Magnetic Circuit Design

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67 TECHNOLOGY INTERNATIONAL 2012 Permanent magnet applications Magnetic circuit design A review of lesser-known applications of rare-earth-based permanent magnets, including magnetocaloric refrigeration systems, watt balances, gyro guidance systems, and advanced magnetic resonance imaging data logging systems M agnetic refrigeration is a technology based on the magnetocaloric effect (MCE) that is seen at specific temperatures in ferromagnetic materials. In recent years an intense effort was dedicated to magnetic refrigeration in the proximity of room temperature, resulting in the discovery of materials with a giant magnetocaloric effect around that temperature. 1,2 It was also demonstrated that magnetic refrigeration technology can achieve a potential efficiency of up to 60% of the Carnot cycle, a substantial improvement over the ~40% efficiency achieved with vapor compression systems. Although not extensively explored so far, magnetic refrigeration may also be a viable method for cooling at elevated temperatures, making it attractive for cooling high-power density rotating machines and electronic devices. At higher temperatures the lifetime of the electronic components may shorten by half with every additional 10°C – especially important for some applications near 120ºC. The MCE can be described as follows: when a magnetic field is applied to a magnetic material, the unpaired spins comprising the material’s magnetic moment align parallel to the magnetic field. The spins give up energy to atomic vibrations in the material’s crystal structure, which increases the material’s temperature. Conversely, upon removal of the field the spins return to random orientations, taking heat out of the atomic vibrations and resulting in a decrease in the material’s temperature. By cycling the material through these hot and cold states and venting away the heat, the system can generate an overall cooling effect. A heat transfer fluid intermediates the cooling of the designated environment. Larger Figure 1: An exploded view of the conceptual design layout Figure 2: The FEA 3D model MCE materials that have been uniquely developed during research efforts for operation at these high temperatures were based on La(Si,Fe,Co) 13 compositions. Second, the flow system for heat- transfer fluid must be designed to safely handle high temperatures and, when using water, possibly high pressure. Finally, a magnetic field source able to produce 1.5T at temperatures around 120°C requires high-temperature permanent magnets. Magnetic circuit design Figure 1 shows the 3D conceptual design of the magnet assembly for the magnetocaloric refrigeration demonstration device (MRDD) for high temperature applications. This device, designed in collaboration with ACA, is intended to demonstrate the feasibility of magnetic refrigeration at high temperatures and test and evaluate various MCE material compositions in a relevant environment. The MRDD shown operates with a 1.5T Halbach magnet assembly mounted on a motor-driven shaft. The frequency of the field variation is 4Hz. The diameter of the magnet assembly is 43cm, its height is 15.8cm, and its estimated weight is 69kg. The air gap is 1.8cm. To produce a magnetic field of 1.5T at 120°C, various magnetic design parameters such as segment dimensions, number of poles, array types, permeability, saturation and magnetic flux leakage need to be carefully considered. The Halbach principle is applied to the magnetic circuit design to improve the magnetic field in the air gap. The key concept of the Halbach array is that the magnetization vector of the PMs should rotate as a function of distance along the array. 5 The 3D model of magnet assembly shown in Figure 2 was obtained using the commercial electromagnetic field FEA software Ansys Maxwell 3D. The permanent magnet blocks used were EEC SmCo 27MGOe with a residual magnetic flux of 10,800G at room temperature. The material of the magnetic pole piece was Hiperco 50 (Permendur). Based on the simulation, the magnetic field distribution is shown in Figure 3, and a uniform field of 1.63T is achieved in the air gap at 24°C; a field of 1.57T is achieved at 120°C. The length of uniform flux density is nearly 10cm, which is long enough to cover the magnetocaloric material bed. Heeju Choi, Melania Marinescu-Jasinski, Jinfang Liu, Michael Walmer & Peter Dent, Electron Energy Corporation variation of the external magnetic field results in a larger magnetic entropy change and a larger MCE. 3 Magnetic field variations of 15kG are sufficient for practical use. Magnetic refrigerator The ideal source of the DC magnetic field necessary to produce the MCE in magnetocaloric materials is a PM system. The magnetic field variation can be achieved by employing a rotary-magnet architecture, 2,4 where a magnet rotates over a set of stationary beds containing the magnetocaloric material, circumferentially arranged. There are major technical challenges unique to the high-temperature environment that must be overcome before a high-temperature magnetic refrigerator can be successfully constructed. First, the magnetocaloric material must retain its properties and long-term chemical stability at high temperatures when simultaneously exposed to a heat-transfer fluid and a strongly varying magnetic field. Astronautics Corporation of America (ACA), which has been working with EEC on the development of high-temperature magnetic refrigeration systems, has found that even at room temperature a number of magnetocaloric materials degrade under long-term exposure to an aqueous fluid and a cycling field, and has been developing high-performance MCE system solutions. The Figure 4 shows the MRDD built at EEC to function at temperatures up to 120°C. Manufacturing feasibility and mechanical safety must be carefully considered in the design of the high-temperature MRDD, especially when having a rotating magnet assembly of 69kg. Table 1 shows the comparison between the magnetic field data as generated by FEA and the test results, revealing a theoretical overestimation of the field of 300-600G. Table 1: FEA magnetic field data comparison Temperature (°C) FEA data (G) Test data (G) 24 16,300 15,710 120 15,700 15,400 180 15,250 NA Magnetic design of a watt balance Currently the only remaining fundamental unit of measure that relies on an actual physical standard is the kilogram. The 1kg worldwide standard, composed of 90% platinum and 10% iridium, stored in a vault in the Bureau International des Poids et Mesures in Paris, has become slightly heavier over the decades due to corrosion and local effects. International standards laboratories would like to base the kilogram on the resolution of Planck’s constant as opposed to this physical sample. Standards laboratories around the world are developing extremely high-precision watt balances to accomplish this goal. The watt balance is an electromechanical weight-measuring instrument, 6 whose operating principle consists of two steps. In the first step an induction coil is moved with nearly constant velocity through a magnetic field and an induced voltage is measured. The ratio of induced voltage to velocity is called the geometry factor and is measured in units of Tesla-meter. In the second step the coil is placed at the center of the magnetic field and a current is passed through the coil to produce a force equivalent to that of a 1kg mass in Earth’s gravitational field (9.81N). The ratio of force to current is equal to the geometry factor that was determined in the first step. To achieve precision in watt balance measurements it is key that the magnetic field is 66 TECHNOLOGY INTERNATIONAL 2012 Permanent magnet applications Figure 3: FEA results: magnetic field distribution at 120°C on the symmetric plane in the air gap (above) Figure 4: The MRDD prototype as mounted on an oven (right) Figure 5: The overall watt balance conceptual design

