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Characteristics of heating in a mechanically vibrated superconducting coil
Cryogenics 37 (I 997) 287-292 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved
PII: SOOll-2275(97)00031-3
001 l-2275/97/$17.00
Characteristics of heating in a mechanically vibrated superconducting coil” Eiji Suzukit, Suzu ki7
Sanetoshi
Saitot,
Junji OhmoriS, Masao Oki§ and Fumio
tRailway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji City, Tokyo 185, Japan *Toshiba Corp., 2-4 Suehiro-cho, Tsurumi-Ku, Yokohama City 230, Japan §Mitsubishi Electric Corp., l-l Tsukaguchi-Honmachi 8-chome, Amagasaki City 661, Japan IHitachi Works, Hitachi Ltd, 3-l-l Saiwai-cho, Hitachi City, lbaraki 317, Japan Received 6 August
1996; revised
16 January
1997
It is revealed that much heat is generated not only by the eddy current due to the relative displacement between conducting elements in a magnet, but also by the mechanical vibrating deformation of a superconducting coil [Suzuki, E., Cryog. Eng., 1994, 29, 495-5031. We measured the characteristics of heating in the mechanically vibrating superconducting coil in resonance using an oil servo actuator. As a result of these experiments, the following facts are made clear: (i) the heat generated in a vibrating superconducting coil is larger in the twisting configuration than in the bending one under the vibrating mode; (ii) the characteristics of the increment in heating in the vibrating coil under the energizing and the de-energizing state are almost the same. We cite as a factor in heating phenomena of the mechanical excitation the frictional heat between the fasteners and a superconducting coil. The size of the displacements in these frictional parts are supposed to be of the order of several micrometres. We intend to make further analyses of the heating phenomena. 0 1997 Elsevier Science Ltd. Keywords: heating phenomena; vibrating deformation
superconducting
To estimate the limits of quenching in a mechanically vibrated superconducting coil, we have constructed equipment which is able to cause a bending or torsional vibrating deformation of the coil and performed various experiments. It is revealed that more than negligible heat generation occurs even when the superconducting coil is mechanically vibrated by a purely mechanical actuating force and suffers bending or torsional deformation. This heat generation is regarded as one of the various heating factors when the superconducting magnet on a maglev vehicle vibrates and heats internally under the influence of electromagnetic disturbances from the ground coils. Additionally, as this heating sometimes causes quenching of a coil, it has an important significance. The amount and features of this heat generation are discussed in this paper.
*Originally Japanese)
published
in
Teion
Kogaku,
1994,
29(10)
(in
coil; resonance; friction; mechanical
Experimental
methods
Test facility 1 shows a view of the test facility. A superconducting coil is hung from the liquid helium tank by stainless steel. We have adopted an actuating mechanism which connects the rod at the centre of a racetrack-shaped coil with the oil servo actuator and sustains two arches of a coil with two fixing rods. In addition to the bending deformation of the coil, another vibrating mode of deformation can occur at the corresponding resonance. Furthermore, we have also performed an experiment in which the torsional deformation of a superconducting coil is caused naturally by acting on the two diagonal points of the coil and reacting at the other two diagonal points. The oil servo actuator can generate a reciprocating force with frequencies broadly ranging from quasi-static to about 400 Hz. This equipment, that operates without electrical actuation, has such features that it enables the vibrating coil to be tested in both the energized and the de-energized Figure
Cryogenics
1997 Volume
37, Number
6
287
Heating
in a mechanically
vibrated
SC coil: E. Suzuki et al. /terns to be measured
The items to be measured in these mechanical actuating tests are as follows:
uperconducting coil
0 acceleration of the surface of the inner vessel; strain on the surface of coil and inner vessel; l temperature of the surface of an inner vessel measured by a copper and constantan thermocouple; l magnetic fluctuation of the coil measured by a search coil. l
Oil servo actuator
Superconducting Figure 1 Vibration (twisting) coil
test
facility
for
mechanically
Results of mechanical
Procedures
Amount
We estimated the increase of heat loss under mechanical vibration by measuring the difference in the rate of the helium gas evaporated in the inner vessel between the vibrated and static states of a coil. The following tests have been carried out in the energized and de-energized states of a coil:
of heat loss
0 Actuating force-l
Z”
A type 3-
5 =
2-
0
+
+ Cl 0
E al t 50
9
+
0
0
0
t
1
150
50
250 Frequency
Figure2 ing test
++
Characteristics
r
350
(Hz)
of heating in the mechanically
bend-
coil coil type
B
C
About 1.0 1400 700 kA 500 A 72 mm wide x 72 mm high
About 1.5 1400 700 kA 500 A 70 mm wide x 72 mm high
Method like laying brick
Method of laying in tight array
Method of laying in tight array
Deformed ellipse shape (machine made) About 5 mm
Near rectangular
Rectangular
A Superconducting wire (NbTi fine multi-filamentary wire) About 1.0 (I 1 copper ratio (2) number of turns 1167 700 kA (3) magnetomotive force (4) running current 600 A (5) se;tii;al dimension of 40 mm wide x 72 mm high
Cryogenics
De-energized state
+ Energized state
Superconducting coil
Z S
Superconducting
288
.47 kN (150 kgf)
%
The superconducting coils used in these experiments are similar to those in the magnets having a pole pitch length of 1350 mm to be adopted for our new maglev test line (Yamanashi Test Line) and they form a short racetrack shape which is 500 mm in width and 1070 mm in length. We can exchange one coil for another in the tests.
