The differences between surface permanent magnet motors and interior permanent magnet motors are the topics that we’ll discuss in this article.
Permanent magnet motors (PM) are a type of AC motor that uses magnets built into or mounted on the motor rotor surface to generate a magnetic field. In electric vehicles, permanent magnet synchronous motors, known for their high torque density and efficiency, are commonly used. The magnets used in these motors are neodymium iron boron magnets, also known as super magnets or powerful magnets.
These magnets are characterized by an extremely concentrated magnetic field, with high magnetic field intensity per square centimeter, giving them strong attractive forces. Despite their small size, the high magnetic field density helps enhance the strength and efficiency of the motor.
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Due to the high efficiency and magnetic field density of permanent magnet motors, motors of the same performance can be one-third the size of traditional motors. Moreover, the high efficiency minimizes energy consumption in electric vehicles. Importantly, the magnetic life of supermagnets can be as long as around 400 years, ensuring sustained efficiency and reliability over time.
Permanent magnet motors can be divided into two types: Interior Permanent Magnet Motors (IPM) and Surface Permanent Magnet Motors (SPM). Both types generate magnetic flux by fixing permanent magnets on or inside the rotor. Surface permanent magnet motors attach magnets to the external surface of the rotor, while interior permanent magnet motors embed magnets inside the rotor.
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The Influence of Hybrid and Electric Vehicles
The major advantage of interior permanent magnet motors lies in their high-speed performance, making them particularly advantageous in the applications of vehicles. On the other hand, the power-speed curve of surface permanent magnet motors roughly forms a hyperbolic shape, gradually rising to a quasi-constant power region within a narrow speed range and then decreasing.
For decades, surface permanent magnet motors have dominated the permanent magnet motor market. However, the rise of hybrid and electric vehicles in recent years has driven an increased demand for interior permanent magnet motors. Interior permanent magnet motors exhibit constant power over a wider speed range and offer good solutions for applications like traction and auxiliary motors.
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In vehicle applications, interior permanent magnet motors have distinct advantages over surface permanent magnet motors. The design of interior permanent magnet motors allows better control of magnetic path magnetization. They can provide a larger range or more consistent torque, and through a technique, designers can change their performance by applying current. The technique mainly involves adjusting the stator magnetic field to partially counteract the influence of the permanent magnet.
Surface Permanent Magnet Motors
Surface permanent magnet motors fix magnets to the rotor’s surface, and their mechanical strength is lower, limiting the motor’s maximum safe mechanical speed. Additionally, regardless of the rotor’s position, the measured inductance values at the rotor terminals remain consistent. So, surface permanent magnet motor design largely relies on the magnetic torque component to generate torque.
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Interior Permanent Magnet Motors
Interior permanent magnet motors embed magnets inside the rotor, offering better mechanical performance and suitability for high-speed operation. These motors also have a relatively higher Lq/Ld ratio. Because of this, interior permanent magnet motors can generate torque by utilizing both magnetic torque and reluctance torque, adapting to various electric vehicle requirements.
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The Vision of IPM and SPM
For high-speed applications like traction motors, interior permanent magnet motors are the optimal choice, using fewer magnet materials and achieving a certain degree of torque. In addition to magnetic torque, these motors use reluctance torque to achieve high torque, responding to the rotation of high-speed motors through vector control.
Simultaneously, by better controlling the magnetization of the magnetic path, interior permanent magnet motors can operate efficiently over a wide speed range. Mechanical reliability is also improved, as magnets do not detach due to centrifugal force, and the rotor is more robust. Under the same power, interior permanent magnet motors can save 30% of energy consumption.