description

A review of lesser-known applications of rare-earth-based permanent magnets,including magnetocaloric refrigeration systems, watt balances, gyro guidancesystems, and advanced magnetic resonance imaging data logging systems.

Transcript of Magnetic Circuit Design

Page 1: Magnetic Circuit Design

67 TECHNOLOGY INTERNATIONAL 2012

Permanent magnet applications

Magnetic circuit designA review of lesser-known applications of rare-earth-based permanent magnets, including magnetocaloric refrigeration systems, watt balances, gyro guidance systems, and advanced magnetic resonance imaging data logging systems

Magnetic refrigeration is a technology based on the magnetocaloric effect (MCE) that is seen at specific temperatures in ferromagnetic

materials. In recent years an intense effort was dedicated to magnetic refrigeration in the proximity of room temperature, resulting in the discovery of materials with a giant magnetocaloric effect around that temperature.1,2 It was also demonstrated that magnetic refrigeration technology can achieve a potential efficiency of up to 60% of the Carnot cycle, a substantial improvement over the ~40% efficiency achieved with vapor compression systems. Although not extensively explored so far, magnetic refrigeration may also be a viable method for cooling at elevated temperatures, making it attractive for cooling high-power density rotating machines and electronic devices. At higher temperatures the lifetime of the electronic components may shorten by half with every additional 10°C – especially important for some applications near 120ºC.

