Magnetic ring arrangement for linear electric machines

Abstract

The invention relates to a magnetic ring arrangement for linear electric machines, the magnet carrier of which has flexible tabs formed by slots open on one side. The magnet carrier can be attached to a moving piston without backlash and can be detachably mounted by sliding locking lugs on the tabs over an axial stop bar on the piston until they are in a detent position. This detent position can then be secured by inserting a retaining ring into a radial groove on the inner surface of the magnet carrier.

Description

Application area

[0001] The invention relates to a magnetic ring arrangement for linear electric machines and in particular a detachable connection of the oscillating magnet of a linear electric machine with the piston of a Stirling cryocooler for cooling the rotor coils of superconducting electric motors with high power density. State of the art

[0002] Superconducting electric motors are considered a key technology for the electrification of heavy transportation vehicles such as ships, trains, trucks, and aircraft. A major challenge lies in cooling the superconducting rotor coils. Unlike the stator coils, these cannot be cooled with liquid hydrogen because no rotary coupling is currently available that functions reliably at high speeds and cryogenic temperatures. Previous rotor cooling systems used stationary thermosiphons with a cryogenic refrigerant, sealed by ferrofluid gaskets. To enable higher rotational speeds, rotationally symmetrical Stirling or pulse tube cryocoolers have been mounted directly on the rotor to cool the rotor coils by heat conduction during rotation. However, the suitability of pulse tube cryocoolers for aerospace applications is questionable, as their efficiency depends on their orientation during operation.In contrast, Stirling cryocoolers offer higher efficiency than pulse tube cryocoolers due to their orientation independence, particularly at low temperatures. However, to date, only commercially available Stirling cryocoolers have been used for cooling superconducting rotor coils, and these were never designed to rotate at high speeds on a shaft. Therefore, they cannot meet the requirements regarding weight, cooling capacity, and reliability. Instead, a rotatably mounted free-piston Stirling engine in an alpha configuration with concentrically arranged pistons is proposed. The inner piston is slidably mounted in the central bore of a hollow cylindrical outer piston. The piston motion is generated by interaction with linear electrical machines. The moving part of the linear electrical machines essentially consists of a magnetic ring arrangement.Documents US 7,692,339 B2, US 6,694,730 B2, US 5,642,088 A, and US 2009 / 0001,823 A1 describe the obvious state of the art in this regard, but do not include a method for a detachably mounted magnetic ring assembly that is guided in the same cylinder as the piston. While US 7,692,339 B2 shows a magnetic ring assembly that is flush with the outer diameter of the piston, the inner stator is also mounted to the piston, and its mass amplifies undesirable leverage forces on the piston guide. US 5,642,088 A proposes mounting the magnets using an adhesive bond. However, volatile substances can outgas from the adhesive, which then condense in the regenerator and thus reduce the efficiency of the Stirling engine. US 6 694 730 B2 describes the clamping of magnets between magnet retaining rings joined by thin rods and spot welds.However, the associated heat input can demagnetize the neodymium magnets that are preferably used if their Curie temperature of about 80 °C is exceeded. Object of the invention

[0003] Therefore, there is a need for a magnetic ring assembly that can be detachably mounted without heat input or adhesives and that can be guided in the same cylinder as the piston. Furthermore, the inner stator of the linear electric machine should be mountable separately from the magnetic ring assembly in order to avoid unnecessarily increasing the moving mass. This problem is solved by a magnetic ring assembly with the features of claim 1. Advantageous embodiments and further developments are the subject of the dependent claims. Example description

[0004] The invention is described and explained below with reference to the illustrations. These illustrations depict: Fig. Figure 1 is a sectional view of a rotating free-piston Stirling cryocooler, whose linear electric machines each utilize a magnetic ring arrangement according to the invention. Fig. Figure 2 is a perspective front view (from below) of an outer piston. Fig. Figure 3 is a sectional view of the outer piston in Fig. 2 along line 3 - 3. Fig. Figure 4 is an exploded view of an outer piston assembly. Fig. Figure 5 is a sectional view of a rotating free-piston Stirling cryocooler, whose linear electric machines each utilize an alternative embodiment of the magnetic ring arrangement according to the invention. Fig. Figure 6 is a perspective front view (from below) of an alternative embodiment of the outer piston. Fig. Figure 7 is a sectional view of the alternative embodiment of the outer piston in Fig. 6 along line 7 - 7. Fig. Figure 8 is an exploded view of an alternative embodiment of the outer piston assembly.

