Methods for charging and using pulsed-power sources

Methods and systems provide pulsed-power to a load utilizing high temperature superconductors (HTS) within multiple pulsed-power devices. According to embodiments described herein, each pulsed-power device includes a HTS mounted on a rotor and an armature coil mounted on a stator. The rotor is positioned to allow a magnetic field within the HTS to induce a voltage in the armature coil when the rotor is rotating and to allow a magnetic field created by passing current through the armature coil to charge the HTS. Current created from the operation of a first pulsed-power device is routed to the armature coil in a second pulsed-power device to charge the associated HTS to a higher value. Subsequently, the second pulsed-power device is operated to produce current that is used to further charge the HTS in the first pulsed-power device. This bootstrapping procedure is repeated until all HTSs are fully charged.

TECHNICAL FIELD

The present disclosure relates generally to pulsed-power systems, and more particularly to the use of high-temperature superconductors within pulsed-power systems.

BACKGROUND

Pulsed-power systems are used to provide stored energy over short intervals in an effort to deliver peak power to a specified load. Pulsed-power systems may be implemented using compensated pulsed alternators (“compulsators”). Compulsators are commonly radial-flux alternators having rotating field windings that are energized by brushed contacts. The rotating magnetic field from the field windings induces a pulsed voltage in stationary armature windings, which in turn deliver a pulsed current to the load. The power of these types of rotating pulsed-power supplies is proportional to the magnetic field that the field windings exert on the armature windings. Heating and other mechanical constraints typically limit the magnetic field supplied by the rotating field windings to about 3 Tesla on the armature windings. This magnetic field limitation prevents conventional compulsators from being used to produce motors and generators having high specific power.

SUMMARY

Methods and systems described herein provide for the use of high-temperature superconductors (HTS) within a pulsed-power source to significantly increase the specific power of the pulsed-power source as compared to conventional pulsed-power systems. The improved specific power allows for devices using pulsed-power systems to appreciate increased power densities, ultimately resulting in decreasing the size of the pulsed-power systems and the devices that use them. To allow for the use of HTSs within the pulsed-power systems without requiring large external equipment for creating the magnetic fields to be trapped within the HTSs, the embodiments described herein additionally provide for bootstrap charging the HTSs using multiple pulsed-power devices.

According to embodiments described herein, a method for charging a pulsed-power system includes providing an initial magnetic field, or charge, to a trapped-flux, high-temperature superconductor (HTS) that is mounted on a rotor within a pulsed-power device. The rotor is spun up to operational speed, which induces a voltage in an associated armature coil to create a pulsed current. The pulsed current is routed to an armature coil of a second pulsed-power device to induce a magnetic field around the armature coil. A HTS mounted on the rotor of the second pulsed-power device is subjected to the magnetic field, and in turn, receives a partial charge. The second pulsed-power device is then spun up to induce a voltage in the corresponding armature coil that creates a pulsed current. This pulsed current is stronger than that produced by the first pulsed-power device due to the stronger magnetic field trapped within the second HTS as compared to that in the first HTS. The pulsed current is routed from the second pulsed-power device to the armature coil in the first pulsed-power device and the first HTS is introduced to the associated magnetic field to increase the charge within the first HTS.

According to further embodiments, a system for providing pulsed current to a load includes a number of linked pulsed-power devices and controlling circuitry. Each pulsed-power device includes a rotor with a mounted HTS and a stator with a mounted armature coil. The power-control circuitry allows each HTS to be charged by routing current from the armature coil of each pulsed-power device to the armature coil of at least one other pulsed-power device to create a magnetic field that is trapped by a corresponding HTS. The power-control circuitry additionally provides pulsed current created while exposing the armature coil of each pulsed-power device to a rotating magnetic field from each HTS during operation of the pulsed-power devices to the load.

Other embodiments provide a method for providing pulsed current to a load. A rotor of a first pulsed-power device is positioned to align a HTS mounted on the rotor with an armature coil mounted on a stator. Current is received in the armature coil to induce a magnetic field that is trapped within the HTS. A rotor of a second pulsed-power device is similarly positioned to align a HTS on the rotor with an armature coil on a stator. The rotor of the first pulsed-power device is rotated to induce a voltage within the corresponding stator from the magnetic field trapped within the HTS. The resulting pulsed current is routed to the armature coil of the second pulsed-power device to induce a magnetic field that charges or further charges the HTS of that device. The rotors with the HTSs of both devices are rotated to induce a voltage in the corresponding armature coils that is used to create a strong pulsed current that is delivered to the load.