The MCE can be described as follows: when a magnetic field is applied to a magnetic material, the unpaired spins comprising the material’s magnetic moment align parallel to the magnetic field. The spins give up energy to atomic vibrations in the material’s crystal structure, which increases the material’s temperature. Conversely, upon removal of the field the spins return to random orientations, taking heat out of the atomic vibrations and resulting in a decrease in the material’s temperature. By cycling the material through these hot and cold states and venting away the heat, the system can generate an overall cooling effect. A heat transfer fluid intermediates the cooling of the designated environment. Larger

Figure 1: An exploded view of the conceptual design layout

Figure 2: The FEA 3D model

MCE materials that have been uniquely developed during research efforts for operation at these high temperatures were based on La(Si,Fe,Co)13 compositions. Second, the flow system for heat-transfer fluid must be designed to safely handle high temperatures and, when using water, possibly high pressure. Finally, a magnetic field source able to produce 1.5T at temperatures around 120°C requires high-temperature permanent magnets.

Magnetic circuit designFigure 1 shows the 3D conceptual design of the magnet assembly for the magnetocaloric refrigeration demonstration device (MRDD) for high temperature applications. This device, designed in collaboration with ACA, is intended to demonstrate the feasibility of magnetic refrigeration at high temperatures and test and evaluate various MCE material compositions in a relevant environment. The MRDD shown operates with a 1.5T Halbach magnet assembly mounted on a motor-driven shaft. The frequency of the field variation is 4Hz. The diameter of the magnet assembly is 43cm, its height is 15.8cm, and its estimated weight is 69kg. The air gap is 1.8cm.

To produce a magnetic field of 1.5T at 120°C, various magnetic design parameters such as segment dimensions, number of poles, array types, permeability, saturation and magnetic flux leakage need to be carefully considered. The Halbach principle is applied to the magnetic circuit design to improve the magnetic field in the air gap. The key concept of the Halbach array is that the magnetization vector of the PMs should rotate as a function of distance along the array.5

The 3D model of magnet assembly shown in Figure 2 was obtained using the commercial electromagnetic field FEA software Ansys Maxwell 3D. The permanent magnet blocks used were EEC SmCo 27MGOe with a residual magnetic flux of 10,800G at room temperature. The material of the magnetic pole piece was Hiperco 50 (Permendur). Based on the simulation, the magnetic field distribution is shown in Figure 3, and a uniform field of 1.63T is achieved in the air gap at 24°C; a field of 1.57T is achieved at 120°C. The length of uniform flux density is nearly 10cm, which is long enough to cover the magnetocaloric material bed.

Heeju Choi, Melania Marinescu-Jasinski, Jinfang Liu, Michael Walmer & Peter Dent, Electron Energy Corporation

variation of the external magnetic field results in a larger magnetic entropy change and a larger MCE.3 Magnetic field variations of 15kG are sufficient for practical use.

Magnetic refrigeratorThe ideal source of the DC magnetic field necessary to produce the MCE in magnetocaloric materials is a PM system. The magnetic field variation can be achieved by employing a rotary-magnet architecture,2,4 where a magnet rotates over a set of stationary beds containing the magnetocaloric material, circumferentially arranged. There are major technical challenges unique to the high-temperature environment that must be overcome before a high-temperature magnetic refrigerator can be successfully constructed. First, the magnetocaloric material must retain its properties and long-term chemical stability at high temperatures when simultaneously exposed to a heat-transfer fluid and a strongly varying magnetic field. Astronautics Corporation of America (ACA), which has been working with EEC on the development of high-temperature magnetic refrigeration systems, has found that even at room temperature a number of magnetocaloric materials degrade under long-term exposure to an aqueous fluid and a cycling field, and has been developing high-performance MCE system solutions. The

Figure 4 shows the MRDD built at EEC to function at temperatures up to 120°C. Manufacturing feasibility and mechanical safety must be carefully considered in the design of the high-temperature MRDD, especially when having a rotating magnet assembly of 69kg. Table 1 shows the comparison between the magnetic field data as generated by FEA and the test results, revealing a theoretical overestimation of the field of 300-600G.

