The “Center” of Permanent Magnet Motors – Permanent Magnets

The world’s first electric motor that emerged in the 1820s was a permanent magnet motor that generated an excitation magnetic field from a permanent magnet. But the permanent magnetic material used then was natural magnet ore (Fe₃O₄), with low magnetic energy density. The motor made of it was huge and was soon replaced by an electric excitation motor.
With the rapid development of various motors and the invention of current magnetizers, in-depth research has been conducted on the mechanism, composition, and manufacturing technology of permanent magnet materials. Various permanent magnet materials have been discovered, including carbon steel, tungsten steel (with a maximum magnetic energy product of about 2.7kJ/m³), cobalt steel (with a maximum magnetic energy product of about 7.2kJ/m³), and so on. Especially with the emergence of aluminum nickel cobalt permanent magnets in the 1930s (with a maximum magnetic energy product of 85kJ/m³) and ferrite permanent magnets in the 1950s (with a maximum magnetic energy product of 40kJ/m³), the magnetic properties have greatly improved. Various micro and small motors have used permanent magnets for excitation. The power of permanent magnet motors ranges from a few milliwatts to tens of kilowatts, and they are widely used in military, industrial, and agricultural production and daily life, resulting in a sharp increase in output. Correspondingly, breakthroughs have been made in the design theory, calculation methods, magnetization, and manufacturing technology of permanent magnet motors during this period, forming a set of analysis and research methods represented by the graphical method of permanent magnet working diagrams. However, the coercivity of aluminum nickel cobalt permanent magnets is relatively low (36-160kA/m), and the residual magnetic density of ferrite permanent magnets is not high (0.2-0.44T), which limits their application range in motors. Until the 1960s and 1980s, rare earth cobalt permanent magnets and neodymium iron boron permanent magnets (collectively referred to as rare earth permanent magnets) were successively introduced. Their high residual magnetic density, high coercivity, high magnetic energy product, and excellent magnetic performance with linear demagnetization curves were particularly suitable for manufacturing motors, thus ushering in a new historical period for the development of permanent magnet motors.

Permanent magnet material

• Motor magnet: The commonly used permanent magnet materials in motors include sintered magnets and bonded magnets, mainly including aluminum nickel cobalt, ferrite, samarium cobalt, neodymium iron boron, etc.
• Aluminum nickel cobalt: Aluminum nickel cobalt permanent magnet material is the earliest widely used permanent magnet material, and its preparation process and technology are relatively mature. Currently, there are factories in Japan, the United States, Europe, Russia, and China for production. Among large-scale production enterprises, Hangzhou Permanent Magnet currently ranks first in China in terms of output, with an annual production capacity of 3000 tons.
• Permanent magnet ferrite materials: In the 1950s, ferrite began to flourish, especially in the 1970s, when strontium ferrite, with good performance in coercivity and magnetic energy machines, was put into production in large quantities, rapidly expanding the use of permanent magnet ferrite. As a non-metallic magnetic material, ferrite has no drawbacks such as easy oxidation, low Curie temperature, and high metal permanent magnetic materials cost, making it highly popular.
• Samarium cobalt material: a permanent magnet material with excellent magnetic properties that emerged in the mid-1960s, and its performance is very stable. Samarium cobalt is particularly suitable for manufacturing motors in terms of magnetic properties. Still, due to its high price, it is mainly used in the research and development of military motors in aviation, aerospace, weapons, and high-tech fields where high performance is not the main factor in price.
• Neodymium iron boron material: Neodymium iron boron magnetic material is an alloy of neodymium, iron oxide, etc., also known as magnetic steel. The advantages of extremely high magnetic energy product and coercive force, as well as high energy density, have made neodymium iron boron permanent magnet materials widely used in modern industry and electronic technology, making it possible to miniaturize, lighten and thin equipment such as instruments, electroacoustic motors, magnetic separation, and magnetization. Due to the high content of neodymium and iron, it is prone to rusting. Surface chemical passivation is currently one of the best solutions.

