Abstract:
Provided are a superconducting magnet apparatus with a switch that automatically connects or disconnects an external power source to a superconducting coil, and a method of controlling the same. The superconducting magnet apparatus includes a superconducting coil that generates a magnetic field when an electric current from an external power source is applied thereto, and a switch that supplies or shuts off an electric current output from the external power source by connecting or disconnecting the superconducting coil to the external power source.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Applications No. 2011-0103792, filed on Oct. 11, 2011 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Apparatuses and methods consistent with exemplary embodiments relate to a superconducting magnet apparatus for generating a magnetic field by receiving an electric current from an external power source, and a control method thereof. 
     2. Description of the Related Art 
     With the development in a coil manufacturing technology using a superconducting magnet as well as the advance of relevant devices, such as an insulating container and a refrigerating device, a superconducting magnet apparatus and applications thereof have been developed. The superconducting magnet apparatus includes a superconducting magnet for a superconducting magnet apparatus or a superconducting magnet for a self levitation vehicle. The superconducting magnet apparatus becomes a persistent current state by receiving an electric current from an external power source through a coil that is cooled to a very low temperature. If the superconducting magnet apparatus has become a persistent current state, the output of the external power source is stopped and the superconducting magnet apparatus is driven in a state of being disconnected to the external power source. 
     The superconducting magnet apparatus requires a current lead when supplying coils with the electric current. The current lead represents a path connecting from a terminal connected to the external power source to a coil existing inside the superconducting magnet apparatus. The current lead is a thermal invasion path along from an ambient temperature terminal, which connects to the external power source, to a very low temperature coil. In a no current state, the current lead becomes an electric heating material. In order to minimize the refrigeration cost of a coil in a superconducting magnet for a superconducting magnet apparatus, the thermal invasion needs to be as small as possible. As a method of reducing the thermal invasion into the superconducting magnet, for a superconducting magnet apparatus operating in a persistent current mode, a demountable current lead is used such that the demountable current lead is separated when a current does not flow, thereby reducing the amount of thermal invasion. However, such a structure of connecting or disconnecting a current lead is handled only by a specialist, and also causes a great workload in a case that an electric current needs to be supplied as an occasion demands. 
     SUMMARY 
     One or more exemplary embodiments provide a superconducting magnet apparatus provided with a switch that is configured to automatically connect or disconnect a superconducting coil with respect to an external power source, and a control method thereof. 
     In accordance with an aspect of an exemplary embodiment, there is provided a superconducting magnet apparatus including a superconducting coil and a switch. The superconducting coil generates a magnetic field by receiving an electric current from an external power source. The switch may selectively connect the external power source to the superconducting coil. 
     The switch may include a bellows-type switch which is set to an on state and an off state by expansion and contraction of a bellows. 
     The bellows-type switch may include the bellows which expands or contracts according to an internal pressure, at least one switch which is switched according to the expansion or contraction of the bellows, a gas tank that supplies the bellows with gas, a gas supply pipe which provides a path for the gas supplied from the gas tank to the bellows, and a gas vent pipe which provides a path for the gas discharged from the bellows. 
     The at least one switch may include a first terminal electrically connected to the external power source and a second terminal electrically connected to the superconducting coil. 
     If the bellows is expanded, the first terminal electrically connected to the external power source may be connected to the second terminal electrically connected to the superconducting coil 
     If the bellows is contracted, the first terminal electrically connected to the external power source may be disconnected from the second terminal electrically connected to the superconducting coil. 
     The bellows-type switch may include the bellows that expands or contracts according to an internal pressure, at least one switch that is switched according to the expansion or contraction of the bellows, a gas transfer pipe which supplies an inner side of the bellows with gas or discharge gas from the bellows, a gas tank which stores the gas that is supplied to the bellows or discharged from the bellows, a heater which heats the gas tank and a heat sink which connects to the bellows to dissipate heat. 
     The heater may be turned on to increase a temperature of the stored gas in the gas tank thereby supplying the gas stored in the gas tank to the bellows, and the heater may be turned off to decrease the temperature of the stored gas in the gas tank thereby discharging the gas in the bellows to the gas tank. 
     The switch may include a shape memory alloy (SMA)-type switch which is set to an on state and an off state according to a temperature. 
     The SMA-type switch may include a first connection terminal which connects to the external power, a second connection terminal which connects to the superconducting coil, a shape memory alloy member which couples to one of the first connection terminal and the second connection terminal, and a heater configured to apply heat to the shape memory alloy member. 
     The shape memory alloy member may remember shapes that correspond to different temperatures. 