(2) thickness of main plate
test
in a coil of type A under mechanical vibration having a constant actuating force. The force acting on the supercon-
Obtain the amplitude of acceleration and the rate of helium gas by sweeping the actuating frequencies with the amplitude of actuating force held constant. Measure the increment of heat loss under a steady actuating force at several frequencies including resonance. Investigate the increment of heat loss and the occurrence of quenching under the increase in the actuating force at a particular frequency (at resonance).
(6) configuration of turning Structure of inner vessel (I) sectional shape
vibration
Figure 2 shows the increment of the evaporated helium gas
the evaporated
Structure of superconducting
test
The superconducting coils which are used in these experiments are three short type coils investigated for our new test line. Table 1 gives the main items in these coils. Each coil structure differs in detail depending on its maker.
state. Various tests were done in the energized and de-energized states of a coil and the results were compared.
Table 1
coils for vibration
bending
1997 Volume
37, Number
6
About 3 mm
(press made)
About 3 mm
Heating in a mechanically Actuating force-100 Energized state
vibrated SC coil: E. Suzuki et al.
kgf Superconducting coil--A
Frequency
(Hz)
Frequency
(Hz)
type
Figure 3 Acceleration of eight points on the coil in the mechanically of the accelerometers in Figure 4
bending test. The numbers Q -
coil from the ground coils for levitation is larger and the frequency in running at a speed of 500 km h-’ corresponds to 308 Hz. A large amount of heat loss is generated at 270-280 Hz, as shown in Figure 2. Figure 3 shows the output of the accelerometers fixed on the surface of the superconducting coil. There are two main resonances having large amplitude peaks in the acceleration at several points of this vibrating coil. Peaks appear at 270-280 Hz with large evaporation and at 150-160 Hz. Figure 4 shows the configurations of the vibrating deformation corresponding to each resonance. From this figure we can recognize the vibration modes in these resonances such as the torsional mode and the bending mode. From these experimental results, it is confirmed that an excessive frictional heat generation in the microscopic slipping parts of a coil occurs notably under torsional deformation. Although we cannot observe directly the parts generating heat loss, the heat loss is supposed to come from friction between the coil and the fasteners, as shown in Figure 5. A microscopic slip between the coil and the fasteners moves each relative to the other and generates heat by the resonant deformation due to the electromagnetic
force in high speed running’. Figure 6 shows the increment of heat loss in the coils of type A under bending actuation and type B under torsional actuation when we increase the actuating force. The heat loss increases out of proportion to the amplitude of the actuating force. We suppose that this heating phenomenon is due to friction and as the actuating force increases, not only the relative movement of slip increases, but also the range of slip extends. Figure 7 shows the increment of heat loss under constant torsional force in the coil of type B at very low frequencies. As the inertial force can be neglected, we suppose that the increment of heat loss is proportional to the number of repetitions if this heating is a frictional one due to quasi-static deformation. However, we could not actually get a completely proportional relationship. Furthermore, we could see a strong resemblance between the characteristics of heat generation when the superconducting coil is energized and when it is de-energized. This means that heat generation due to the friction on a boundary surface does not depend upon a hoop force of the energized coil and we cannot as yet ascertain the reason.
