What is the difference between inline and surface mount design of NdFeB magnets for permanent magnet motors?
A permanent magnet (PM) motor is a type of AC motor that uses permanent magnets embedded or attached to the surface of the motor rotor. Electric vehicles use permanent magnet synchronous motors with high torque density and high efficiency, using permanent magnets that are neodymium magnets, also known as super magnets. This name is due to the very concentrated magnetic field per square centimeter, which makes them very attractive, their small size, but their high magnetic field density favors strength and efficiency.
Considering the characteristics of permanent magnet motors and the high magnetic field density, the size of the motor is designed to be one third of the size of a motor with the same performance. In addition, the high efficiency minimizes the power consumption of electric vehicles and, last but not least, the magnetic lifetime of super magnets is estimated to be about 400 years, ensuring their efficiency and reliability over time.
Permanent magnet motors (also known as PMs) can be divided into two main categories: embedded permanent magnets (IPM) and surface permanent magnets (SPM), both of which generate magnetic flux through permanent magnets fixed to or inside the rotor. Surface permanent magnets are motors with permanent magnets attached to the circumference of the rotor, and embedded permanent magnets are motors with permanent magnets embedded in the rotor called IPMs.
The biggest advantage of the embedded design is the high-speed performance, which gives it an advantage in traction motors in vehicle applications. The power-speed curve of a surface permanent magnet motor is roughly hyperbolic, rising to a quasi-constant power region over a narrow speed range and then falling.
Surface PM motors have dominated the PM motor market for decades; however, in recent years, the emerging hybrid and electric vehicle markets have boosted demand for inline PM motors. With advantages such as near-constant power over a wide speed range and magnet retention design, embedded PM motors offer a good solution for applications such as traction and auxiliary motors.
In vehicle applications, embedded permanent magnet technology offers significant advantages over surface permanent magnet motors. The embedded PM design allows for more control over the magnetization of the magnetic circuit. In contrast, embedded PM motors offer a greater range of more or less consistent torque using a technique called magnetic field weakening, where the designer can apply current to alter performance, and magnetic field weakening primarily involves adjusting the stator field to partially counter the effects of the PMs.
Surface Permanent Magnets
Surface permanent magnet motors, which have magnets fixed to the outside of the rotor surface, are mechanically weaker than embedded permanent magnet motors, and the weakened mechanical strength limits the maximum safe mechanical speed of the motor. In addition, the magnetic convexity (Ld ≈ Lq) of these motors is consistent with the inductance measured at the rotor terminals regardless of the rotor position. Due to the close magnetic convexity ratio, surface permanent magnet motor designs rely heavily on the magnetic torque assembly to generate torque.
Embedded Permanent Magnets
Unlike surface permanent magnet motors, which have permanent magnets embedded in the rotor, the placement of the permanent magnets makes embedded permanent magnet motors mechanically sound and suitable for high speed operation. These motors also have their relatively high magnetic convexity ratio (Lq>Ld) to define them. Due to their more pronounced magnetic convexity, embedded PM motors are able to generate torque by using the magnetic torque and reluctance torque components of the motor, making them adaptable to various electric vehicles.
Conclusion
For high-speed applications such as traction motors, the embedded permanent magnet motor is the best choice, using less magnet material and gaining some torque from the pronounced magnetic convexity ratio of the rotor. In addition to magnetic torque, reluctance torque is used to achieve high torque, and it can respond to high speed motor rotation by vectoring both types of torque. Together with better control of the magnetization of the magnetic circuit by controlling the current, it is able to operate efficiently over a wide speed range.
Mechanical safety is improved, and the fact that the magnets do not fall off due to centrifugal force, unlike embedded permanent magnets, is an added benefit to the robustness of the rotor. Compared to conventional motors, embedded PM motors can save up to 30% of energy consumption for the same power.