DETAILED DESCRIPTION

The following detailed description is directed to methods and systems for utilizing high temperature superconductors to provide pulsed-power to a load. As discussed briefly above, magnetic fields exerted on armature windings by the field windings of a typical compulsator are not typically strong enough to produce motors with the high specific power required for many applications. A solution according to the embodiments provided herein includes the use of HTSs in the place of traditional brushed field windings within a compulsator. The magnetic fields trapped in a superconducting single grain monolith, for example, could be several times higher than what is possible with permanent magnets or electromagnets. Because the specific power of a motor or generator is a linear function of a magnetic field, the use of HTSs within a compulsator as described in the embodiments below can significantly increase the power density within motors and generators that utilize compulsators. As will become clear from the disclosure below, the use of HTSs creates a brushless system that exerts very high magnetic fields on armature windings to produce pulsed-power devices with power densities significantly higher than that of contemporary devices.

However, to utilize HTSs in a compulsator to create a pulsed-power device with high specific power, the magnetic field required to create the trapped flux state within the HTS must be higher than the trapped field. Typically, a discharge from a substantial capacitor into windings surrounding a HTS would be used to initially charge the HTS. In order for the pulsed-power system to be utilized in a vehicle, weapon, or other environment having space, size, and/or weight limitations, the pulsed-power system cannot utilize the relatively large external current sources typically required to fully charge a HTS.

Embodiments of the disclosure provided below replace field windings within two linked compulsators with HTSs to create a pulsed-power system. The various embodiments describe bootstrap charging the HTSs within one compulsator with the output current from the other compulsator. Because applications of pulsed-power systems within aerospace, ground, or sea mobile platforms often utilize counter-rotating pairs of compulsators so that the platform movement is not affected, these types of systems can be modified with HTSs according to the embodiments provided herein to enable pulsed-power systems in mobile platforms with significantly higher specific power than typical mobile systems.

In the following detailed description, references are made to the accompanying drawings that form a part hereof, and which are shown by way of illustration, specific embodiments, or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, aspects of a pulsed-power system will be described.FIG. 1shows a pulsed-power system100according to embodiments described herein. The pulsed-power system100includes a first pulsed-power device102and a second pulsed-power device102. It should be appreciated that more than two pulsed-power devices102may also be used within the pulsed-power system100. As will become clear from the description of various embodiments below, each pulsed-power device102operates as a compulsator with the field windings replaced by bulk HTSs. The two pulsed-power devices102are electrically linked such that the current output from one pulsed-power device102may be directed through the armature windings of the other pulsed-power device102during the bootstrap charging process described below. After the charging process is complete, the two pulsed-power devices102together may be used to provide pulsed current to the load104. Known power control circuitry112is used to provide the current in pulses.

The pulsed-power system100additionally includes a direct current (DC) current source106, motor108, and cooling system110. The DC current source106may be a small capacitor, battery, flywheel, superconducting inductor, or other current source that is used in the manner described below to provide the initial charge to the first pulsed-power device102and/or the second pulsed-power device102. The DC current source106may be connected to the first pulsed-power device102, the second pulsed-power device102, or one or more DC current sources106may be connected to both the first and second pulsed-power devices102.

The motor108, if used within the pulsed-power system100, may be mechanically attached to the rotor of the first and/or second pulsed-power device102to spin the rotor up to the operational rotational speed. It should be appreciated that the power requirements for the motor will be modest since the average period between pulses is much longer than the pulse length. According to an alternative embodiment, the motor108is eliminated from the pulsed-power system100. In this embodiment, current is sent through the armature windings of the pulsed-power devices102to spin the corresponding rotor. In doing so, the pulsed-power device102works as a brushless synchronous motor while spinning up to speed.

In order to trap magnetic flux, a HTS must be at a temperature below its critical temperature. Depending upon the type of HTS, this critical temperature varies. As an example, a bulk HTS consisting of single-grain Y—Ba—Cu—O (YBCO), and its rare-earth analogs, the critical temperature is of the order of 90 K. According to various embodiments provided herein, there are several methods for cooling the HTSs. The cooling system110depicted inFIG. 1represents the cooling components required by any one of these methods. A first cooling technique is to cool the rotor while it is stationary, using thermal conduction between the rotor and a cold head. The heat capacity of the rotor will keep the rotor sufficiently cold for the lifetime of its mission. This cooling method is useful for very high-speed operation when the rotor must spin in vacuum.