ducting
Cryogenics
@ correspond to the positions
1997 Volume
37, Number
6
289
Heating
in a mechanically
vibrated
SC coil: E. Suzuki et al. (a) Characteristics of A type coil actuated by bending force
Energized slate Actuating force-l
.47 kN (150 kgf) n
zE -“.ol -0.02 5 E : 54 6
-0.03 -0.04 -0.05 -0.06 -0.07 -0.06 -0.09 -0.10
L I 0
Actuating frequency-150
Hz
150 Hz (bending resonance) +
+ 270 Hz (twisting resonance)
+
’
’
0.2
’
0.4
Longitudinal
’
1 0.6
0.6
position
1.0
I-
_)
40
60
(I$
60
Actuating
100
force
120
140
(kgf)
(b) Characteristics of B type coil actuated by twisting force
$;;I
I
2 g g
4
...;.&
‘I
50
0.04 0.03 0.02 0.01
90
110
Actuating Figure 6
E o : -0.01 s -0.02 .; -0.03 -0.04 -0.05 1 ! 0
,
, 0.2
,
, 0.4
,
Longitudinal
) 0.6
+ Configuration
,
, 0.6
position
q Displacement
Figure 4
70
I
Increment
of heat load
130
150
(kgf)
versus actuating
force
,
1.o
(m)
of upper side
Displacement of lower side of vibration
force
Twisting force 4.9 kN (500 kgf) 6 type coil (de-energized state)
mode
i
6
10 Frequency
Figure7
Increment
14
16
(Hz)
of heat load at low frequency
Persistent current switch
Temperature
Figure 5
290
Structure
Cryogenics
of superconducting
1997 Volume
of coil
We utilized some sensors (carbon glass resistor) for the purpose of locating the heating spot of the mechanically vibrated superconducting coil and investigated the variation of the temperatures under vibration. Figure 8 shows the experimental result of the variation of the temperature on the de-energized coil of type B which is vibrated by a torsional actuating force (150 kgf = 1.47 kN) having various frequencies. The indication of these sensors reveals that the temperature rises around the upper edge of this racetrackshaped coil, especially at 320 Hz. Figure 9 shows the temperature rise on the de-energized coil when the actuating force increases at 300 Hz. We note the temperature rise around the upper edge, especially at the resonance of torsional vibration under an actuating force larger than 100 kgf (0.98 kN). We interpret this as that heat generation does not occur there, but the evapor-
coil
37, Number
rise on the surface
6
Heating in a mechanically 2.
B type coil (de-energized state) twisting force 1.47 kN (150 kgf) 5.1 5.0
E8 CR12
4.9
0
CR13
3.
IZi CR15 CGCl
2 2
4.7 4.6
m
CCC2
$
4.5
I
CGC4
z
4.4
F
4.3
3
4.2 4.1 4.8 i 170
320
350
frequency
Amount
(Hz)
Figure8 Temperature on the surface of inner vessel and coil versus actuating frequency. The sensors CRl5, CGCl, CGC2 are attached on the upper side of the coil B type coil (de-energized state)
4.7 -
0 CR12 + CR13
g
4.6-
0
$
4.5-
CR15 ACGCI xCGC21 v CGC4
4.4-
g
4.3 -
,/$
Upper side
/.* L( A
E c
1” -v
4.2 4.1 -
Outer surface--there is no mark on the straight part of the coil but there are horizontally slipping marks in the vicinity of the boundary between the straight part and the arch. Side surface-there are pressed marks on the inner side parts of the arches (ll-18,23-28,42-48,51-56), but no slipping mark on them.
We suppose that the main heat generation occurs at the parts having the frictional marks due to slipping (see points 1 and 2 above).
Actuating
?
vibrated SC coil: E. Suzuki et al.
Actuating frequency-300
Hz
A
50
I
70
90
Actuating
110
130
I
I
150
force (kgf)
Figure9 Temperature on the surface of inner vessel and coil versus actuating force. The sensors CRl5, CGCI, CGC2 are attached on the upper side of the coil
ated gas beyond disposal simply stays in the space of the upper edge and raises the temperature there. In the case of the bending or the torsional deformation under an actuating force smaller than 100 kgf (0.98 kN), we note the increase in the evaporated gas but did not observe a rise in temperature. Under these circumstances, it seems to be difficult to identify the parts generating the heat only by the indication of the temperature sensor.