[0005] Fig. Figure 1 shows a sectional view of a rotating free-piston Stirling cryocooler in Alpha configuration 200, which utilizes a magnetic ring arrangement according to the invention. The Alpha configuration is characterized in that a piston is arranged in both the compression chamber 242 and the expansion chamber 244, and both act simultaneously as working and displacer pistons. This means that both pistons contribute to the periodic compression and expansion of the working gas. In the compression chamber 242, an outer piston 302 of an outer piston assembly 300 is arranged, which has the form of a hollow cylinder and whose outer surface 304 (see Figure 1) Fig. 2 - 4) is guided in a housing of the outer piston 202. In the expansion chamber 244, an inner piston 402 of an inner piston assembly 400 is arranged, which is guided in the inner surface 306 of the outer piston 302. The compression chamber 242 is connected to the expansion chamber 244 by an ambient heat exchanger 222, a regenerator 226, and a cold heat exchanger 218. According to the known Stirling principle, the heat exchangers 218 and 222 ensure heat absorption and heat release to the environment, respectively. The regenerator 226 increases the efficiency of the process by increasing the temperature difference between the compression chamber 242 and the expansion chamber 244 through the intermediate storage of heat. Furthermore, the inner piston 402 moves concentrically to a cylinder liner 206, which spatially separates the compression chamber 242 from the expansion chamber 244 and directs the gas flow between these chambers through the heat exchangers 222 and 218 and the regenerator 226.A leak at the sealing gap between cylinder liner 206 and inner piston 402 does not cause a pressure loss in the Stirling process, but only a short-circuit flow between the hot and cold sides, or between compression chamber 242 and expansion chamber 244. Since a sufficiently small sealing gap also acts as a regenerator, the sealing gap between cylinder liner 206 and inner piston 402 can be made larger than the sealing gap between outer piston 302 and inner piston 402, which reduces manufacturing costs and simultaneously prevents the inner piston 402 from seizing. The outer piston 302 and inner piston 402 are preferably guided without contact by means of a self-sustaining gas bearing (not shown), which utilizes the compression pressure of the pistons.The inner piston assembly 400 can only be guided in the central region of the inner piston 402, while a large portion of the moving mass is concentrated in the rear region on the side facing the buffer chamber 246. Therefore, the inner piston 402 has a counterweight 406 on the side facing the expansion chamber 244. The counterweight 406 serves to shift the center of mass of the inner piston assembly to the center of the guided region when the two pistons 302 and 402 are in the neutral position. This prevents the inner piston 402 from being tilted by leverage forces, which could negatively affect its guidance. The two pistons 302 and 402 are each connected to separate linear electric machines 500A and 500B, preferably moving magnet motors or generators, which are already known in the prior art.The linear electric machines 500A and 500B can each generate a magnetic flux in the outer stator 510 and the inner stator 502, both made of a ferromagnetic material, by means of an alternating current-energized coil 504. To prevent the movement of the inner piston 402 from dividing the buffer chamber 246 into two separate gas chambers with different spring stiffnesses, axial through-holes 412 are provided in the underside of the inner piston 402. Furthermore, a local constriction 405, which can be designed, for example, as a throttle valve and is located on the underside of the inner piston 402, ensures that an unintended pressure equalization between the cavity 408 and the buffer chamber 246 cannot occur quickly enough during the oscillating piston movement.In the air gap between the outer stator 510 and the inner stator 502 is a magnetic ring assembly 520, which is attached to the outer piston 302 and the inner piston 402 by a locking mechanism. The magnetic ring assembly 520 consists of radially magnetized ring magnet segments 522, a magnet carrier 528, and a retaining ring 548. The ring magnet segments 522 generate the oscillating piston movement through interaction with the magnetic flux in the air gap and are preferably made of neodymium. The magnet carrier 528 is preferably made of a material with low magnetic permeability, such as stainless steel or titanium. The air gaps of the linear electric machines 500A and 500B also form an interface for energy transfer between the rotating and stationary parts of the Stirling engine 200.The magnetic flux is transferred from the stationary outer stator 510 through the housing of the outer piston 202 to the rotating inner stator 502. The housing of the outer piston 202 is preferably made of a material with low magnetic permeability and also has the smallest possible wall thickness, since the magnetic resistance in the magnetic circuit increases significantly with increasing air gap. A small wall thickness of the pressurized housing of the outer piston 202 is primarily achieved by a small outer diameter, which in turn is achieved by arranging the outer stator 510 outside the pressure vessel. In the de-energized state, the two pistons 302 and 402 are in one of two detent positions, in which the ring magnet segments 522 are each aligned centrally at one of the two poles of the outer stator 510. Therefore, no additional support for the two pistons 302 and 402 is required.The rotating part of the Stirling engine 200 is supported within the stationary part by means of rolling bearings 232A and 232B. Furthermore, an airflow between the rotating and stationary parts of the Stirling engine 200 is generated by a flow tube 208 with radial inlet openings 248 and a fan 228. This convectively dissipates the waste heat from a heat sink 224, which is thermally connected to the ambient heat exchanger 222 via heat pipes 223, as well as the waste heat from the coil 504 in the linear electric machines 500A and 500B. In the area of ​​the rolling bearings 232A and 232B, the airflow is guided through axial ventilation openings 252A and 252B in the housing of the outer piston 202. The heat sink 224 is attached to the rotating part of the Stirling engine 200 by threaded bolts 239 and nuts 237. A seal 236 seals the working gas from the environment, preferably by means of a metal C-ring or an O-ring.