     The shape memory alloy member may be a two-way shape memory alloy member that remembers shapes that correspond to two temperatures, respectively. 
     If a heat is applied to the shape memory alloy member by the heater, the shape memory alloy member may reach to a predetermined temperature and expands, and if the shape memory alloy expands, the first connection terminal may be connected to the second connection terminal. 
     If a heat is not applied to the shape memory alloy member by the heater, the temperature of the shape memory alloy member may cool to a room temperature or maintain a room temperature and contract, and if the shape memory alloy contracts, the first connection terminal may disconnect from the second connection terminal or remain disconnected from the second connection terminal. 
     In accordance with an aspect of another exemplary embodiment, there is provided a method of controlling a superconducting magnet apparatus. The method includes providing a superconducting coil which generates a magnetic field when an electric current from an external power source is applied to. The superconducting magnet apparatus includes a switch that is configured to selectively connect the external power source to the superconducting coil. The method also includes supplying an electric current to the superconducting magnet apparatus from the external power source by switching on the switch, and shutting off the electric current to the superconducting magnet apparatus from the external power source by switching off the switch. 
     The switch may include a bellows-type switch, an ON/OFF state of which is adjustable by expansion and contraction of a bellows. 
     The switch may include an SMA-type switch, which is set to an on state and an off state adjustable according to a temperature. 
     In accordance with an aspect of another exemplary embodiment, a switch which selectively connects an external power source to a superconducting coil of a superconducting magnet apparatus may include a bellows, a first fixed terminal electrically connected to the external power source, a first movable terminal electrically connected to the super conducting coil, a first support member; and a first elastic member, where the first fixed terminal is fixed to the first support member and the first movable terminal is coupled to the first elastic member and moves according to expansion and contraction of the bellows. 
     The switch may also include a second fixed terminal electrically connected to the external power source, a second movable terminal electrically connected to the super conducting coil, a second support member; and a second elastic member, where the second fixed terminal is fixed to the second support member and the second movable terminal is coupled to the second elastic member and moves according to expansion and contraction of the bellows. 
     If the bellows of the switch expands, the first movable terminal make a contact with the first fixed terminal and the second movable terminal make a contact with the second fixed terminal. On the other hand, if the bellows of the switch contracts, the first movable terminal disconnects from the first fixed terminal by a tension of the first elastic member and the second movable terminal disconnects from the second fixed terminal by a tension of the second elastic member. 
     As described above, the supply or the shutdown of an electric current to a superconducting magnet apparatus is controlled by a switch, thereby reducing the workload. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a view schematically illustrating a superconducting magnet apparatus according to an exemplary embodiment; 
         FIG. 2  is a perspective view illustrating a switch provided in the superconducting magnet apparatus according to the exemplary embodiment; 
         FIG. 3  is an exploded perspective view illustrating the switch provided in the superconducting magnet apparatus according to the exemplary embodiment; 
         FIG. 4  is a cross-sectional view illustrating the switch in an off-state provided in the superconducting magnet apparatus according to the exemplary embodiment; 
         FIG. 5  is a cross-sectional view illustrating the switch in an on-state provided in the superconducting magnet apparatus according to the exemplary embodiment; 
         FIGS. 6 and 7  are views illustrating a concept of operation of the switch provided in the superconducting magnet apparatus according to the exemplary embodiment; 
         FIG. 8  is a view schematically illustrating a superconducting magnet apparatus according to another exemplary embodiment; and 
         FIGS. 9 and 10  are views illustrating a switch provided in the superconducting magnet apparatus according to another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the exemplary embodiments of, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  is a view schematically illustrating a superconducting magnet apparatus according to an exemplary embodiment. 
     A superconducting magnet apparatus includes a superconducting coil  100 , a housing  200 , a switch  300 , a cryogenic refrigerating device  400 , and a gas tank  500 . The superconducting coil  100  operates in a superconducting state while maintaining a cryogenic temperature. The housing  200  is provided in the form of a ring to accommodate the superconducting coil  100 . The switch  300  is disposed at one side of the housing  200  to perform a switching operation to connect or disconnect the superconducting coil  100  with respect to an external power source  350 . The cryogenic refrigerating device  400  is disposed at one side of the housing  200 . The gas tank  500  is configured to supply the switch  300  with gas. Helium (H) in a liquid state is filled in the housing  200  to keep the superconducting coil  100  at a cryogenic temperature. 
