What Are Motor Magnets?

There are many kinds of motors while their working principle is same that magnetic field is formed once setting up an electric circuit and the mutual conversion of electric energy and kinetic energy is realized by electromagnetic induction.

What are the benefits of permanent magnet motor?

The permanent magnet motor is an energy-saving motor features high efficiency and simple structure which magnetic field is formed by permanent rare earth magnets without magnetic coil and exciting current. Compared with the traditional excitation motor, the permanent magnet motor has the remarkable advantages of simple structure, reliable operation, small size, light weight, low loss and high efficiency which is wildly used in the fields of aerospace, national defense, industrial & agricultural production and daily life. With the development of high-performance permanent magnet materials and the rapid development of control technology, the application of permanent magnet motors will be wider and wider.

What are motor magnets?

Motor magnets are the magnets applied in various types of motors and related permanent magnet rotors and stators. Motor magnet materials include ferrite permanent magnets, AlNiCo permanent magnets, SMCO permanent magnets, NdFeB permanent magnets, etc. According to different motor design principles and actual applications, the shape of motor magnets refer to rectangle, disc, arc, ring, segment, etc.

What are the relationship between motor magnets and permanent magnet motors?

The development of permanent magnet motor closely related to the development of motor magnet. The world's first motor that appeared in the 1820s was a permanent magnet motor with the magnetic field formed by permanent magnets. However, the permanent magnet material used at the time was natural magnetic iron ore (Fe3O4). Due to its low magnetic properties, the motor made from it was relatively bulky and was soon replaced by excitation motor.

With the development of science and demands of various motors, scientists have conducted in-depth researches on the mechanism, composition and manufacturing technology of permanent magnet materials and have successively discovered carbon steel and tungsten steel((BH)max 2.7 kJ/m3 ), cobalt steel ((BH)max 7.2kJ/m3) and other permanent magnet materials. Various micro and small motors are made from permanent magnets again as magnets especially the AlNiCo permanent magnets in the 1930s ((BH) max can reach to 85kJ/m3) and the ferrite permanent magnets in the 1950s ((BH) max can reach to 40kJ/m3) have an improved magnetic property. However, the coercivity force of AlNiCo permanent magnets is low (Hcb: 36-160 kA/m), and the remanence density of ferrite permanent magnets is not high (Br: 0.2-0.44 T), which limits their application in motors. Samarium cobalt permanent magnet materials came out until the 1960s. The intermetallic compound SM2CO17 permanent magnet material formed by rare earth element samarium and metal cobalt, zirconium and iron, which maximum magnetic energy product can reach to about 150 times that of carbon steel, 8 to 10 times that of permanent ferrite magnet, and 3 to 5 times that of permanent AlNiCo magnets. With the advantages of high remanence density, high coercivity, high magnetic energy, low temperature coefficient, and stable magnetic strength, SM2CO17 permanent magnet material is very suitable for the manufacture of motors, especially for low-speed torque motors, starting motors, sensors and magnetic systems like magnetic bearings while its disadvantages are that products are fragile and at high cost. Whatever the disadvantages, the appearing of SMCO permanent magnet materials has led the development of permanent magnet motors to a new historical period. In the 1980s, the third generation of rare earth permanent magnet materials came out that ND2FE14B is the main phase of the NdFeB permanent magnet material with a small amount of neodymium-rich phase and have higher remanence, coercivity and maximum energy product than permanent samarium cobalt magnet materials. It is the most popular magnet material used in permanent magnet motors with good mechanical properties, low alloy density which is conducive to make light, thin, small and ultra-miniaturized magnetic components. However, due to its higher magnetic temperature coefficient and poorer corrosion resistance, its application in high temperature and harsh environments is limited.

What are the benefits of rare earth motor magnets?