of relative movement
in slipping
Three superconducting coils for testing have a structure in which the coil is securely tightened through the fasteners of the inner vessel. Therefore both the coil and the fasteners vibrate as one body in the ordinary vibration, but it is supposed that they can move relative to each other at resonance. We set search coils on the inner vessel of a type C coil as shown in Figure I I and investigated their output in order to measure the amount of these relative displacements. When this coil was vibrated by an actuating force of 150 kgf ( 1.47 kN), the amounts of this relative movement between the coil and the inner vessel were calculated to be several microns at resonance through the actuating frequency and the magnetic variation. Also, we assume the deformation of the coil in consideration of the heat generation and the accelerations in the torsional vibrating mode of the type B coil and we obtain the shearing force by differentiating the deformation three times. We have a reasonable value of the slipping movement by the shearing force estimated as several tens of micrometres. The area having a large shearing force corresponds to the part matching a knot of the vibrating mode in which the curvature of the deformation of the coil varies. Assuming this heating phenomenon in mechanical vibration as the frictional slipping caused by the shearing force, then this coincides with the experimental results that as more heat is generated in the torsional mode having many knots of the vibrating deformation than in the bending mode*.
Conclusions Investigation Supposition
of the parts generating
heat
The heat loss is supposed to come from the above-mentioned friction between the coil and the fasteners, i.e. the parts generating heat. To find out where friction occurs on the racetrack-shaped coil, we disassembled the inner vessel of a coil of type C which has been mechanically actuated and examined the interior. Figure 10 represents the frictional marks on the inner, outer and side surfaces of Kapton tapes which surround the coil and are pressed by fasteners. The features characterizing the spots having the frictional marks are as follows:
We found that a large heat generation occurs in the vibrating deformation of the mechanically actuated superconducting ‘coil in addition to the heating phenomenon caused by the eddy current due to the relative displacement of the electrical conductive bodies (i.e. the outer vessel or radiation shield plate of a superconducting magnet) in the strong magnetic field of the superconducting coil. Various experiments have made the following facts clear: 1.
2.
1.
Inner surface-there are marks having strong slip and contact on both arches (14-29, 43-58). Also, the marks indicating horizontal slipping are recognized on the part of every fastener when the coil is vibrated.
The heat generation in a vibrating superconducting coil is larger in the torsional deformation mode than in the bending vibrating mode. The characteristics of heating in the vibrating coil under the energized and de-energized states are almost the same. It is necessary
to be sure about which physical quantity
Cryogenics
1997 Volume
37, Number
6
291
Heating
in a mechanically
vibrated
SC coil: E. Suzuki et al. Surface of outside of tape
Figure 10
Aspect of the surface of Kapton tapes existing between fasteners and coil of type C
Attachment for search coil
/
Search coil
l
ate mechanically the superconducting coil at room temperature, although this needs high accuracy. Actuating the superconducting coil with a spacer made of another material between the coil and the fasteners.
Regretably, we could not find references directly concerning the behaviour of a mechanically vibrated superconducting coil, but there are several papers3” of a related nature which may be of interest. Superconducting
Acknowledgement Figure 11 Fixture of search coil detecting the relative displacement between superconducting coil and inner vessel
is related to this heat loss (e.g. deforming rate or acceleration of a coil); at present we cannot find any standardized estimation. Although we can guess at the parts generating frictional heat on the basis of various experimental results obtained so far, we cannot as yet obtain confirmation. Hence, we intend to make further investigations of the analysis of the heating factor and study further the following: l
l
292
Actuating mechanically the coil while holding a noncontact gap between the coil and the fasteners for the purpose of removing the cause of heat generation. Detecting the pattern of the heating parts when we actu-
Cryogenics
1997 Volume
37, Number
6
This research has been subsidized by the Ministry of Transport of Japan.
References 1.
2.
3.
4.
5.
Saito, S., Suzuki, E. and Nemoto, K., 49th Meeting on Cryogenic and Superconductivity, Tsukuba, Japan, May 1993, preprint paper no. Cl23 (in Japanese). Saito, S., Suzuki, E., Ohmori, J. and Chandratilleke, G.R., 50th Meeting on Cryogenics and Superconductivity, Kagoshima, Japan, November 1993, preprint paper no. D3-22 (in Japanese). Yazawa, T., Urata, M., Chandratilleke, G.R. and Maeda, H., Racetrack coil instability resulting from friction heat generation at fixtures. IEEE Trans. Appl. Supercond, 1993, 3(l), 312-315. Dotsenko, V.I., Kislyak, IF. and Chaykovskaya, N.M., Stability of superconducting composites during external friction. Cryogenics, 1990, 30, 894-899. Knight, G.W. and Evans, D., Evaluation of structural integrity of superconducting magnet coils. Cryogenics, 1991, 31, 292-297.