     If the superconducting coil  100  generates heat, the helium in a liquid state filled in the housing  200  undergoes a phase transition into a gas state by absorbing heat. The helium in a gas state has a low density relative to the helium in a liquid state, and moves upward by the difference in density. The helium in a gas state is cooled by the cryogenic refrigerating device  400  disposed at one side of the housing  200 , and thus is transformed into a liquid state. In this manner, the superconducting coil  100  disposed in the housing  200  continuously maintains the cryogenic state. 
       FIG. 2  is a perspective view illustrating a switch provided in the superconducting magnet apparatus according to the exemplary embodiment.  FIG. 3  is an exploded perspective view illustrating the switch provided in the superconducting magnet apparatus according to the exemplary embodiment.  FIG. 4  is a view illustrating a switch in an on-state provided in the superconducting magnet apparatus according to the exemplary embodiment.  FIG. 5  is a view illustrating a switch in an off-state provided in the superconducting magnet apparatus according to the exemplary embodiment. 
     Referring to  FIG. 2 , the switch  300  is a bellows-type switch  300 . The bellows-type switch  300  includes a support bracket  301  fixed to the housing  200 , a bellows  302  that expands or contracts according to the internal pressure, a first switch  310  and a second switch  320  that are switched according to the expansion/contraction of the bellows  302 , a gas supply pipe  303  to supply the inside the bellows  302  with gas, and a gas vent pipe  304  to discharge the gas that exists in the bellows  302 . 
     Referring to  FIG. 3 , the first switch  310  provided on the switch  300  includes a fixed terminal  311  fixed to a first support  305  and a first movable terminal  312  that is configured to move according to the expansion or the contraction and is coupled to a first elastic member  306 . 
     The first fixed terminal  311  and the first movable terminal  312  of the first switch  310  are conductors. The first fixed terminal  311  of the first switch  310  is electrically connected to the external power source  350 . The first movable terminal  312  of the first switch  310  is electrically connected to the superconducting coil  100 . Accordingly, in an on-state of the switch  300 , the first movable terminal  312  makes contact with the first fixed terminal  311  to form a current path connecting from the external power source  350  to the superconducting coil  100 . In an off-state of the switch  300 , the first movable terminal  312  does not make contact with the first fixed terminal  311 , and thus shuts off the electric current flowing from the external power source  350  to the superconducting coil  100 . 
     The first movable terminal  312  of the first switch  310  is coupled to the first elastic member  306 . The first elastic member  306  has a tension and tends to return to its original state when the bellows  302  is contracted. If the bellows  302  is expanded, the first movable terminal  312  moves and makes contact with the first fixed terminal  311 . If the bellows  302  is contracted, the first movable terminal  312  returns to its original state by the tension of the first elastic member  306  and then releases the contact with the first fixed terminal  311 . 
     The second switch  320  provided on the switch  300  includes a second fixed terminal  321  fixed to a second support  307  and a second movable terminal  322  that is configured to move according to the expansion or contraction of the bellows  302  and is coupled to a second elastic member  308 . 
     The second fixed terminal  321  and the second movable terminal  322  of the second switch  320  are conductors. The second fixed terminal  321  of the second switch  320  is electrically connected to the external power source  350 . The second movable terminal  322  of the second switch  320  is electrically connected to the superconducting coil  100 . Accordingly, in an on-state of the switch  300 , the second movable terminal  322  makes contact with the second fixed terminal  321  to form a current path connecting from the external power source  350  to the superconducting coil  100 . In an off-state of the switch  300 , the second movable terminal  322  does not make contact with the second fixed terminal  321 , and thus shuts off the electric current flowing from the external power source  350  to the superconducting coil  100 . 
     The second movable terminal  322  of the second switch  320  is coupled to the second elastic member  308 . The second elastic member  308  has a tension and tends to return to its original state when the bellows  302  is contracted. If the bellows  302  is expanded, the second movable terminal  322  moves and makes contact with the second fixed terminal  321 . If the bellows  302  is contracted, the second movable terminal  322  returns to its original state by the tension of the second elastic member  308  and then releases the contact with the second fixed terminal  321 . 
     The first switch  310  and the second switch  320  are simultaneously set on or off according to the expansion or the contraction of the bellows  302 . 
     Referring to  FIG. 4 , the first switch  310  and the second switch  320  are in an off state according to the contraction of the bellows  302 . As shown in a region “A” of  FIG. 4 , a state transition of the first switch  310  and the second switch  320  into an off state represents that the first movable terminal  312  is released from the connection with respect to the first fixed terminal  311  and that the second movable terminal  322  is released from the connection with respect to the second fixed terminal  321 . 
