Magnetics Business & Technology - Spring 2016 - (Page 10)

FEATURE ARTICLE Some Design Considerations Using Permanent Magnets By Jinfang Liu, Chief Operating Officer | Electron Energy Corp. Permanent magnets are critical components for many applications including motors, generators, computers, instrumentation, automation, oil exploration, aerospace and defense systems. Figure 1 shows the history of the permanent magnet development in the last 100 years. The maximum energy product, or (BH)max, has increased from a few MGOe to more than 50 MGOe. Commercially popular magnets nowadays include neodymium iron born (NdFeB), samarium cobalt (SmCo), Alnico, and ceramic magnets (also known as hard ferrite). NdFeB and SmCo are collectively known as Rare Earth magnets, which exhibit very high magnetic properties. Ceramic magnet is the least expensive option on the market. grain boundary phase for unprotected NdFeB magnets, and intergranular decomposition could happen in an environment with high temperatures and high humidity. A simple passivation treatment will help protect NdFeB magnets in a temporary dry storage facility. For most of the applications, effective surface protection is necessary. Popular surface coating options include NiCuNi plating, Ni plating, zinc plating, aluminum ion vapor deposition and epoxy coating. Figure 2. Maximum Operating Temperature Versus Maximum Energy Product, (BH)max Figure 1. Development of Permanent Magnets in the Last 100 Years If the total weight and overall size of the system are not a constraint at all, one may consider ceramic magnets because of their low cost. Ceramic magnets are still widely used today even though their maximum (BH)max is only about 4 MGOe. If the total weight and size are important, rare earth magnets are recommended because of their superior magnetic performance. Of course there are many factors to be considered when designing systems using permanent magnets. The following briefly describes a few important design considerations. 1. Operating Environment Operating temperature has a significant impact on the selection of permanent magnets. Figure 2 shows the relationship between maximum operating temperature and maximum energy product, (BH)max, for permanent magnets. SmCo, Alnico and ceramic magnets can be used at elevated temperatures, while NdFeB can only be used at relatively lower temperatures. NdFeB magnets should be considered when you design a high performance system if the maximum operating temperature is below 150°C, while SmCo magnets would become a first choice if the maximum operating temperature is above 180°C. SmCo, Alnico and ceramic magnets have very good corrosion resistance and, therefore, surface coatings are generally not necessary. Surface coating may be recommended for SmCo magnets when the maximum operating temperature is above 400°C. NdFeB magnet does not have good corrosion resistance without surface protection. Corrosion generally starts from neodymium-rich 10 Magnetics Business & Technology * Spring 2016 2. Reversible Temperature Coefficient of Residual Induction Residual induction, Br, changes with temperature for permanent magnets. The reversible temperature coefficient (RTC) of Br is defined as: where ∆Br is the change of residual induction and ∆T is the change of temperature. The typical values of reversible temperature coefficient, a, for the major classes of permanent magnets are -0.2 percent/°C for ceramic magnets, -0.03 to -0.02 percent/°C for Alnico magnets, -0.11 percent/°C for NdFeB, -0.04 percent/°C for SmCo5 and -0.035 percent/°C for Sm2Co17-type magnets. The magnetic properties decrease with temperature relatively faster for NdFeB magnets. When designing a magnetic system, one can estimate the magnetic properties at the operating temperature based on the reversible temperature coefficient. Figure 3 shows the maximum energy product, (BH)max, versus temperature for NdFeB and SmCo magnets. Some popular NdFeB magnet grades, N46H, N42SH, N40UH and N35EH, are included Figure 3 to compare with SmCo2:17-33. The maximum energy product at room temperature for N46H, N42SH, N40UH, and N35EH are 46, 42, 40 and 35 MGOe, respectively, while SmCo2:1733 has only 33 MGOe at room temperature. When the operating temperature is at 150°C, the (BH)max for SmCo2:17-33 magnet is actually comparable to most of the NdFeB grades and it is higher than that for N35EH. This phenomenon is directly related to the difference in reversible temperature coefficient of residual induction of these materials. One may want to choose SmCo magnets when www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2016

Editor's Choice
Permanent Magnets in a Changing World Market
Design Considerations Using Permanent Magnets
Insulated Iron Powders, SMC, Current State and Future Possibilities
Research & Development
The Design of a Permanent Magnet Motor for a Direct Drive Fan Application
Tailoring of Pole Shapes of Multipolar Injection Molded Magnets
Electromagnetics
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: Seventeen Mistakes People Make with Permanent Magnets

Magnetics Business & Technology - Spring 2016