PII: SOOll-2275(97)00031-3
001 l-2275/97/$17.00
Characteristics of heating in a mechanically vibrated superconducting coil” Eiji Suzukit, Suzu ki7
Sanetoshi
Saitot,
Junji OhmoriS, Masao Oki§ and Fumio
tRailway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji City, Tokyo 185, Japan *Toshiba Corp., 2-4 Suehiro-cho, Tsurumi-Ku, Yokohama City 230, Japan §Mitsubishi Electric Corp., l-l Tsukaguchi-Honmachi 8-chome, Amagasaki City 661, Japan IHitachi Works, Hitachi Ltd, 3-l-l Saiwai-cho, Hitachi City, lbaraki 317, Japan Received 6 August
1996; revised
16 January
1997
It is revealed that much heat is generated not only by the eddy current due to the relative displacement between conducting elements in a magnet, but also by the mechanical vibrating deformation of a superconducting coil [Suzuki, E., Cryog. Eng., 1994, 29, 495-5031. We measured the characteristics of heating in the mechanically vibrating superconducting coil in resonance using an oil servo actuator. As a result of these experiments, the following facts are made clear: (i) the heat generated in a vibrating superconducting coil is larger in the twisting configuration than in the bending one under the vibrating mode; (ii) the characteristics of the increment in heating in the vibrating coil under the energizing and the de-energizing state are almost the same. We cite as a factor in heating phenomena of the mechanical excitation the frictional heat between the fasteners and a superconducting coil. The size of the displacements in these frictional parts are supposed to be of the order of several micrometres. We intend to make further analyses of the heating phenomena. 0 1997 Elsevier Science Ltd. Keywords: heating phenomena; vibrating deformation
superconducting
To estimate the limits of quenching in a mechanically vibrated superconducting coil, we have constructed equipment which is able to cause a bending or torsional vibrating deformation of the coil and performed various experiments. It is revealed that more than negligible heat generation occurs even when the superconducting coil is mechanically vibrated by a purely mechanical actuating force and suffers bending or torsional deformation. This heat generation is regarded as one of the various heating factors when the superconducting magnet on a maglev vehicle vibrates and heats internally under the influence of electromagnetic disturbances from the ground coils. Additionally, as this heating sometimes causes quenching of a coil, it has an important significance. The amount and features of this heat generation are discussed in this paper.
*Originally Japanese)
published
in
Teion
Kogaku,
1994,
29(10)
(in
coil; resonance; friction; mechanical
Experimental
methods
Test facility 1 shows a view of the test facility. A superconducting coil is hung from the liquid helium tank by stainless steel. We have adopted an actuating mechanism which connects the rod at the centre of a racetrack-shaped coil with the oil servo actuator and sustains two arches of a coil with two fixing rods. In addition to the bending deformation of the coil, another vibrating mode of deformation can occur at the corresponding resonance. Furthermore, we have also performed an experiment in which the torsional deformation of a superconducting coil is caused naturally by acting on the two diagonal points of the coil and reacting at the other two diagonal points. The oil servo actuator can generate a reciprocating force with frequencies broadly ranging from quasi-static to about 400 Hz. This equipment, that operates without electrical actuation, has such features that it enables the vibrating coil to be tested in both the energized and the de-energized Figure
Cryogenics
1997 Volume
37, Number
6
287
Heating
in a mechanically
vibrated
SC coil: E. Suzuki et al. /terns to be measured
The items to be measured in these mechanical actuating tests are as follows:
uperconducting coil
0 acceleration of the surface of the inner vessel; strain on the surface of coil and inner vessel; l temperature of the surface of an inner vessel measured by a copper and constantan thermocouple; l magnetic fluctuation of the coil measured by a search coil. l
Oil servo actuator
Superconducting Figure 1 Vibration (twisting) coil
test
facility
for
mechanically
Results of mechanical
Procedures
Amount
We estimated the increase of heat loss under mechanical vibration by measuring the difference in the rate of the helium gas evaporated in the inner vessel between the vibrated and static states of a coil. The following tests have been carried out in the energized and de-energized states of a coil:
of heat loss
0 Actuating force-l
Z”
A type 3-
5 =
2-
0
+
+ Cl 0
E al t 50
9
+
0
0
0
t
1
150
50
250 Frequency
Figure2 ing test
++
Characteristics
r
350
(Hz)
of heating in the mechanically
bend-
coil coil type
B
C
About 1.0 1400 700 kA 500 A 72 mm wide x 72 mm high
About 1.5 1400 700 kA 500 A 70 mm wide x 72 mm high
Method like laying brick
Method of laying in tight array
Method of laying in tight array
Deformed ellipse shape (machine made) About 5 mm
Near rectangular
Rectangular
A Superconducting wire (NbTi fine multi-filamentary wire) About 1.0 (I 1 copper ratio (2) number of turns 1167 700 kA (3) magnetomotive force (4) running current 600 A (5) se;tii;al dimension of 40 mm wide x 72 mm high
Cryogenics
De-energized state
+ Energized state
Superconducting coil
Z S
Superconducting
288
.47 kN (150 kgf)
%
The superconducting coils used in these experiments are similar to those in the magnets having a pole pitch length of 1350 mm to be adopted for our new maglev test line (Yamanashi Test Line) and they form a short racetrack shape which is 500 mm in width and 1070 mm in length. We can exchange one coil for another in the tests.