     Referring to  FIG. 5 , the first switch  310  and the second switch  320  are in an on state according to the expansion of the bellows  302 . As shown in a region “B” of  FIG. 5 , a state transition of the first switch  310  and the second switch  320  into an on state represents that the first movable terminal  312  is connected to the first fixed terminal  311  and that the second movable terminal  322  is connected to the second fixed terminal  321 . 
     The first fixed terminal  311  and the second fixed terminal  321  are primarily fixed to the first support  305  and the second support  307 , respectively, and are secondarily fixed to a first fixing member  330  and a second fixing member  340 , respectively, to prevent the first fixed terminal  311  and the second fixed terminal  321  from rotating. In addition, the first fixed member  330  and the second fixed member  340  have guide members  335  and  345  fixed thereto. The guide members  335  and  345  are provided at inner sides of the first fixed member  330  and the second fixed member  340 . The guide members  335  and  345  are provided with guide slots  336  and  346 , respectively, and each provided with a plurality of connecting holes  348  into which a connecting member  347  is inserted. 
     Guide protrusions  313  and  314  are provided at one side of the first movable terminal  312  and one side of the second movable terminal  322 , respectively. The movement of the guide protrusions  313  and  314  are guided along the guide slots  336  and  346  provided in the guide members  335  and  345 , respectively. 
     The bellows  302  is supplied with gas through the gas supply pipe  303 . The gas supply pipe  303  is connected to the gas tank  500  to supply gas. A gas valve  361  is installed inside the gas supply pipe  303 . According to the on/off state of the gas valve  361 , the gas stored in the tank  500  is supplied to the bellows  302  through the gas supply pipe  303  or blocked from being supplied to the bellows  302  through the gas supply pipe  303 . 
     The gas that exists in the bellows  302  is discharged through the gas vent pipe  304 . A gas valve  362  is installed on the gas vent pipe  304 . According to the operation of the gas valve  362 , the gas supplied to the bellows  302  is discharged or blocked from being discharged. Meanwhile, the on/off state of the gas vales  361  and  362  is adjusted according to the operation by an actuator (not shown). 
       FIGS. 6 and 7  are views illustrating a concept of operation of the switch provided in the superconducting magnet apparatus according to the exemplary embodiment. 
     Referring to  FIG. 6 , the first fixed terminal  311  and the second fixed terminal  321  are connected to the external power source  350  while in a fixed state, and the first movable terminal  312  and the second movable terminal  322  are connected to the superconducting coil  100 . If the bellows  302  provided between the first movable terminal  312  and the second movable terminal  322  is expanded, the first movable terminal  312  and the second movable terminal  322  are connected to the first fixed terminal  311  and the second fixed terminal  321 , respectively. In this case, a current path is formed between the external power source  350  and the superconducting coil  100  to transfer an electric current such that the current output from the external power source  350  is supplied to the superconducting coil  100 . 
     Referring to  FIG. 7 , if the bellows  302  provided between the first movable terminal  312  and the second movable terminal  322  is contracted, the connection between the first movable terminal  312  and the first fixed terminal  311  and the connection between the second movable terminal  322  and the second fixed terminal  321  are released. In this case, the current path to transfer an electric current between the external power source  350  and the superconducting coil  100  is blocked, thereby unable to output the electric current from the external power source  350 . 
     According to the above described embodiments, the first fixed terminal  311  and the second fixed terminal  321  are connected to the external power source  350 , and the first movable terminal  312  and the second movable terminal  322  are connected to the superconducting coil  100 . However, according to another exemplary embodiment, the first fixed terminal  311  and the second fixed terminal  321  are connected to the superconducting coil  100 , and the first movable terminal  312  and the second movable terminal  322  are connected to the external power source  350 . 
       FIG. 8  is a view schematically illustrating a superconducting magnet apparatus according to another exemplary embodiment. 
     A structure of supplying the bellows  302  with gas is different from the embodiment illustrated on  FIG. 2 . The embodiment illustrated on  FIG. 2  includes the gas supply pipe  303  and the gas vent pipe  304 . An electronic valve (not shown) is provided on each of the gas supply pipe  303  and the gas vent pipe  304 . According to the on/off of the electronic valve provided on each of the gas supply pipe  303  and the gas vent pipe  304 , a control of supplying gas from the gas tank  500  or a control of discharging gas to the bellows  302  is performed. 
     Referring to  FIG. 8 , the switch  300  includes the gas tank  500 , a gas transfer pipe  510  that is configured to supply the bellows  302  with gas of the gas tank  500  or to discharge the gas of the bellows  302  to the gas tank  500 , a heater  520  to increase the kinetic energy of gas in the gas tank  500  by heating the gas tank  500 , a controller  600  to control the on/off of the heater  520 , a heat sink  700  connected to the gas tank  500  to dissipate heat of the gas tank  500 , and a heat transfer member  800  connecting the gas tank  500  to the heat sink  700  to transfer heat. 