A second cooling method includes cooling the stator and letting heat from the rotor radiate to the stator. One limitation to this method is that radiation heat transfer is low at low temperatures. A third cooling method includes allowing gas to flow in the rotor interior and/or in the gap between the rotor and stator. This gas is then cooled using a cryocooler. It should be appreciated that the cooling system110may service all or multiple pulsed-power devices102in the pulsed-power system100or each pulsed-power device102may have a dedicated cooling system110. In addition to cooling the pulsed-power devices102, the cooling system110may be used to cool the armature windings in order to reduce their resistance and increase the current capacity in them. It should be understood that when charging the HTSs within the pulsed-power devices102, the precise procedure for cooling while applying a magnetic field may be chosen according to the desired properties of the HTS and the desired trapped magnetic field within. For example, known techniques such as field cooling, adiabatic zero-field cooling, and/or pulsed zero-field cooling may be utilized to charge the HTSs within the pulsed-power devices102described herein.

According to various embodiments described herein, there are two major topologies for the pulsed-power devices102, radial and axial. In addition, there are various possible configurations within each topology. Turning now toFIG. 2, a radial-flux, internal rotor pulsed-power device102A will be described. The radial-flux, internal rotor pulsed-power device102A includes an internal rotor assembly202that rotates within an external stator assembly204. The internal rotor assembly202is shown separately inFIGS. 3A and 3B, while alternative embodiments of the external stator assembly204are shown inFIGS. 4 and 5.

The internal rotor assembly202includes a shaft206connected to an internal rotor208. A number of bulk HTSs210are secured to the exterior surface of the internal rotor208. Banding material212wraps around the outer surface of the HTSs210to provide a mechanical restraint against the centrifugal forces when the internal rotor208is rotating at high speeds. The radial-flux, internal rotor pulsed-power device102A is shown in a configuration that includes four axial rows of HTSs210evenly spaced around the circumference of the internal rotor208, with three HTSs210in each row. Bulk HTSs210in each row are magnetized radially, with bulk HTSs210in the same row magnetized in the same direction. It should be understood that any number of HTSs210may be utilized and positioned at any location around the exterior surface of the internal rotor208depending on the desired characteristics of the radial-flux, internal rotor pulsed-power device102A and the type of HTS210used.

Because three HTSs210are shown in each row around the circumference of the internal rotor208, three rings of banding material212are shown, one for the first, second, and third HTSs210in each row, respectively, as best seen inFIGS. 3A and 3B. The banding material may be a metal, a fiber composite material, or any other suitable material. The HTSs210may consist of any type of superconductor materials and may be any size without departing from the scope of this disclosure. The choice of HTS210will be made according to the application for which the pulsed-power device102will be used. According to one embodiment, the HTSs210are approximately 2.6 cm diameter YBCO disks internally impregnated with Bi—Pb—Sn—Cd alloy. A trapped magnetic field generally between 9 T and 17 T may be possible between generally 46 K and 29 K for HTSs210of this type.

Looking now atFIGS. 2,4, and5, aspects of the external stator assembly204will be described according to various embodiments presented herein. The external stator assembly204includes an external stator214and armature windings, or armature coils216. The armature coils216consist of axially oriented cables made of a large number of parallel strands of fine-filament Litz wire to minimize eddy currents. Alternatively, the armature coils216may consist of superconducting wires. It should be understood that any type of material commonly used for armature coils216may be used without departing from the scope of this disclosure.

Each armature coil216corresponds to a row of HTSs210and may include an end turn at one end of the external stator214as shown inFIGS. 2 and 4. Alternatively, the external stator214may include two armature coils216corresponding with each row of HTSs210, each armature coil216entering and leaving at one end of the pulsed-power device102, with the end turns of both armature coils216located in the middle of the external stator214, as shown inFIG. 5. With this embodiment, more than one pulsed-power device102may be utilized to simultaneously bootstrap charge a row of HTSs210on the pulsed-power device102. Additionally, this embodiment simultaneously induces voltage in two sets of armature coils216for each row of HTSs210, a feature that can be useful depending on the application of the radial-flux, internal rotor pulsed-power device102A.

In either embodiment, the external stator214provides a mechanical constraint for the armature coils216against the forces applied to them from the HTSs210. As seen inFIG. 2, the radial-flux, internal rotor pulsed-power device102A configuration provides a gap between the HTSs210and the armature coils216that is interrupted by the banding material212. The properties of the banding material212and the corresponding HTSs210and armature coils216must be taken into account when establishing the gap distance. Moreover, because the internal rotor208may tend to expand at high rotational velocities, the external stator assembly204, and/or the armature coils216may be designed to move slightly radially outward as the rotational velocity increases in an effort to maintain a constant gap width, if desired.