(2) thickness of main plate
test
in a coil of type A under mechanical vibration having a constant actuating force. The force acting on the supercon-
Obtain the amplitude of acceleration and the rate of helium gas by sweeping the actuating frequencies with the amplitude of actuating force held constant. Measure the increment of heat loss under a steady actuating force at several frequencies including resonance. Investigate the increment of heat loss and the occurrence of quenching under the increase in the actuating force at a particular frequency (at resonance).
(6) configuration of turning Structure of inner vessel (I) sectional shape
vibration
Figure 2 shows the increment of the evaporated helium gas
the evaporated
Structure of superconducting
test
The superconducting coils which are used in these experiments are three short type coils investigated for our new test line. Table 1 gives the main items in these coils. Each coil structure differs in detail depending on its maker.
state. Various tests were done in the energized and de-energized states of a coil and the results were compared.
Table 1
coils for vibration
bending
1997 Volume
37, Number
6
About 3 mm
(press made)
About 3 mm
Heating in a mechanically Actuating force-100 Energized state
vibrated SC coil: E. Suzuki et al.
kgf Superconducting coil--A
Frequency
(Hz)
Frequency
(Hz)
type
Figure 3 Acceleration of eight points on the coil in the mechanically of the accelerometers in Figure 4
bending test. The numbers Q -
coil from the ground coils for levitation is larger and the frequency in running at a speed of 500 km h-’ corresponds to 308 Hz. A large amount of heat loss is generated at 270-280 Hz, as shown in Figure 2. Figure 3 shows the output of the accelerometers fixed on the surface of the superconducting coil. There are two main resonances having large amplitude peaks in the acceleration at several points of this vibrating coil. Peaks appear at 270-280 Hz with large evaporation and at 150-160 Hz. Figure 4 shows the configurations of the vibrating deformation corresponding to each resonance. From this figure we can recognize the vibration modes in these resonances such as the torsional mode and the bending mode. From these experimental results, it is confirmed that an excessive frictional heat generation in the microscopic slipping parts of a coil occurs notably under torsional deformation. Although we cannot observe directly the parts generating heat loss, the heat loss is supposed to come from friction between the coil and the fasteners, as shown in Figure 5. A microscopic slip between the coil and the fasteners moves each relative to the other and generates heat by the resonant deformation due to the electromagnetic
force in high speed running’. Figure 6 shows the increment of heat loss in the coils of type A under bending actuation and type B under torsional actuation when we increase the actuating force. The heat loss increases out of proportion to the amplitude of the actuating force. We suppose that this heating phenomenon is due to friction and as the actuating force increases, not only the relative movement of slip increases, but also the range of slip extends. Figure 7 shows the increment of heat loss under constant torsional force in the coil of type B at very low frequencies. As the inertial force can be neglected, we suppose that the increment of heat loss is proportional to the number of repetitions if this heating is a frictional one due to quasi-static deformation. However, we could not actually get a completely proportional relationship. Furthermore, we could see a strong resemblance between the characteristics of heat generation when the superconducting coil is energized and when it is de-energized. This means that heat generation due to the friction on a boundary surface does not depend upon a hoop force of the energized coil and we cannot as yet ascertain the reason.