Corrosion resistance, maximum operating temperature, processing performance, demagnetization curve shape, and price comparison of commonly used permanent magnet materials for motors (Figure)

Performance	Aluminum nickel cobalt	Ferrite	Rare earth cobalt	NdFeB
Residual Magnetism/T	1.3	0.42	1.05	1.16
Correction Force/(Ka/M)	60	200	780	850
Demagnetization Curve Shape	Bend	Upper straight, lower curved	Straight line	Straight line (bending at high temperature)
Residual Magnetic Temperature Coefficient/(%/K)	-0.02	-0.18	-0.03	-0.12
Corrosion Resistance	Strong	Strong	strong	Easy to oxidize
Magnetization	Magnetization after installation	Magnetized and mounted (also magnetized after mounting)	Magnetization post installation	Installation after magnetization
Maximum Operating Temperature ° C	550	200	300	150
Processing Performance	Minor grinding, electric discharge machining	Special prop slicing and small amount of grinding	Small amount of EDM	Good processability
Price	Secondary	Low	Very high	High
Application Occasions	Places where high temperature stability is required for instruments and meters	Low performance and volume requirements, low price requirements	High performance, high temperature, high temperature stability, where price is not a major consideration	High performance, high volume requirements. Occasions where temperature is not high

The relationship between magnetic steel performance and motor performance

1. The influence of residual magnetism
For DC motors, under the same winding parameters and testing conditions, the higher the residual magnetism, the lower the no-load speed, and the smaller the no-load current; The greater the maximum torque, the higher the efficiency at the highest efficiency point. In actual testing, the remanence standard of magnetic steel is generally determined by the level of no-load speed and the maximum torque.
For the same winding parameters and electrical parameters, the reason why the residual magnetism is higher, the no-load speed is lower, and the no-load current is smaller is that the running motor generates sufficient reverse induced voltage at a relatively low speed, which reduces the algebraic sum of the electromotive force applied to the winding.
2. The influence of coercive force
During the operation of the motor, there is always the influence of temperature and reverse demagnetization field. From the perspective of motor design, the higher the coercive force, the smaller the thickness direction of the magnetic steel. The smaller the coercive force, the greater the thickness direction of the magnetic steel. But after the magnetic steel exceeds a certain coercive force, it is useless because other motor components cannot work stably at that temperature. The coercivity can meet the requirements and is based on meeting the requirements under the recommended experimental conditions without wasting resources.
3. The influence of squareness
Squareness only affects the straightness of the efficiency curve in motor performance testing. Although the straightness of the motor efficiency curve has yet to be listed as an important indicator standard, it is very important for the driving distance of hub motors under natural road conditions. Due to different road conditions, motors can only sometimes operate at the maximum efficiency point, which is one of the reasons why some motors have low maximum efficiency and long driving distances. A good hub motor should have high maximum efficiency and an efficiency curve as horizontal as possible, with a smaller slope of efficiency reduction being better. With the maturity of the market, technology, and standards for hub motors, this will gradually become an important standard.
4. The impact of performance consistency
Inconsistent residual magnetism: Even some with particularly high performance could be better, as the magnetic flux in each unidirectional magnetic field section is inconsistent, resulting in asymmetric torque and vibration.
Inconsistent coercive force: Especially for some products, the coercive force is too low, which can easily cause reverse demagnetization, resulting in the inconsistent magnetic flux of each magnetic steel and causing motor vibration. This effect is more significant for brushless motors.

The Influence of the Shape and Tolerance of Magnetic Steel on the Performance of Electric Motors