     The controller  600  controls the on/off of the heater  520 . When the bellows  302  is expanded to turn the switch  300  in an on-state, the controller  600  turns on the heater  520 . Upon turning on the heater  520 , heat is transferred to the gas tank  500  so that the kinetic energy of gas is increased by the heat transferred to the gas tank. Upon the increase in the kinetic energy of gas, the gas stored in the gas tank  500  moves to the bellows  302 . Upon the supply of gas to the bellows  302 , the switch  300  is set to the on-state through the above described mechanism illustrated in  FIG. 2 . 
     When the bellows  302  is contracted to turn the switch  300  in an off-state, the controller  600  turns off the heater  520 . Upon turning off the heater  520 , heat of the gas tank  500  is transferred to the heat sink  700  through the heat transfer member  800 , and then dissipated. As the temperature of the gas tank  50  is decreased due to dissipation, the internal gas pressure is lowered. Upon the decrease of the internal gas pressure, the gas stored in the bellows  302  is transferred to the gas tank  500 . In this case, the bellows  302  is contracted, and the switch  300  is set to the off-state through the above described mechanism illustrated in  FIG. 2 . 
       FIGS. 9 and 10  are views illustrating a switch provided in the superconducting magnet apparatus according to another embodiment. 
     The switch  300  is a shape memory alloy type switch  300 , an on/off state of which is adjusted according to the change of temperature. The shape memory alloy type switch  300  includes first connection terminals  901   a  and  901   b  connected to the external power source  350 , second connection terminals  902   a  and  902   b  connected to the superconducting coil  100 , a shape memory alloy member  910  coupled to the first connection terminals  901   a  and  901   b  or the second connection terminals  902   a  and  902   b  and configured to remember a shape, a heater  520  to apply heat to the shape memory alloy member  910 , and a controller  600  to control the on/off of the heater  520 . Meanwhile, the shape memory alloy member  910  has a two-way shape memory effect that remembers both a shape at a low temperature and a shape of a high temperature. 
     When the electric current needs to be transferred to the superconducting coil  100  from the external power source  350 , the controller  600  applies heat to the shape memory alloy member  910  by operating the heater  520 . If the temperature of the shape memory alloy member  910  increases and reaches to a predetermined temperature, the shape memory alloy member  910  expands, and if the temperature of the shape memory alloy member  910  decreases and reaches to a predetermined temperature, the shape memory alloy member  910  contracts. The shape memory alloy member  910  remembers a shape of the shape memory alloy member  910  when the shape alloy member  910  expands, and a shape of the shape memory alloy member  910  when the shape alloy member  910  contracts. Accordingly, the shape memory alloy member  910  is expanded by heat applied by the heater  520 , thereby connecting the first connection terminals  901   a  and  901   b  to the second connection terminals  902   a  and  902   b . As the first connection terminals  901   a  and  901   b  are connected to the second connection terminals  902   a  and  902   b , a closed loop circuit is formed between the external power source  350  and the superconducting coil  100 , thereby able to transfer the electric current between the external power source  350  and the superconducting coil  100 . 
     When the electric current needs to be stopped from being transferred to the superconducting coil  100  from the external power source  350 , the controller  600  prevents heat from being applied to the shape memory alloy member  910  by stopping the operation of the heater  520 . Accordingly, the heat transferred to the heater  520  is blocked, and the temperature of the shape memory alloy member  910  decreases to a predetermined temperature, and thus the shape memory alloy member  910  is contracted, thereby releasing the connection between the first connection terminals  901   a  and  901   b  and the second connection terminals  902   a  and  902   b . If the first connection terminals  901   a  and  901   b  are connected to the second connection terminals  902   a  and  902   b , a closed loop circuit is not formed between the external power source  350  and the superconducting coil  100 , thereby stopping the supply of electric current. 
     Meanwhile, the description of the embodiment illustrated on the  FIGS. 9 and 10  has been made in relation that the shape memory alloy member  910  is coupled to the second connection terminals  902   a  and  902   b . However, according to another exemplary embodiment, the shape memory alloy member  910  may be coupled to the first connection terminals  901   a  and  901   b.    
     While exemplary embodiments have been particularly shown and described above, it would be appreciated by those skilled in the art that various changes may be made therein without departing from the principles and spirit of the present inventive concept as defined by the following claims.