Table 1: FEA magnetic field data comparison

Temperature (°C)

FEA data (G) Test data (G)

24 16,300 15,710

120 15,700 15,400

180 15,250 NA

Magnetic design of a watt balanceCurrently the only remaining fundamental unit of measure that relies on an actual physical standard is the kilogram. The 1kg worldwide standard, composed of 90% platinum and 10% iridium, stored in a vault in the Bureau International des Poids et Mesures in Paris, has become slightly heavier over the decades due to corrosion and local effects. International standards laboratories would like to base the kilogram on the resolution of Planck’s constant as opposed to this physical sample. Standards laboratories around the world are developing extremely high-precision watt balances to accomplish this goal.

The watt balance is an electromechanical weight-measuring instrument,6 whose operating principle consists of two steps. In the first step an induction coil is moved with nearly constant velocity through a magnetic field and an induced voltage is measured. The ratio of induced voltage to velocity is called the geometry factor and is measured in units of Tesla-meter. In the second step the coil is placed at the center of the magnetic field and a current is passed through the coil to produce a force equivalent to that of a 1kg mass in Earth’s gravitational field (9.81N). The ratio of force to current is equal to the geometry factor that was determined in the first step.

To achieve precision in watt balance measurements it is key that the magnetic field is

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Permanent magnet applications

Figure 3: FEA results: magnetic field distribution at 120°C on the symmetric plane in the air gap (above)

Figure 4: The MRDD prototype as mounted on an oven (right)

Figure 5: The overall watt balance conceptual design

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Permanent magnet applications Permanent magnet applications

stable in time, independent of the current in the coil, heating, hysteresis and the external magnetic fields. The present accuracy record is held by the US National Institute of Standards and Technology (NIST), with a relative uncertainty of 3.6 x 10-8 and experiments are continuing toward an end goal of 1 x 10-8.7

For this application, in addition to the stability, it is required that the magnetic field be constant within ±0.01% along the travel distance of the coil, typically 8cm. The desired magnet system is envisioned as using two PM rings embedded in an iron yoke to deliver a radial magnetic field with a flux density of at least 5,000G over a 3cm wide, cylindrical air gap, of which a central 8cm has a magnetic field that meets the required uniformity within ±0.01%.

Magnetic circuit designFigure 5 shows the 3D section view of watt balance conceptual design that consists of two components – a PM (EEC SmCo 27MGOe grade) and magnetic steel. The outer diameter is 60cm, the height is 45cm and its estimated weight is 850kg. The arrow indicates the magnetization direction of the PM, and the rest of the parts are magnetic steel 1018. For the magnetic field simulation the commercial electromagnetic FEA software Vector Field Opera 3D is preferred because it makes it possible to assign finer meshing in the precision air gap area. Figure 6 shows the calculated flux density distribution on the surface with a uniform field achieved through the precision air gap. The peak field in the yoke is below the saturation level of magnetic steel 1018. The calculated magnetic field in the precision air gap is shown in Figure 7. All design requirements are satisfied because the average field is 5,516G and the variation is ±0.35G, which indicates a uniformity of ±0.0063%.

However, the field data of magnet assemblies differ in reality, especially at this level of precision, due to two possible effects related to the magnetic residual flux and permeability. First, if 5.7% of the magnet volume has its magnetic residual flux decreased by 5%, then the magnetic field in the air

gap is reduced by 0.1G, which is 0.0018% of the average field near 5,500G. However, the magnetic residual flux change can be minimized by choosing magnets with very uniform properties. Second, the magnetic permeability of magnetic steels is very important because the magnetic field uniformity depends on the material itself, especially for a large and complicated assembly consisting of several components and subsystems. When increasing the permeability of steel by 50%, which is the worst-case scenario, the magnetic field changes by 4.5G (0.08% of the average field). However, the material permeability cannot differ by 50% and the effect can be minimized by using the same batch and production procedures.