Abovementioned advantages of motor magnets: high remanence density, high coercivity, and high-energy product, which make the permanent magnet motors smaller and lighter while the motor efficiency is not reduced or even higher. In addition, compared with common motors, the rare earth permanent magnet motors have features of no coil and iron core, no energy loss, no heating ,smaller size & weight and higher efficiency which is an ideal replacement for common motors . Rare earth permanent magnet motors in the automobile industry already replaced the traditional motors, which reduces the volume by 40% to 70%, increase the efficiency by more than 50%, and save copper and electricity.

Its remarkable energy-saving effect and the advantages of reducing carbon emissions are in line with the general trend of energy conservation and environmental protection in the world. Therefore, high-efficiency rare earth permanent magnet motors are replacing traditional motors in large quantities, and the demands for direct-drive permanent magnets, wind power generations, energy-saving home appliances, energy-saving elevators, inverter air conditioners, hybrid vehicles, new energy vehicles, and high-efficiency motors for rail transit are booming. The huge demand is still growing, and new materials are still not available in terms of device miniaturization requirements, such as hard disk voice coil motor (VCM), DVD optical drive/player motor, mobile phone vibration motor, small & light automobiles.

Therefore, rare earth motor magnets are widely used in motors at present, and the demand is sting increasing.

How to choose motor magnets?

When choosing motor magnets, we must start from to lea the following aspects:

1. What is the relationship between the performance of the permanent magnet motor and the remanence of the motor magnets?

In short, the greater remanence the motor magnet has the stronger magnetic strength the magnet will have. For a DC permanent magnet motor, under the same winding parameters and test conditions, the higher the magnetic force is, the lower the no-load speed and no-load current will be and the greater the maximum torque is, the higher efficiency it will have. In the actual test, the standard remanence of the motor magnets is generally judged by the level of no-load speed and the maximum torque.

2. How does the coercivity（Hcj） of the motor magnets affect the motor performance?

The working temperature affects the progress of reverse demagnetization during the process of operation. From the perspective of motor design, the higher the coercive force of the motor magnet is, the stronger the ability to resist the reverse demagnetization will have, there is no point preferring a certain high coercivity as other components of the motor will be fail to work stably at a higher temperature. Therefore, Based on the required motor performance, the manufacturing cost of the motor cut by reducing the thickness of motor magnets. On the contrary, the smaller the coercive force is, the greater the thickness of the magnets should be to increase the resistance to reverse demagnetization.

3. How does the squareness（Hk/Hcj RATE） of the motor magnets affect the performance of the motor?

The squareness（Hk/Hcj RATE） of the motor magnet determines the straightness of the test curve of motor performance under extreme conditions. The better the squareness is, the higher the working limit of the motor will have. Under the same operating limits, the better the squareness, the better the stability of the motor will have. Based on the required application conditions, the purchase cost of magnets is cut by reducing the magnet length.

4. What effect does the consistency of motor magnets performance have on motors?

A permanent magnet motor usually has multiple pairs of magnets inside. If the remanence of each magnet is different, that is the magnetic flux of each magnet becomes nonuniform, the motor will be shaking caused by asymmetric torque, and the running sound of motors will be much louder. If the consistency of coercive force is not good, especially the coercive force of some magnets is too low, the speed of reverse demagnetization will be much higher, and eventually the motor will be shaking as the magnetic flux of each magnet become nonuniform

5. How to choose the temperature resistance grade for the motor magnets?

It depends on the actual temperature of the motor operation environment and is fine if it does not exceed the temperature limit. Demagnetization happens if the working temperature is over the limit, which leads to the failed motor operation caused by the disappearance of the magnetic field. Therefore, normally temperature resistance grade for motor magnets will be higher than the designed one.

Conclusion:

Under the general trend of energy saving and environmental protection in the world, the industrial production of rare earth permanent motor magnets with high remanence density, high coercivity and high magnetic strength is an important precondition for the large-scale application of permanent magnet motors.

April 21 2022 Posted by Mag Spring® – Green magnetic solution expert

TAGS: What are motor magnets? What are the benefits of rare earth motor magnets? How to choose motor magnets? motor magnets,