Magnetics Business & Technology - Summer 2011 - (Page 6)

FEATURE ARTICLE By Jinfang Liu and HeeJu CHoi • eLeCtron energy Corp. Designing Permanent Magnet Systems for Critical Applications Rare earth magnets are the most powerful permanent mag- pling systems can be found in many applications including cheminets available and are used in numerous applications including cal, biological, pharmaceutical and other industrial systems. voice coil motors, hard disc drives, DVD players, CD players, The torque of magnetic couplers is related to the magnetic cell phones, loudspeakers, hybrid vehicles, wind turbines, mo- properties of permanent magnets, magnet dimensions, the numtors, generators, medical devices and aerospace applications. ber of poles and the air gap between the driver and driven compoWorldwide demand for permanent magnet systems has trended nents. Just like a motor or a generator design, the number of magup significantly in the last decade driven net poles will have a significant impact on Number of Poles 6 8 12 14 by automotive, consumer electronics and the torque. An optimum number of poles Torque [Nm] 3.4 12.2 14.4 10.2 alternative energy industries. China curwould help maximize the torque, assuming rently supplies about 95 percent of the all other design parameters are the same. Table 1. Torque vs the number of poles of a magrare earth needs worldwide. With grow- netic coupler determined by finite element analysis The torque characteristics of a rotary coing domestic demands for rare earth and axial magnetic coupler with the assembly environmental issues, China has been curdimension of ID 24 mm, OD 58 mm and 25 tailing its exports in recent years, which mm long, for instance, are investigated by prompted fears of rare earth shortage in the finite element method using Maxwell 3D world. The tight rare earth magnet supply electromagnetic field solver. The coupling situation makes efficient design even more torque increases as the number of alternatcritical. In view of these complexities, it ing poles increases and reaches a peak of is very important to understand the critical 14.4 Nm at 12 poles, but decreases when factors when designing the magnetic cirthe number of poles is 14 for this specific cuit. In this article we will provide a few coupler design as shown in Table 1. case studies for critical applications. Both sintered and bonded rare earth magnets can be used in the permanent magnetSelection of Permanent Magnets ic couplings. If the application has a moderate torque requirement in a moderate Understanding the application environoperating environment (<150°C), bonded ment is essential for satisfactory perfor- Figure 1. 3D model of a 12-pole magnetic coupler NdFeB type magnets are a good choice. mance of the device and is an important step in the design process, which determines the appropriate mag- If the application has a high torque requirement in a moderate netic parameters used for analytical and / or numerical analysis. operating environment (<150°C), sintered NdFeB type magnets SmCo type magnets have the best thermal stability, while NdFeB could be considered. If the application requires high torque at magnets have the highest maximum energy product, (BH)max. higher temperatures (>150°C), sintered SmCo magnets are the Sintered NdFeB magnets offer high magnetic properties at room best choice [1,2,3]. temperature, but can only be used at relatively low temperatures. In comparison, standard SmCo magnets have impressive mag- Halbach Arrays netic properties at elevated temperatures up to 300°C. Ultra high Permanent magnet Halbach arrays are very useful for a variety temperature SmCo magnets produced by Electron Energy Corp. of applications, including high field magnet sources, magnetizing (EEC) can be used in applications with temperature up to 550°C. and de-magnetizing systems, and permanent magnet motors and Permeance coefficient, also known as the operating point or load generators. Halbach progressive magnetization design can maxiline, is determined by the magnetic circuit and the dimensions of mize the coupling torque. Halbach progressive magnetization is the magnet. The load line of a given permanent magnet is a straight based on the Halbach theorem, which states that if the magnetizaline passing through the origin of the demagnetization curve with tion of an infinite line source oriented perpendicular to its axis is a slope of negative Bd/Hd for a stand alone magnet, meaning there rotated about that axis, the field it produces remains everywhere are no other magnets or magnetic field sources around it. The per- constant in magnitude and is everywhere rotated by the same anmeance coefficient should be carefully considered during the mag- gle in the opposite sense. For an infinite dipole, the tangential and netic circuit design process to have an optimal design. radial field components are: Hq = l sinq / 2pr2 (1) Magnetic Coupling Systems Hr = l cosq / 2pr2 (2) Magnetic couplings are used to transmit rotational and/or linear l is the moment per unit length, and q is the angle measured from motion without direct contact. Magnetic couplers in a mixer, for the dipolar axis. Therefore the magnitude of H is: example, could eliminate the needs for seals, which would im|H| = (Hq2 + Hr2)1/2 = l/2pr2 (3) prove the reliability and safety aspects of such machines because |H| is independent of dipolar orientation. The field orientation seals are prone to deterioration over time, causing leaks. There angle a with respect to the axis is twice q. The Halbach principle are rotary couplers, face to face couplers and linear couplers. Cou- has been used extensively in a variety of applications [5], includ- 6 Magnetics Business & Technology • Summer 2011

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2011

Magnetics Business & Technology - Summer 2011
Editor’s Choice
Designing Permanent Magnet Systems for Critical Applications
Growth in the Sensor Market Led by Magnetically Actuated Reed and Hall Sensors
Extending the Possibilities of Iron
Research & Development
Magnets • Materials • Measurement
Application • Component Developments
Guest Column: Cue-less About Q of an Inductor?
Industry News
Calendar of Events
Advertising Index
Spontaneous Thoughts: Sputnik Moments

Magnetics Business & Technology - Summer 2011