Gyro guidanceThe basis for magnetic guidance of directional drilling is the creation of a magnetic field of simple geometry in the vicinity of the drill bits. The strength of this magnetic field should be greater than the Earth’s magnetic field and should be greater than any time-dependent magnetic fields produced by nearby earth structures or metal infrastructure. The steering tool mounted behind the drill bit consists of three accelerometers and three magnetometers that measure the magnetic field to calculate the azimuthal direction of the borehole. These sensors measure the required angles of inclination, azimuth and toolface. Both magnetometers and accelerometers give voltage outputs that can be used to calculate the required directional angle.

Combined with the length of pipe downhole, the azimuth and inclination of the borehole give the operator the position of the drill bit, thereby creating positive steerability to keep the bit on the pre-project planned track.8,9 This gyro guidance system enables the horizontal directional drill to reduce the surface disturbance by 95%. The horizontal drilling can lead to an increase in reserves in place by 2% of the original oil in place according to US Department of Energy. The production ratio of horizontal versus vertical wells is 3.2:1 while the cost ratio is only 0.33:1.10

The Sm-Co magnet is a critical part of the spinning flywheel in this miniature two-axis dynamically tuned gyroscope. The magnet ring creates a magnetic field interacting with torquing coils to enable control of the flywheel. The gyro measures the rotation of the Earth accurately enough to find true north during drill steering, surveying and other underground navigation tasks. The environment is particularly harsh and 1,000G shocks and 150°C ambient temperatures are common. n

AcknowledgementsThe support of the US Department of Defense, under FA-8650-10-C-2101, and the support of the US National Institute of Standard and Technology, under SB134111CN0137, are gratefully acknowledged. The authors would also like to thank Carl Zimm and Dr Steven Jacobs at Astronautics Corporation of America for their valuable discussions and input.

References1) Yu B., Liu M., Egolf P., Kitanovski A. Review of

magnetic refrigerator and heat pump prototypes built before the year 2010, International Journal of Refrigeration, 33 (2010); 1029-1060

2) Zimm C., Boeder A., Chell J., Sternberg A., Fujita A., Fujieda S., Fukamichi K. Design and performance of a permanent magnet rotary refrigerator, International Journal of Refrigeration, 29 (2006); 1302-1306

3) Pecharsky V., Gschneidner K. Advanced magnetocaloric materials: What does the future hold?, International Journal of Refrigeration, 29 (2006); 1239-1249

4) US patent 6,668,5605) Gieras J. F., Wang R.-J., Kamper M. J. Axial Flux

Permanent Magnet Brushless Machines, Kluwer Academic Publishers, London, UK, (2004)

6) Kibble, B. P., Sanders, J. H., Wapstra, A. H. Atomic Masses and Fundamental Constants 5, New York: Plenum (1975); 545-51

7) Steiner, R. L., Williams, E. R., Liu, R., Newell, D. B. Uncertainty Improvements of the NIST Electronic Kilogram, IEEE Trans. Instrum. Meas. 56 (2): 592–596, doi:10.1109/TIM.2007.890590

8) ElGizawy M. L. Continuous Measurement While Drilling Surveying System Utilizing MEMS Inertial Sensor’ PhD Dissertation, Department of Geomatics Engineering, University of Calgary (Feb 2009)

9) Djurkov A., Cloutier J., Mintchev M. P. Mathematical Model and Simulation of a Pneumatic Apparatus for In-Drilling Alignment of an Inertial Navigation Unit during Horizontal Well Drilling, International Journal Information Technologies and Knowledge (2008); Vol. 2, 147-156

10) Innovative Technology Summary Report, Horizontal Wells, prepared for US Department of Energy (Sept 1998)

11) Subterranean Navigation, Engineering Technology, Ingenia Issue 45 (December 2001)

Figure 6: Surface contours analysis (above left)

Figure 7: Magnetic flux density plot in the precision air gap (above)