ducting
Cryogenics
@ correspond to the positions
1997 Volume
37, Number
6
289
Heating
in a mechanically
vibrated
SC coil: E. Suzuki et al. (a) Characteristics of A type coil actuated by bending force
Energized slate Actuating force-l
.47 kN (150 kgf) n
zE -“.ol -0.02 5 E : 54 6
-0.03 -0.04 -0.05 -0.06 -0.07 -0.06 -0.09 -0.10
L I 0
Actuating frequency-150
Hz
150 Hz (bending resonance) +
+ 270 Hz (twisting resonance)
+
’
’
0.2
’
0.4
Longitudinal
’
1 0.6
0.6
position
1.0
I-
_)
40
60
(I$
60
Actuating
100
force
120
140
(kgf)
(b) Characteristics of B type coil actuated by twisting force
$;;I
I
2 g g
4
...;.&
‘I
50
0.04 0.03 0.02 0.01
90
110
Actuating Figure 6
E o : -0.01 s -0.02 .; -0.03 -0.04 -0.05 1 ! 0
,
, 0.2
,
, 0.4
,
Longitudinal
) 0.6
+ Configuration
,
, 0.6
position
q Displacement
Figure 4
70
I
Increment
of heat load
130
150
(kgf)
versus actuating
force
,
1.o
(m)
of upper side
Displacement of lower side of vibration
force
Twisting force 4.9 kN (500 kgf) 6 type coil (de-energized state)
mode
i
6
10 Frequency
Figure7
Increment
14
16
(Hz)
of heat load at low frequency
Persistent current switch
Temperature
Figure 5
290
Structure
Cryogenics
of superconducting
1997 Volume
of coil
We utilized some sensors (carbon glass resistor) for the purpose of locating the heating spot of the mechanically vibrated superconducting coil and investigated the variation of the temperatures under vibration. Figure 8 shows the experimental result of the variation of the temperature on the de-energized coil of type B which is vibrated by a torsional actuating force (150 kgf = 1.47 kN) having various frequencies. The indication of these sensors reveals that the temperature rises around the upper edge of this racetrackshaped coil, especially at 320 Hz. Figure 9 shows the temperature rise on the de-energized coil when the actuating force increases at 300 Hz. We note the temperature rise around the upper edge, especially at the resonance of torsional vibration under an actuating force larger than 100 kgf (0.98 kN). We interpret this as that heat generation does not occur there, but the evapor-
coil
37, Number
rise on the surface
6
Heating in a mechanically 2.
B type coil (de-energized state) twisting force 1.47 kN (150 kgf) 5.1 5.0
E8 CR12
4.9
0
CR13
3.
IZi CR15 CGCl
2 2
4.7 4.6
m
CCC2
$
4.5
I
CGC4
z
4.4
F
4.3
3
4.2 4.1 4.8 i 170
320
350
frequency
Amount
(Hz)
Figure8 Temperature on the surface of inner vessel and coil versus actuating frequency. The sensors CRl5, CGCl, CGC2 are attached on the upper side of the coil B type coil (de-energized state)
4.7 -
0 CR12 + CR13
g
4.6-
0
$
4.5-
CR15 ACGCI xCGC21 v CGC4
4.4-
g
4.3 -
,/$
Upper side
/.* L( A
E c
1” -v
4.2 4.1 -
Outer surface--there is no mark on the straight part of the coil but there are horizontally slipping marks in the vicinity of the boundary between the straight part and the arch. Side surface-there are pressed marks on the inner side parts of the arches (ll-18,23-28,42-48,51-56), but no slipping mark on them.
We suppose that the main heat generation occurs at the parts having the frictional marks due to slipping (see points 1 and 2 above).
Actuating
?
vibrated SC coil: E. Suzuki et al.
Actuating frequency-300
Hz
A
50
I
70
90
Actuating
110
130
I
I
150
force (kgf)
Figure9 Temperature on the surface of inner vessel and coil versus actuating force. The sensors CRl5, CGCI, CGC2 are attached on the upper side of the coil
ated gas beyond disposal simply stays in the space of the upper edge and raises the temperature there. In the case of the bending or the torsional deformation under an actuating force smaller than 100 kgf (0.98 kN), we note the increase in the evaporated gas but did not observe a rise in temperature. Under these circumstances, it seems to be difficult to identify the parts generating the heat only by the indication of the temperature sensor.
