How to Choose Permanent Magnets?

How to Choose Permanent Magnets?


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


One of the most important parameters to be considered when selecting permanent magnets is the operating temperature. It can be imagined that the magnetic field is not constant, but changes with the change of temperature, either stronger or weaker. The following figure shows the available temperature range for each of the five commercial materials. Conveniently, most applications use magnets between - 40  °C and 150  °C, and all listed materials can be used within this range.
Because of its low cost, ferrite (hard ferrite, ceramics) magnets are still widely used. It should be noted, however, that ferrite magnets lose 25% of their flux output at temperatures of  20 to 150 °C. They also have relatively low energy products, usually 10% rare earth magnets. Therefore, rare earth magnets are necessary where the size and weight of devices are important, such as hard disk drives and portable electronic devices (such as mobile phones and hearing aids).


The price and availability of raw materials are at the core of today's discussion on permanent magnets. These factors are influenced by the general availability of raw materials, geographical distribution, recoverability of extracted ores and open market transactions. Of all the elements used in magnetic materials, rare earth elements are the most problematic, based on geographical constraints of origin. China still produces 90% of rare earth oxides and more than 95% of rare earth metals and alloys every year. China also produces and consumes 80% of permanent magnets. In the United States, the production of NdFeB is meaningless. Smco is an alternative to neodymium magnets, which has fewer source constraints and performs better in high temperature applications.


The strength of magnets can be quantified by one of two key indicators. The first is residual induction. If the permanent magnet is placed on the steel block, the force required to remove the magnet from the steel block is proportional to Br. BR is a magnet characteristic issued by the manufacturer of magnets.
The second key indicator is the maximum energy product (bh), which can be used to calculate the performance of the motor. (bh) Max is proportional to br2. Each type of device can benefit from B (that is, Br) or B2 (that is, (Bh) Max or Br2), as described in this application table.


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