[0006] The following section describes the magnetic ring arrangement 520 using the following examples: Fig. 2 - 4 are described. The magnetic ring arrangement 520 is preferably identical in construction to the outer piston assembly 300 and the inner piston assembly 400 and is therefore only described using the outer piston assembly 300 as an example. First, the ring magnet segments 522 are slid over the magnet carrier 528 until the lower axial surface 526 of the ring magnet segments 522 engages in an axial groove 532 in the magnet carrier 528. In doing so, tabs 538 on the magnet carrier 528 are bent inwards. These tabs are formed by axial slots 536 and have locking lugs 542 at their open ends with chamfers 546 facing the open end and chamfers 544 facing the closed end. Another function of the axial slots 536 is to reduce the eddy currents induced by the linear electric machines 500A and 500B in the magnet carrier 528 and thus increase the efficiency of the linear electric machines 500A and 500B.The locking lugs 542 also have a radial retaining ring groove 534 on their inner side for receiving a retaining ring 548. For assembly, the magnet carrier 528 with mounted ring magnet segments 522 is now moved via an axial stop bar 322 (see . Fig. 2 or Fig. 3) pushed on the outer piston 302. In doing so, the chamfer 546, which faces the open end, bends the tabs 538 inwards until the locking lugs 542 have passed the axial stop bar 322 and are then bent outwards again by spring forces. However, in the assembled state, the tabs 538 do not return completely to their initial position, but only until the chamfer 544, which faces the closed end, abuts a similar chamfer 324 on the axial stop bar 322. The remaining spring force of the tabs 538 is generated by the inclined plane on which the chamfers 324 and 544 meet (see Fig. 1) A force component that pulls the magnet carrier 528 towards the outer piston 302 until the upper axial surface 524 of the ring magnet segments 522 rests against the axial stop bar 322 without play. Additionally, a radial stop bar 323 on the outer piston 302, together with the axial groove 532 in the magnet carrier 528, ensures that the ring magnet segments 522 are fixed in the radial direction. To prevent the magnet carrier 528 from becoming dislodged by the oscillating piston movement, a retaining ring 548 is inserted into the radial retaining ring groove 534, preferably a Hoopster ring. ®A retaining ring from the Smalley company is used. The radial retaining ring groove 534 is designed to be so deep that the installed retaining ring 548 is flush with the inner diameter of the magnet carrier 528. This allows for a small air gap in the linear electric machines 500A and 500B and prevents the magnetic ring assembly 520 from colliding with the inner stator 502 during the oscillating piston movement. To further minimize the air gap, it is advantageous for the tubular part of the magnet carrier 528, located between the axial groove 532 and the locking lugs 542, to have the smallest possible wall thickness, preferably no greater than 0.5 mm. It is also advantageous for the outer diameter of the magnet carrier 528 to be slightly smaller than the outer diameter of the outer surface 304 of the outer piston 302.This prevents grinding of the magnet carrier 528 in the housing of the outer piston 202 caused by manufacturing tolerances.

