performance is given in Table I. The primary goal of this experiment was to demonstrate the levitation of a 10 cm scale device with 1-2 cm scale YBCO samples. This concept relies on the fact that for a low to zero magnetic drag device, only the source of magnetic field need be symmetric.9) Thus, a neodymium-iron-boron ring magnet of 8 cm in diameter was used with a maximum field of 0.4 Tesla and approximately 5% uniformity in the field in the circumferential direction (see also Ref. 4 for a similar device). Stress analysis of the rare-earth ring magnet under centrifugal forces showed that an allowable tensile stress of 4 107 N/m2 would limit the speed to under 15,000 RPM. Thus, a magnet containment structure was designed using the differential thermal contraction between the aluminum structure and the ring magnet.

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Force versus current at distance 3mm 21°K I i i 1 /i i; i j 1 i! j i i i i ii\! \ ii\ i :::::::::j i j i i j j :j i: :/ :: :j :j :; :j :i :j ij :i :j :j :j Ij :;::::::::::

.:

L:::::;::::j:::i

~~iiii:lii~~~;i~:;:r:iiti~~~-~ .i-__..: ....................f .........f ....i..

250--

I

::

;

‘:

:

:

:

: i .

i

; j

Ii,, i j

:

: ; :

--

:““‘....j....5....I....T....j....T.’ ;*\ ; : ; i j j -iiijjiii ij;:::::,::: ; ; i i : i : : ; : : j i i j : :/::::ij::: : : : ; : : : : : : :: :: :{ :: :: :; :: i i:iiii:: :)ii ii ;;: ij ..A

. . . . . . . . . i....;

2ooi; i j j f j; j: i i

0

50

100

I

. . . . i....;

j

I

. . . . i

j

. . . .

j

L.... f . . ..A

ii

. . . . . . . . . i

. . . .

j

IS0 200 current [A]

250

300

Force versus current at distance 20 mm 20°K 70 60 50 40 30 20

10 0 100

150 current [Aj

200

250

300

Figure 4. Measured force between the Nb3Sn coil and YBCO superconductor for different currents in the co9.

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Turbine

/Rotor Permanent

magnet

Superconductors In

out

II

I_

I

rn2 A .

Brass

holder

Figure 5. High speed rotor experimental set-up.

This concept is called a discrete-element, rotary superconducting bearing. It implies that one does not have to fabricate large samples of bulk YBCO or other high temperature superconductor in order to levitate large rotors.

six YBCO

samp!es prccessed with

0 u

-*l~-qi~ex!: **A”.&

p:otocol were used. Four samples

were processed by Dr. H. Hojaji of the Catholic University of America, and two samples were processed by Dr. M. Murakami of ISIEC, Tokyo. The specific processes have been reported elsewhere.

TABLE I. CHAIWCTERISTICS OF THE LEVITATED ROTOR Mass:

0.85 kg

Magnet:

Neodymium-iron-boron; average peak field: 0.4 T magnetic field uniformity < 5% O.D. = 9 cm I.D. = 7 cm allowable tensile stress 4 lo7 N/m2 polarization: axial

Bearing:

6 samples of YBCO coolant: liquid nitrogen maximum levitated force: 27.9 N vertical stiffness at 3 mm gap: 10.3 N/mm lateral stiffness: 1.4 N/mm pitch frequency: 0.25 Hz (15 rpm)

Stored energy:

at 28 000 t-pm: 4.4 kJ

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The arrangement of YBCO pellets and ring magnet are shown in Figure 6. Both vertical and lateral force measurements were obtained for this configuration. The maximum vertical force achieved was 27.9 N at zero gap. The vertical stiffness at a 3 mm gap was around 104 N/m, and the lateral stiffness was close to 1.4 103 N/m.

UA2

4

Figure 6. Arrangement of YBCO stator bearing components. The dotted line shows the ring magnet position. The axes shown are directions for lateral stiffness measurements.

The magnets were placed in an enclosed brass cryostat which had a liquid nitrogen flow through the pellet chamber. The rotor was placed above the cryostat and generally operated between 78% and room temperature. The rotor was driven by a small turbine using dry nitrogen gas. Speeds of up to 28,000 RPM were obtained. However, the vacuum created by the spinning rotor between the rotor and the brass cryostat seemed to create a negative lift and aerodynamic heating as well as some magnetic losses contributed to eventual quenching the YBCO samples at these speeds. Speed decay measurements were made using a small laser and a photo cell. A typical decay history is shown in Figure 7. The early time history seems to be exponential and is believed driven by aerodynamic forces. The later time history appears linear suggesting a constant drag

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Time [set] Figure 7. The decay of the angular frequency of the 0.85 kg rotor

versus time.

vacuum we believe the decay rule for this rotor-bearing would be 2.1 Hz per minute, so that a 30,000 RPM rotor should have a 150 minute spin down. The maximum linear speed at the ring magnet at 28,000 RPM was 117 m/s.

ACKNOWLEDGEMENTS This research was supported in part by the U.S. NASA Goddard Space Center under Dr. Y. Flom. The authors also wish to thank the IGC Corp. of Guilderland, NY for the NbgSn wire, Dr. M. Murakami of ISTEC, Tokyo, and Dr. H. Hojaji of The Catholic University of America, for the YBCO samples, Dr. J.E.C. Williams of the MIT Magnet Laboratory for advice on superconducting leads. Finally, we wish to thank R. Takahata of Koyo-Seiko Co., Osaka, for supplying the ring magnet.

REFERENCES 1.

Moon, F.C. and Chang, P.-Z., “High speed rotation

of magnets

on high Tc

superconducting bearings,” Appl. Phys. Lea. s, (4) 397-399 (1990). 2. Dill, J.D., Rao, D.K., and Decker, R., “A feasibility study for the application of high temperature superconducting

bearings to rocket engine turbo pumps,” Proc. Conf.

Advanced Earth to Orbit Propulsion Technology, May 15-17, 1990, NASA Marshall Space Flight Center.

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3. O’Connor, I..,.“Active magnetic bearings give systems a lift,” Mechanical Engineering m (2), 52-57 (1992). 4. Takahata, R., Ueyama, H., Yotsuya, T., “Load carrying capacity of supe~onducting magnetic bearings,” Proc. ISEM-91, International Symposium on Electra-Mechanics, Sendai, Japan, January 28-30, 1991. 5. Fukuyama, H,, K. S&i, T. Takizawa, 5, Aihara, M. Murakami, H. Takaichi, S. Tanaka, ‘~Sup~r~onducting

magnetic

bearing

using

MPMG

YBaCuQ,”

Advances

in

Superconductivty IV, H. Hayakawa, N. Koshizuku feds.), Springer-Verlag, Tokyo, pp. 1092- 1096. 6. Rao, D., “A 15 lb. rotor at speeds up to 9,000 RPM using superconducting bearings,” Press release MTI Corp., Latham, New York, May 6,1992. 7. Moon, F.C., “Xevitation studies in high Tc superconductors at lower temperature and high fields,” Advances in Superconductivity

IV, H. Hayakawa,

N. Koshizuka (eds.),

Springer-Verlag, Tokyo, 1049- 1054. 8. Chang, P.-Z., “Mechanics of Superconducting Magnetic Bearings,” Ph.D. Dissertation, Cornell University, Ithaca, New York, January 1991. 9. Moon, F.C., -on (expect@+

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J. Wiley & Sons, 1993