FIG. 6shows a pulsed-power device102configuration in which the banding material212is not positioned within the gap between the HTSs210and the armature coils216. According to this embodiment, a radial-flux, external rotor pulsed-power device102B is configured with an external rotor assembly602and an internal stator assembly604.FIG. 7illustrates an exploded view of the radial-flux, external rotor pulsed-power device102B whereby the internal stator assembly604is removed from the external rotor assembly602. According to this embodiment, the external rotor assembly602includes an external rotor608that rotates around the stationary internal stator assembly604.

The HTSs210are mounted on the inside surface of the external rotor608in four rows of three HTSs210each, similar to the configuration of HTSs210on the exterior of the internal rotor assembly202discussed above. Because the HTSs210are mounted on the inside surface of the external rotor608, the rotor provides the required mechanical constraint to hold the HTSs210in place while the external rotor608is spinning and further banding material212is not necessary. Without the banding material212, the radial-flux, external rotor pulsed-power device102B may allow for a closer gap distance between the armature coils216and the rotating HTSs210.

The internal stator assembly604includes an internal stator614and the armature coils216mounted on an outside surface of the internal stator614. As with the radial-flux, internal rotor pulsed-power device102A discussed above, there are four armature coils216, one for each row of HTSs210; however, an alternative embodiment similar to that shown inFIG. 5may allow for two armature coils216for each row of HTSs210. According to one embodiment, additional material616is added to the exterior of the external rotor608to increase the flywheel energy storage capabilities of the radial-flux, external rotor pulsed-power device102B. It should be understood that the external rotor608, the radial-flux, external rotor pulsed-power device102B may be used not only to supply pulsed power but also to store the required energy for multiple pulses as may be desired in various commercial and military applications.

FIGS. 8-10illustrate an alternative embodiment of the pulsed-power device102in which the rotors and stators are configured axially rather than radially. The axial-flux pulsed-power device102C shown inFIG. 8includes two sets of rotor assemblies802and stator assemblies804configured as disks. However, any number of rotor assemblies802and stator assemblies804may be utilized depending on the operational parameters of the axial-flux pulsed-power device102C. As seen inFIG. 9, each rotor assembly802includes a rotor disk808attached at the center to the shaft206. Four HTSs210are mounted on the rotor disk808at evenly spaced intervals and magnetized axially. Although not shown inFIG. 8, various embodiments of the axial-flux pulsed-power device102C may include banding material212to secure the HTSs210against the centrifugal forces present during operation.FIG. 9shows the banding material212. If any banding material212is necessary to secure the HTSs210against the centrifugal forces during operation, the banding material would not interfere with the gap between the HTSs210and the armature coils216due to the axial configuration of the axial-flux pulsed-power device102C.

FIG. 10shows an example of a stator assembly804. Each stator assembly804includes a stator disk814and an armature coil216for each HTS210mounted on the rotor disk808. Each armature coil216enters and leaves the adjacent HTS210region radially with a circumferential end turn proximate to the center of the stator disk814. During operation, the shaft206rotates the rotor disks808. When rotating, the trapped magnetic fields within the HTSs210induce a voltage in the armature coils216to produce a pulsed current output. It should be appreciated that the HTSs210and the armature coils216may be mounted on either side of the rotor disks808and stator disks814, respectively, such that the rotor disks808are either inside or outside the stator disks814.

The operation of the pulsed-power devices102is similar across all three configurations described above: the radial-flux, internal rotor pulsed-power device102A; the radial-flux, external rotor pulsed-power device102B; and the axial-flux pulsed-power device102C. According to all three embodiments, two pulsed-power devices102are electrically linked. The rotors of the pulsed-power devices102rotate such that the bulk HTSs210, which contain trapped magnetic fields much higher than those available with conventional compulsators that utilize field windings or even permanent magnets, pass in close proximity to the armature coils216on the adjacent stators. In doing so, the magnetic field from each HTS210induces a voltage in the armature coils216. The power control circuitry112allows the resulting current from the pair of pulsed-power devices102to flow from the devices in pulses.

The pulsed-power devices102described above are utilized in tandem to charge the HTSs210within the pulsed-power devices102by providing the required trapped magnetic fields within the HTSs210. Because the current required to create a magnetic field within a HTS210is less than the resultant pulsed current output from the pulsed-power device102when the rotors are rotating at operational speed, the two pulsed-power devices102can be used to bootstrap charge each other. A small charge is applied to the first pulsed-power device102to partially charge the HTSs210to a level corresponding to the small charge. The pulsed current from the first pulsed-power device102is then routed to the second pulsed-power device102to charge the corresponding HTSs210to a level higher than that of the HTSs in the first pulsed-power device102. The procedure is then repeated from the second pulsed-power device102to the first and so forth until the HTSs210of both pulsed-power devices102are fully charged.