[0007] In the following, an alternative embodiment of the invention will be described with reference to Fig. Sections 5-8 are described. Identical components are identified by the same reference numerals and are therefore not described again. The magnetic ring assembly 720 consists of a radially magnetized ring magnet 723, a magnet carrier 728, and a retaining ring 548. First, the ring magnet 723 is slid over the magnet carrier 728 until the lower axial surface 726 of the ring magnet 723 abuts an axial stop 732 on the magnet carrier 728. This causes tabs 738 on the magnet carrier 728 to bend inwards. These tabs are formed by axial slots 736 and have locking lugs 742 at their open ends with chamfers 746 facing the open end and chamfers 744 facing the closed end. The locking lugs 742 also have a radial retaining ring groove 734 on their inner side for receiving a retaining ring 548. For assembly, the magnet carrier 728 with the mounted ring magnet 723 is now moved via an axial stop bar 622 (see Fig. 6 or Fig. 7) pushed on the outer piston 602. In doing so, the chamfer 746, which faces the open end, bends the tabs 738 inwards until the locking lugs 742 have passed the axial stop bar 622 and are then bent outwards again by spring forces. However, in the assembled state, the tabs 738 do not return completely to their initial position, but only until the chamfer 744, which faces the closed end, abuts a similar chamfer 624 on the axial stop bar 622. The remaining spring force of the tabs 738 is generated by the inclined plane on which the chamfers 624 and 744 touch (see Fig.5) A force component that pulls the magnet carrier 728 towards the outer piston 602 until the upper axial surface 724 of the ring magnet 723 rests against the axial stop bar 622 without play. Since the ring magnet 723 is closed, no additional radial stop bar is required. This allows the ring magnet 723 to have a greater radial thickness than the ring magnet segments 522, which increases the power density of the linear electric machines 500A and 500B. To prevent the magnet carrier 728 from becoming dislodged by the oscillating piston movement, a retaining ring 548 is additionally inserted into the radial retaining ring groove 734, preferably a Hoopster ring. ®A retaining ring from the Smalley company is used. The radial retaining ring groove 734 is designed to be so deep that the installed retaining ring 548 is flush with the inner diameter of the magnet carrier 728. This allows for a small air gap between the linear electric machines 500A and 500B and prevents the magnetic ring assembly 720 from colliding with the inner stator 502 during the oscillating piston movement. To further minimize the air gap, it is advantageous for the tubular portion of the magnet carrier 728, located between the axial groove 732 and the locking lugs 742, to have the smallest possible wall thickness, preferably no greater than 0.5 mm. It is also advantageous for the outer diameter of the magnet carrier 728 to be slightly smaller than the outer diameter of the outer surface 604 of the outer piston 602.This prevents grinding of the magnet carrier 728 in the housing of the outer piston 202 caused by manufacturing tolerances. Reference symbol list 200 Stirling cryocoolers 202 Housing of the outer piston 204 Inner piston housing 206 Cylinder liner 208 Flow tube 212 stationary mounting flange 214 rotating mounting flange 218 cold heat exchanger 222 Ambient heat exchangers 223 Heatpipe 224 Heat sink 226 Regenerator 228 Fan 232A Rolling bearing 232B Rolling bearing 236 Seal 237 Mother 239 threaded bolts 242 Compression chamber 244 Expansion area 246 Buffer space 248 radial ventilation opening 252A axial ventilation opening 252B axial ventilation opening 300 outer piston assembly 302 outer piston 304 outer surface 306 inner surface area 322 axial stop bar 323 radial stop bar 324 phase 400 inner piston assembly 402 inner piston 405 local bottleneck 406 Counterweight 408 Cavity 412 axial ventilation opening 500A Linear electric machine of the outer piston 500B Linear internal piston electric machine 502 inner stator 504 coil 510 outer stator 520 magnetic ring arrangement 522 Ring magnet segment 524 axial area 526 axial area 528 magnetic carriers 532 axial groove 534 Retaining ring groove 536 axial slot 538 tab 542 Rastnase 544 phase 546 phase 548 retaining ring 600 alternative outer piston assembly 602 outer piston 604 outer surface 622 axial stop bar 624 phase 720 alternative magnetic ring arrangement 723 Ring magnet 724 axial area 726 axial area 728 magnetic carriers 732 axial stop bar 734 Retaining ring groove 736 axial slot 738 tab 742 Rastnase 744 phase 746 phase QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 7 692 339 B2

