03 August 2000
Motor Magnetics
by Jake Ring
Permanent magnet (e.g., brushless DC or linear) motor use in factory automation has grown significantly. Annual market segment expansion, coupled with price decreases in electronics and permanent magnet materials, have presented designers with a growing demand for an increasingly cost-effective motor.
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Permanent magnets have steadily become indispensable components in a variety of products. |
Yet many continue to view permanent magnets as a commodity item, and aren't aware of the benefits of certain high-energy magnets that can help reduce an application's total system cost. Designers require a better understanding of how permanent magnet technology has evolved in order to select a material or grade that's optimal for their applications.
Aluminum-nickel-cobalt (AlNiCo) magnets, introduced in the late 1920s, were surpassed by ferrite magnets (c. 1950); the latter remain the most commonly used magnet material today. Although ferrites have consistently been cost-effective, advancing technology in numerous industries demands higher energy and more efficiency from magnet applications.
Enter rare earth compositions. Samarium-cobalt (SmCo5) reached the market in the late 1960s, offering a maximum energy product that was higher by a factor of five, the highest known coercivity, and excellent thermal stability. A second-generation development (Sm2Co17) further increased the energy product level. However, cobalt's unstable supply and samarium's relatively high price made for an expensive product. With uses limited to small or thermally demanding situations, samarium's negative attributes paved the way for the introduction in the 1980s of the rare earth compound neodymium-iron-boron (NdFeB, or Neo).
No other magnetic material possesses higher energy density than Neo, which enables motors to have smaller size, greater torque, and greater efficiency. This has revolutionized an engineer's ability to improve designs across nearly all industries.
Surprisingly, Neo magnets gained acceptance early despite their initial high cost. The benefits of smaller, lighter motors in high-tech products outweighed the component cost considerations. This total system value approach enabled innovative designs and gave companies such as Matsushita, Toshiba, Seagate, and Canon an additional technological edge in their respective markets. Most modern electronic office equipment owes its current design to Neo magnets' enabling technology. The growth of these applications and the accompanying productivity and process improvements has driven magnet costs substantially lower.
Material Advances
Over time, Neo magnets' properties have improved as their cost has decreased. Today, most new products and applications are designed with them, to the exclusion of other materials. Like many new discoveries, however, Neo magnets' capabilities and properties were both undervalued and underestimated. Constant research and development, plus the material's use in new applications, has provided a better understanding of its capacity and limits. Simply knowing what environments the magnet can endure and what it can accomplish has helped define a broader range of products in which it can be used.
Until recently, design engineers didn't consider using Neo magnets in many applications due to the high-temperature coefficient of the remanence and coercivity as well as the low corrosion resistance. We've since learned that NdFeB has better thermal stability and corrosion resistance than originally thought, even without coating.
Although the component cost still remains higher than ferrite, the effect of a Neo magnet's smaller volume, yet greater energy and flexibility, permits it to lower a product's total cost. This has encouraged the redesign of older, existing products (e.g., conventional DC motors). When considering the total application cost, designers must account for manufacturing, assembly, and product return.
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A hot-pressed Neo magnet can hold tight tolerances without grinding. |
For example, a DC motor's brushes have several limitations, including brush life, brush residue, maximum speed, and electrical noise. With a brushless DC (BLDC) motor, these factors are alleviated, with the added benefit of having no sparks. BLDC motors are potentially cleaner, faster, more efficient, less noisy, and more reliable. While they do require electronic control, even this cost can be reduced with an optimized magnetic circuit design using high-energy Neo magnets.
Figure 1 compares a BLDC motor design using ferrite to one optimized for Neo magnets. For an equivalent 4-pole motor the Neo-based design not only is reduced in size and weight for the same output power, but also has greater efficiency. In addition, its overall system cost is similar because both manufacturing and assembly time and expected warranty costs are less. Thus, NdFeB isn't just an improvement over SmCo, but is now replacing more and more ferrite applications.
Neo magnets can help reduce specification constraints, especially in motion controls. For the design application above, a smaller current rating electronic drive amplifier is used since with torque angle control, higher speeds can still be obtained even with Neo magnets' higher flux. As the costs of drive electronics decrease, such opportunities to use lower rating components help lower the overall system cost.
Processing
Neo magnets continue to play an essential role in technological advances in an increasing range of industries by offering diverse magnet manufacturing processes. Rare-earth magnet processing is accomplished by either powder metallurgy (crushing, pressing, and sintering) or rapid quenching and pressing. Sumitomo Special Metals Co., Ltd. and Magnequench International, Inc. hold the patents for those respective processes.
Powder metallurgy consists of pulverizing the alloy into fine powder before pressing it in a magnetic field to align the individual particles. It's then compressed into the desired shape (typically a block). Sintering takes place at temperatures close to 1,100°C; the magnets are then aged at lower temperatures to improve their magnetic properties. Because of shrinkage and warpage during the sintering process, powder metallurgy magnets must be produced in larger shapes and then ground to their final size.
In rapid quenching, molten alloy is ejected onto a chilled, rotating metallic wheel. The alloy cools as it hits, rapidly solidifying into ribbon-like flakes, which are crushed to form a powder. Compared to that produced by the powder metallurgy process, the rapidly quenched powder is considerably more coarse and has an ideal microstructure for making magnets. The powder is used to produce bonded or hot-pressed magnets that are used in various motion control applications. Combining the powdered alloy with an epoxy or thermoset resin, then casting it through either compression or injection molding techniques, produces bonded magnets. Hot-pressed magnets are produced by cold pressing the powdered alloy to a form and then hot pressing at around 750°C to increase density. Unlike sintering, this process employs fewer steps, doesn't use any orienting magnetic field, and yet still achieves similar magnetic properties. In either case, bonded or metal magnets can be made in the final shape without expensive grinding.
Injection molded high-energy Neo magnets are the industry's most recent development. This allows manufacturers to create intricate, customized magnet subassemblies for individual applications. It also makes the magnet an intrinsic part of the structure, eliminating the need for gluing or other assembly steps. Further, it aids flexibility by allowing varied, complex shapes and tight tolerances not easily achieved by other manufacturing processes. This has enabled innovative designs in computer storage devices, power hand tools, and automotive applications that have higher power and efficiencies at a total cost low enough to justify replacing ferrite material.
What Does This Mean for Motor Designers?
Only fifteen years after Neo magnets' discovery, efforts are underway to develop emission-free, permanent magnet motors that will replace two-stroke internal combustion engines in lawn and garden equipment. Designs are also under development for 42-V applications in the next breed of low-emission, fuel-efficient automobiles. To achieve milestones of this magnitude, the industry is committing to reducing the price of NdFeB-quenched powders by 7 to 8% each year until 2005. In addition, Magnequench's recently opened Technology Center offers engineers a facility dedicated to providing technical application assistance in magnetic design, ensuring design optimization from concept through material selection to production-level prototyping and magnetization.
Neo magnet materials allow engineers greater creativity in design while defining a more robust product. Consequently, inherent benefits are experienced from start to finish in every application. No other permanent magnet offers the highest performance at the lowest total cost. MC
Additional Information
Figures and Graphics
Author Information
Jake Ring is the vice president of marketing for Magnequench International, Inc., of Anderson, Ind. Ring earned his bachelor of science in math and computer science from Vanderbilt University and his MBA from Washington University in St. Louis, Mo. After working for Emerson Electric Co., he joined Magnequench in 1999 to implement the brand development of the Neo magnets and powders as well as the Technology Center in Research Triangle Park, NC. Contact him at tel: (888) 335-0258.
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