Choosing the Right Magnet Material for Electric Motor Design

Introduction 

Magnets are materials that produce a magnetic force field around them, which can either pull or repel certain materials such as iron and nickel. Based on their composition and source of magnetism, they can be classified as permanent magnets and regular magnets (temporary magnets). Permanent magnets (PMs) are the type of magnets that can retain their magnetism once magnetized. Whereas regular or temporary magnets behave like permanent magnets in the presence of a magnetic field but lose their magnetic properties if the magnetic field disappears. Electromagnets are an example of a regular magnet; they are wound coils of wire which act as a magnet when an electric current is supplied through the coil. By varying the magnitude and direction of the supply current, the strength of the electromagnet can be altered. Let us look at the different types of magnets. 

Magnets Comparison and their Applications 

There are four main types of magnets: ceramic (also called ferrite), AlNiCo, Samarium Cobalt (SmCo), and Neodymium (NdFeB). The last type is used mostly in traction motors for hybrids and EVs. Each magnet has its benefits and selecting the optimal magnet depends on your application requirements.

Key Characteristics of Magnet Material [1] 

Both rare-earth magnets that are SmCo and NdFeB are extremely strong however, SmCo has better operating temperature and resistance to corrosion. It is mainly made of an alloy of samarium, cobalt, and iron. It is very stable at high operating temperatures and has better resistance to demagnetization. But it is more prone to break when dropped or snapped together. NdFeB magnets are more vulnerable to corrosion, but this can be easily solved by adding a protective layer of copper-nickel plating. NdFeB magnets are the strongest and produce the highest energy. It was developed in the 1980s by combining neodymium, iron, and boron. In the early decades, the NdFeB magnets were very expensive however, they have recently become the most affordable and cost-effective rare earth magnet due to the expansion of the mining operations and the loosening of patent restrictions. AlNiCo and Ceramic magnets produce less energy compared to rare earth magnets and they are cheaper. They exhibit higher corrosion resistance and have better operating temperature capacity. In addition, these magnets can lose their magnetic strength permanently when exposed to extremely cold temperatures.

Commercially, NdFeB magnets come in various grades such as N42, N52, or N42SH. Depending on the application, the N52 grade is recommended for the highest magnetic strength at room temperature. Moreover, the N42 grade NdFeB magnet is the best magnet which provides a great balance between magnetic strength, performance at higher operating temperatures, and cost. We can easily replace the N52 magnet with the N42 by slightly increasing its size. For very high temperatures, consider the N42SH grade magnet. Here are some applications of all types of magnets. 

Magnet Applications

Key Criteria to Select Magnets for Electric Motor Design 

1. The expected design life of the equipment where the magnet is fitted, 
2. Shape and size of magnets,
3. What does the magnet have to hold, lift, or attract,
4. Corrosion resistance,
5. Maximum operating temperature,
6. Demagnetizing resistance,
7. The magnetic strength required,
8. Corrosion resistance,
9. Cost. 

Different Magnet Shapes [3] 

Design Challenges 

Designing an electric machine is a challenging task as its electromagnetic, thermal, and mechanical performances are coupled and need to be taken into consideration at the same time. Based on the design constraints and user demands, the motor designer has many choices and criteria to take care of. 

• Selection of motor topology,
• Optimizing design parameters and solving multiple design scenarios – Ceramic, AlNiCo, NdFeB and SmCo,
• Usage of new materials to improve machine efficiency,
• Electromagnetic performance under no-load and on-load conditions,
• Comparing the results of all the motors. 

So, estimating these parameters and losses that generate heat in the motor are the major challenges while designing the motor. Another characteristic of the motor is the high-speed operation during the flux weakening mode and how to obtain its efficiency plot at different torque and speed. Also, determining the structural deformation and stress caused by the stator and rotor due to magnetic force is very important. So, to make these design processes simple and easier, EMWorks has various motor solution products.