[0002] US 6 694 730 B2

[0002] US 5 642 088 A

[0002] US 2009 / 0001 823 A1

[0002]

Claims

[1] Magnetic ring arrangement for linear electrical machines, the magnet carrier (528) of which is made of a material with low magnetic permeability, such as stainless steel or titanium, has axial slots (536) to reduce eddy current losses and the ring magnet (522) of which can be made of individual segments, characterized by , that the axial slots (536) of the magnet carrier (528) are open on one side and thereby create flexible tabs (538), wherein the magnet carrier (528) has locking lugs (542) at its open axial end with chamfers (546) directed towards the open end, which bend the tabs (538) inwards when slid over an axial stop bar (322), wherein the magnet carrier (528) has locking lugs (542) at its open axial end with chamfers (544) directed towards the closed end, with which the tabs (538) snap into a similar chamfer (324) without play after passing the axial stop bar (322). [2] Magnetic ring arrangement for linear electrical machines according to claim 1, characterized by , that the ring magnet segments (522) are fixed by the magnet carrier (528) with an axial groove (532) and by the outer piston (302) with a radial stop bar (323). [3] Magnetic ring arrangement for linear electric machines according to claim 2, characterized by , that the magnet carrier (528) has at its axial end with the open axial slots (536) on the inner surface a radial retaining ring groove (534) for receiving a retaining ring (548). [4] Magnetic ring arrangement for linear electric machines according to claim 3, characterized by , that at least one retaining ring (548) is inserted into the radial retaining ring groove (534) which is sold by the company Smalley under the brand name Hoopster Ring. [5] Magnetic ring arrangement for linear electrical machines according to claim 1, characterized by, that a one-piece and closed ring magnet (723) is used, which is fixed by a magnet carrier (728) and an outer piston (602) without the radial stop bar (323). [6] Magnetic ring arrangement for linear electric machines according to claim 5, characterized by , that the magnet carrier (728) has at its axial end with open axial slots (736) on the inner surface a radial retaining ring groove (734) for receiving the retaining ring (548). [7] Magnetic ring arrangement for linear electric machines according to claim 6, characterized by , that at least one retaining ring (548) is inserted into the radial retaining ring groove (734) which is sold by the company Smalley under the brand name Hoopster Ring.

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