Turning now toFIGS. 11A and 11B, an illustrative routine1100for bootstrap charging two pulsed-power devices102will now be described in detail. The routine1100will be described with respect to the pulsed-power system100shown inFIG. 1. The routine1100begins at operation1102, where the HTSs210of both pulsed-power devices102are cooled to a temperature below the critical temperature associated with the HTSs210. It should be appreciated that the HTSs210of the first pulsed-power device102may be cooled first prior to receiving an initial charge, followed by cooling the HTSs210of the second pulsed-power device102at a subsequent time when they are to receive the initial charge.

The HTSs should be cooled to their corresponding operating temperatures to maximize the magnetic field that can be trapped according to the operational parameters of the pulsed-power device102. As discussed above, the cooling system110used to cool the HTSs210may utilize any one or more of several cooling techniques, including using thermal conduction between the rotor and a cold head, cooling the stator and relying on radiation heat transfer from the rotor to the stator, and utilizing a cryocooler to cool gas within the rotor or in the gap between the rotor and stator.

From operation1102, the routine1100continues to operation1104, where the first rotor is locked into a position that allows the magnetic field from the armature coils216to be centered on the HTSs210or otherwise applied to provide a maximum magnetizing effect. The routine1100continues to operation1106, where a current from the DC current source106is provided to the armature coils216. This should occur when the HTSs210are at operational temperature or as the temperature is lowered through the critical temperature to the operational temperature. When the HTSs210are at operational temperature, the current from the DC current source106is removed at operation1108.

From operation1108, the routine1100continues to operation1110, where this initial charging procedure is optionally performed for the second pulsed-power device102. Doing so is not necessary, as the first pulsed-power device102will be used to charge the second pulsed-power device102. However, providing an initial charge to the second pulsed-power device102using the DC current source106while the first pulsed-power device102is being initially charged, then the first pulsed current sent from the first pulsed-power device102will increase the initial charge in the second pulsed-power device102, potentially speeding the overall process.

The routine1100continues from operation1110to operation1112, where the second rotor, which is the rotor in the second pulsed-power device102, is locked into place for charging as described above with respect to the first pulsed-power device102. It should be noted that stopping the rotor of one of the pulsed-power devices102to align the armature coils216with the HTSs210may not be desired. According to an alternative embodiment, the pulsed current that is sent through the armature coils216may be timed according to the speed of the rotor to charge the HTSs210at the moments in which they are aligned with the armature coils216. From operation1112, the routine1100continues to operation1114, where the first pulsed-power device102is spun up to operational speed in order to provide the pulsed current to the second pulsed-power device102for charging purposes. Spinning the first pulsed-power device102up to speed can be accomplished using the motor108. Alternatively, current is sent through the armature coils216in order to spin the rotor. In this embodiment, the first pulsed-power device102works as a brushless synchronous motor while spinning up to speed.

The routine1100continues to operation1116, where one or more current pulses from at least the first pulsed-power device102are routed to the armature coils216of the second pulsed-power device102. This pulsed current provides an initial charge to the HTSs210in the second pulsed-power device102that is greater than that in the HTSs210of the first pulsed-power device102. If an initial charge was provided to the second pulsed-power device102from the DC current source106, then this pulsed current from the first pulsed-power device102increases that initial charge. As stated above, these current pulses may be timed such that stopping the rotor in the second pulsed-power device102is not necessary.

It should be understood that the pulsed power system100may include more than two pulsed-power devices102. In embodiments having more than two pulsed-power devices102, each pulsed-power device102may be charged using two or more pulsed-power devices102. Pulses from more than one pulsed-power device102may be combined in parallel and pass through the armature coils216of the pulsed-power device102being charged. The maximum current possible through the armature coils216of the pulsed-power device102being charged would be limited according to the temperature management of the armature coils216.

From operation1116, the routine1100continues to operation1118, where the rotor of the first pulsed-power device102is locked into position to accept a charge. At operation1120, the second pulsed-power device102is spun up to speed for delivering a pulsed current to the first pulsed-power device102. From operation1120, the routine1100continues to operation1122, where current pulses from the second pulsed-power device102are routed through the armature coils216of the first pulsed-power device102to further charge the HTSs210of the first pulsed-power device102. The routine1100continues from operation1122to operation1124, where a determination is made as to whether the HTSs210in both pulsed-power devices102are fully charged. If the HTSs210are fully charged, then the routine1100ends. However, if the HTSs210are not fully charged, then the routine1100returns to operation1112and the bootstrap charging continues as described above.

By utilizing the embodiments described above, electric motors and generators with high specific power may be created for use in weapons systems, launch systems, and any number and type of commercial and military aircraft systems. The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.