Design Solutions for Electric Motors by EMWorks 

EMWorks Solutions for Electric Machines 

First, we have the MotorWizard tool. It is a template-based motor design software that allows users to accurately solve both electric and magnetic problems. It includes electrostatic, magnetostatic, and transient solvers equipped with integrated analytical and finite element-based solvers.

Second, we have the EMWorks2D tool. It is a 2D electromagnetic simulation software that uses finite elements to solve magnetic, electric, and transient problems. EMWorks2D allows you to study the effects of the geometric or simulation parameter changes on the design. It also allows coupling a transient magnetic study to mechanical motion and thermal.

Third, we have the EMS tool. It enables users to do both electric and magnetic simulations using the complete 3D geometry. EMS is a true multi-physics software that enables users to couple the magnetic and electric design to Circuit, Motion, Thermal, and Structural analyses on the same model in a hassle-free integrated environment.

EMWorks2D Design Specifications

To have reasonable comparative results, there are many ways one can compare these different models. We can compare the machine based on power density but as the power is the product of torque and speed, one can increase the speed of the machine and can say my motor has a higher power density compared to others. One can play with the magnet materials' properties to improve the performance of the machine. Some designers also fix the input DC supply voltage or current density of the motor and then compare the performances. So, in short various parameters can be kept fixed as a constraint by clients or users and other parameters can be varied to compare motor performance.

In this blog, the different magnet materials are used to compare the 12 slots and 8 pole motor performance as listed in the table below. Keeping the supply current, stator outer diameter, stack length, core material, and base speed the same for all designs. 

Schematic Drawing of Designed Surface-Mounted PM Motor

EMWorks2D Model Settings and Results 

To begin the performance analysis in EMWorks 2D software, first, different study settings needed to be done such as materials, boundary conditions, mesh, and windings. Once we click on the material tab, we can assign different materials to the motor parts from the material database. Then, we can set the outer boundary as air, select rotor parts for torque and force calculation, and then set the mesh control of the motor. The next step is winding settings, you can choose either current-driven or voltage-driven, set the amplitude and frequency of the supply, number of turns, its diameter, and select the direction of the current. After we are done with these settings, we run the analysis; once it is completed, we can check the performance curves using the result tab. 

Model Settings in EMWorks2D

Here is the electromagnetic performance of all four designs. Back EMF waveforms were obtained at the base speed of 1000 rpm and with zero current excitation which is basically  at no load. Other important results while designing a motor are the cogging torque and their ripple magnitude. Cogging torque occurs due to the interaction between the slotted structure of the stator core and the rotor PMs. There are various techniques that have been proposed for minimizing this cogging torque, such as skewing, magnet shaping, and shifting. 

Back EMF

Cogging Torque

Next is the magnet flux linkage which is the rate of a magnetic field linking through a given conductor. This waveform was also obtained at the base speed of the motor with no load. Similarly, the slot current density should be limited to the type of cooling technology used in the motor. For example, if it is a liquid-cooled type then 8 to 25 A/mm2 is good enough. Whereas if it forced air-cooled type motors then the current density should not exceed more than 8 A/mm2. 

No-Load Flux Linkage

Input Current  and Current Density

The electromagnetic torque is obtained at the base speed and with a full-rated load current. Other important plots are the flux density and current density plots. Using these instantaneous plots, the motor designer can predict the behavior of the flux path and the region where it is saturating.

Electromagnetic Torque

Magnetic Flux Density Plots

Performance Comparison

To sum up, we can see in the table that the base speed, input current excitation, stator outer diameter, and stack length are kept the same for all four designs. To compare them, we have calculated the volume of each motor to obtain its torque density. It was found that the torque density is highest for the NdFeB design with 25.58 kNm/m3. 

Performance Comparison of Different Motor Topologies

Conclusion

Different topologies of surface-mounted permanent magnet motors were simulated, and their electromagnetic and thermal performance was obtained using EMWorks2D software. It was observed that the PM motor with a NdFeB type magnet yielded the highest torque density, and the lowest torque density was achieved with a ceramic type of magnet. In a nutshell, the selection of the motor topology is a trade-off between the constraints and the performance requirement of the end product. 

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