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5 Key Parameters to Consider When Choosing a Permanent Magnet

Jan. 19, 2022

Custom Block NdFeB Magnet


Every type of permanent magnet has a unique set of characteristics that can affect how it performs in various applications. When evaluating magnet options for your project, pay attention to the following:


1. Temperature

One of the most important parameters to consider when choosing a permanent magnet is the temperature of use. As one might imagine, the magnetic field is not constant; it changes with temperature, getting either stronger or weaker. The figure below shows the usable temperature range for each of the five commercially available materials. Conveniently, most applications utilize magnets between -40°C and 150°C, and all listed materials are used within that range.


Because of their low cost, ferrite (hard ferrite, ceramic) magnets are still widely used.  Note, however, that ferrite magnets lose 25% of their flux output when heated from 20 to 150 °C.  They also have relatively low-energy products, typically 10% of rare earth magnets. So where device size and weight are important, such as hard disk drives and portable electronics (e.g. cell phones, hearing aids), rare earth magnets are necessary.


2. Magnetic Output

How strong a magnet is can be quantified by one of two key metrics. The first is residual induction, BR. If a permanent magnet is placed on a steel block, the force required to remove the magnet from the block is proportional to BR. BR is one of the magnet characteristics published by magnet producers.


The second key metric is maximum energy product, (BH)MAX. The maximum energy product is useful in calculating the performance of motors. (BH)MAX is proportional to BR2. Each type of device benefits especially from either B (that is, BR) or B2 (that is,(BH)MAX or BR2) as described in this table of applications.


3. Availability

Issues of raw material price and availability are at the core of today’s discussion on permanent magnets. These factors are influenced by the general availability of the raw materials, geographic distribution, ease of recovery from mined ore, and open market trading. Of all the elements used in magnetic materials, rare earth elements have been the most problematic based on geographic limitations on sourcing. China still produces 90 percent of the rare earth oxides used each year and more than 95 percent of the rare earth metals and alloys. China also produces and consumes 80 percent of all permanent magnets. There is inconsequential production of neodymium-iron-boron (NdFeB) in the US. One alternative to neodymium magnets is samarium cobalt (SmCo), which has fewer sourcing limitations and performs better in higher temperature applications.


4. Magnet Grade

Selecting a grade is the next step, once you have decided which permanent magnet material is best for your application. Generally, a grade indicates the Maximum Energy Product of a magnet. For instance, Grade 32 implies the (BH)max is about 32 MGOe. A higher grade of permanent magnet has a better performance. However, a higher grade is usually associated with a higher cost. Taking sintered Nd-Fe-B magnets as an example, the price of Grade 45 is twice and even more than that of Grade 33. Other property parameters, such as Br and Hci, also need to be considered in selecting a grade. One way to select the suitable grade for your application is 'trial and error'. You can purchase several magnets with different grades (some suppliers have these magnets available on their shelves) and try each grade until you find one right for your application.


5. Shape and Size of Magnets

The properties of the magnet will differ greatly according to the size and shape (Disc, Cylinder, Block & Bar, Cube, Ring & Tube, Sphere & Ball, Arc & Wedge, Other). Take into consideration whether the magnet will fit in the object in which you wish to place it, whether the magnet is too thick or too thin, and then select the size which would be easiest to handle. For round neodymium magnets, we have a wide range of Dia.\Thick from 1/32inch to over 3inch in diameter.


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Permanent magnet manufacturers have a first-class production line, high precision, and good quality products, welcome to discuss! Our permanent magnets have been widely used in the fields of electroacoustics, motors, instruments, meters, automatic control, etc.