1. The influence of magnetic steel thickness
When the inner or outer magnetic circuit coils are fixed, as the thickness increases, the air gap decreases, and the effective magnetic flux increases. The obvious manifestation is that under the same residual magnetism, the no-load speed decreases, the no-load current decreases, and the maximum efficiency of the motor increases. However, there are also unfavorable aspects, such as an increase in the commutation vibration of the motor and a relative steepening of the efficiency curve of the motor. Therefore, the thickness of the motor magnetic steel should be as consistent as possible to reduce vibration.
2. The influence of magnetic steel width
For densely arranged brushless motor magnets, the total cumulative gap cannot exceed 0.5mm. If it is too small, it will not be able to be installed, and if it is too small, it will lead to motor vibration and efficiency reduction. This is because the position of the Hall element measuring the position of the magnet does not correspond to the actual position of the magnet, and consistency in width must be ensured; otherwise, the efficiency of the motor will be low, and the vibration will be large.
For brushless motors, there is a certain gap between the magnetic steels, which is reserved for the mechanical commutation transition zone. Although there are gaps left, most manufacturers have strict magnetic steel installation procedures to ensure the accurate installation position of the motor magnetic steel to ensure installation accuracy. If the width of the magnetic steel exceeds, it will not be able to be installed; If the width of the magnetic steel is too small, it will cause misalignment of the magnetic steel positioning, increase the vibration of the motor, and reduce efficiency.
3. The influence of chamfer size and nonchamfer of magnetic steel
If there is no chamfer, the magnetic field change rate at the edge of the motor’s magnetic field is large, causing pulsation of the motor. The larger the chamfer, the smaller the vibration. However, chamfering generally has a certain loss of magnetic flux, and for some specifications, when chamfering to 0.8, the magnetic flux loss is 0.5-1.5%. When the residual magnetism of a brush motor is low, appropriately reducing the chamfer size is beneficial for compensating for residual magnetism, but the pulse vibration of the motor increases. Generally speaking, when the residual magnetism is low, the tolerance in the length direction can be appropriately enlarged, which can improve the effective magnetic flux to a certain extent and keep the motor performance unchanged.

Relevant precautions for permanent magnet motors

1. Magnetic Circuit Structure and Design Calculations
To fully utilize the magnetic properties of various permanent magnet materials, especially the excellent magnetic properties of rare earth permanent magnets, and manufacture high-performance permanent magnet motors, it is necessary to establish new design concepts, reanalyze and improve the magnetic circuit structure by simply applying the structure and design calculation methods of traditional permanent magnet motors or electric excitation motors. With the rapid development of computer hardware and software technology, as well as the continuous improvement of modern design methods such as electromagnetic field numerical calculation, optimization design, and simulation technology, and through the joint efforts of the academic and engineering communities of electric motors, breakthroughs have been made in the design theory, calculation methods, structural technology, and control technology of permanent magnet motors, A complete set of analysis and research methods, as well as computer-aided analysis and design software, has been developed that combines numerical calculations of electromagnetic fields with analytical solutions of equivalent magnetic circuits and is constantly being improved.
2. Control issues
After being manufactured, permanent magnet motors can maintain their magnetic field without needing external energy, making it extremely difficult to adjust and control their magnetic field from the outside. It is difficult for permanent magnet generators to adjust their output voltage and power factor externally, and permanent magnet DC motors cannot adjust their speed by changing the excitation. These limits the application range of permanent magnet motors. However, with the rapid development of power electronic devices and control technologies such as MOSFETs and IGBTs, most permanent magnet motors can be applied without magnetic field control and only with armature control. When designing, it is necessary to combine three new technologies: rare earth permanent magnet materials, power electronic devices, and microcomputer control to enable permanent magnet motors to operate under new operating conditions.
3. Irreversible demagnetization problem
If designed or used improperly, a permanent magnet motor may experience irreversible demagnetization or loss of excitation under the armature reaction generated by the impact current or severe mechanical vibration when the temperature is too high (neodymium iron boron permanent magnet) or too low (permanent ferrite magnet), resulting in reduced motor performance and even inability to use. Therefore, it is necessary to research and develop methods and devices for checking the thermal stability of permanent magnet materials suitable for motor manufacturers, to analyze the anti demagnetization ability of various structural forms, to adopt corresponding measures to ensure that permanent magnet motors do not lose excitation during design and manufacturing.
4. Cost issues
Ferrite permanent magnet motors, especially micro permanent magnet DC motors, have been widely used due to their simple structure and process, reduced mass, and lower total cost than electric excitation motors. Due to the current high price of rare earth permanent magnets, the cost of rare earth permanent magnet motors is generally higher than that of electric excitation motors, which needs to be compensated with its high performance and operational cost savings. In some situations, such as voice coil motors for computer disk drives, using neodymium iron boron permanent magnets improves performance, significantly reduces volume and mass, and reduces overall cost. In the design process, it is necessary to not only compare performance and price based on specific usage scenarios and requirements but also innovate structural processes and optimize the design to reduce costs.