of relative movement
in slipping
Three superconducting coils for testing have a structure in which the coil is securely tightened through the fasteners of the inner vessel. Therefore both the coil and the fasteners vibrate as one body in the ordinary vibration, but it is supposed that they can move relative to each other at resonance. We set search coils on the inner vessel of a type C coil as shown in Figure I I and investigated their output in order to measure the amount of these relative displacements. When this coil was vibrated by an actuating force of 150 kgf ( 1.47 kN), the amounts of this relative movement between the coil and the inner vessel were calculated to be several microns at resonance through the actuating frequency and the magnetic variation. Also, we assume the deformation of the coil in consideration of the heat generation and the accelerations in the torsional vibrating mode of the type B coil and we obtain the shearing force by differentiating the deformation three times. We have a reasonable value of the slipping movement by the shearing force estimated as several tens of micrometres. The area having a large shearing force corresponds to the part matching a knot of the vibrating mode in which the curvature of the deformation of the coil varies. Assuming this heating phenomenon in mechanical vibration as the frictional slipping caused by the shearing force, then this coincides with the experimental results that as more heat is generated in the torsional mode having many knots of the vibrating deformation than in the bending mode*.
Conclusions Investigation Supposition
of the parts generating
heat
The heat loss is supposed to come from the above-mentioned friction between the coil and the fasteners, i.e. the parts generating heat. To find out where friction occurs on the racetrack-shaped coil, we disassembled the inner vessel of a coil of type C which has been mechanically actuated and examined the interior. Figure 10 represents the frictional marks on the inner, outer and side surfaces of Kapton tapes which surround the coil and are pressed by fasteners. The features characterizing the spots having the frictional marks are as follows:
We found that a large heat generation occurs in the vibrating deformation of the mechanically actuated superconducting ‘coil in addition to the heating phenomenon caused by the eddy current due to the relative displacement of the electrical conductive bodies (i.e. the outer vessel or radiation shield plate of a superconducting magnet) in the strong magnetic field of the superconducting coil. Various experiments have made the following facts clear: 1.
2.
1.
Inner surface-there are marks having strong slip and contact on both arches (14-29, 43-58). Also, the marks indicating horizontal slipping are recognized on the part of every fastener when the coil is vibrated.
The heat generation in a vibrating superconducting coil is larger in the torsional deformation mode than in the bending vibrating mode. The characteristics of heating in the vibrating coil under the energized and de-energized states are almost the same. It is necessary
to be sure about which physical quantity
Cryogenics
1997 Volume
37, Number
6
291
Heating
in a mechanically
vibrated
SC coil: E. Suzuki et al. Surface of outside of tape
Figure 10
Aspect of the surface of Kapton tapes existing between fasteners and coil of type C
Attachment for search coil
/
Search coil
l
ate mechanically the superconducting coil at room temperature, although this needs high accuracy. Actuating the superconducting coil with a spacer made of another material between the coil and the fasteners.
Regretably, we could not find references directly concerning the behaviour of a mechanically vibrated superconducting coil, but there are several papers3” of a related nature which may be of interest. Superconducting
Acknowledgement Figure 11 Fixture of search coil detecting the relative displacement between superconducting coil and inner vessel
is related to this heat loss (e.g. deforming rate or acceleration of a coil); at present we cannot find any standardized estimation. Although we can guess at the parts generating frictional heat on the basis of various experimental results obtained so far, we cannot as yet obtain confirmation. Hence, we intend to make further investigations of the analysis of the heating factor and study further the following: l
l
292
Actuating mechanically the coil while holding a noncontact gap between the coil and the fasteners for the purpose of removing the cause of heat generation. Detecting the pattern of the heating parts when we actu-
Cryogenics
1997 Volume
37, Number
6
This research has been subsidized by the Ministry of Transport of Japan.
References 1.
2.
3.
4.
5.
Saito, S., Suzuki, E. and Nemoto, K., 49th Meeting on Cryogenic and Superconductivity, Tsukuba, Japan, May 1993, preprint paper no. Cl23 (in Japanese). Saito, S., Suzuki, E., Ohmori, J. and Chandratilleke, G.R., 50th Meeting on Cryogenics and Superconductivity, Kagoshima, Japan, November 1993, preprint paper no. D3-22 (in Japanese). Yazawa, T., Urata, M., Chandratilleke, G.R. and Maeda, H., Racetrack coil instability resulting from friction heat generation at fixtures. IEEE Trans. Appl. Supercond, 1993, 3(l), 312-315. Dotsenko, V.I., Kislyak, IF. and Chaykovskaya, N.M., Stability of superconducting composites during external friction. Cryogenics, 1990, 30, 894-899. Knight, G.W. and Evans, D., Evaluation of structural integrity of superconducting magnet coils. Cryogenics, 1991, 31, 292-297.