Improvements in the performance of magnets in permanent magnet motors have significantly promoted their development. This article will discuss the details of magnets in permanent magnet motors.
Definition of Permanent Magnet Motor
A permanent magnet motor generates the magnetic field using permanent magnets, eliminating the need for excitation coils or excitation currents. It features a simple structure and high efficiency, making it an excellent energy-saving motor.
With the advent of high-performance permanent magnetic materials and advancements in control technology, permanent magnet motors have significant advantages over traditional electrically excited motors. These include a more straightforward structure, reliable operation, smaller size, lighter weight, reduced losses, higher efficiency, and flexible motor shapes and sizes. Consequently, they find a wide range of applications.
Movement of Magnets in Permanent Magnet Motors
The magnets can be located on the rotor or the stator.
When the magnets in permanent magnet motors are located on the rotor, the magnets rotate, and the coils are stationary. This structure is used in permanent magnet synchronous motors and brushless DC motors.
When the magnets in permanent magnet motors are on the stator, the magnets are stationary, and the coils rotate. This configuration is typically used in permanent magnet DC motors.
Types of Magnets Used in Permanent Magnet Motors
Common permanent magnetic materials used in motors include ferrite magnets, samarium-cobalt magnets, and neodymium-iron-boron magnets.
| Neodymium-Iron-Boron Magnets: Neodymium-iron-boron is the most widely used permanent magnet material in permanent magnet motors. It has high magnetic energy density and coercivity, often referred to as high-strength magnets. Motors utilizing neodymium-iron-boron components are small and lightweight, with a high cost-performance ratio.
| Samarium-Cobalt Magnets: Samarium-cobalt magnets have very stable magnetic properties and greater temperature resistance than neodymium-iron-boron magnets. They are particularly suitable for manufacturing motors, but due to their high cost, they are mostly used in high-tech fields such as aerospace and military applications, where motor performance is prioritized over cost.
| Alnico Magnets: Alnico permanent magnets are one of the earliest widely used permanent magnetic materials, with a mature preparation process and technology.
| Ferrite Magnets: Ferrite materials are resistant to oxidation, have a high Curie temperature, and are low-cost, making them suitable for medium and low-end motors. However, they are relatively large and do not have the magnetic performance of neodymium-iron-boron or samarium-cobalt magnets.
Position of Magnets in Permanent Magnet Motors
Depending on how the permanent magnets are arranged, motors can be classified into Interior Permanent Magnet Motors (IPM) and Surface Permanent Magnet Motors (SPM).
| Surface Permanent Magnet Motors: The permanent magnets face the air gap directly, making processing and installation easier, which offers benefits like a simpler structure, relatively small axial size, and high transmission efficiency.
| Interior Permanent Magnet Motors: The permanent magnets are placed inside the core, which allows for more magnets to be placed, improving the gap magnetic density. These motors have a smaller diameter, the power density is very high, and the moment of inertia is low.
Usages of Magnets in Permanent Magnet Motors
Magnets in permanent magnet motors may use one type or multiple types.
When only one type of permanent magnetic material is utilized in a motor, it is called a single-material structure, which is the case for the majority of motors.
If two or more types of permanent magnetic materials are used in the same motor, it is referred to as a hybrid structure, which improves motor performance and reduces manufacturing costs.
Shapes of Magnets in Permanent Magnet Motors
When designing permanent magnets, it is essential to ensure that the magnets in permanent magnet motors produce sufficient magnetic flux and magnetomotive force in the magnetic circuit. The shape of the magnets varies with the type of permanent magnetic material used.
| Tile Shape: Tile-shaped magnets are widely used in permanent magnet motors. There are typically two types: concentric tile-shaped and equal radius tile-shaped. Since the neodymium-iron-boron material is usually rectangular, wire cutting methods are commonly used for processing. The material utilization rate of the concentric tile shape is relatively low, generally around 40%-50%. Using an equal radius tile-shaped structure can improve material utilization and reduce wire cutting costs.
| Ring Shape: This consists of a single, continuous circular ring that is straightforward in structure and convenient for processing and assembly, widely used in permanent magnet DC motors. The disadvantage is low material utilization and the presence of a magnetic field on the geometric neutral axis, which is not advantageous for commutation in permanent magnet DC motors.
| Rectangular Shape: Consisting of one or several rectangular permanent magnets, rectangular magnets offer a simple structure, ease of processing, and high material utilization. They are commonly used in built-in permanent magnet synchronous motors. When designing the shape of permanent magnets, the length-to-thickness ratio of rectangular magnets should avoid exceeding 20.
Factors Affecting Motors Due to Magnets in Permanent Magnet Motors
| Impact of Remanence: For DC motors, under the same winding parameters and testing conditions, higher residual magnetism results in lower no-load speeds and currents. Conversely, it increases maximum torque and enhances efficiency at peak efficiency points. In practical tests, the level of residual magnetism is generally assessed based on no-load speed and maximum torque.
| Impact of Coercivity: During operation, temperature and reverse demagnetizing fields consistently affect the motor. From a design perspective, higher coercivity allows for thinner materials to be used in the magnetic steel. Conversely, lower coercivity requires thicker magnetic steel. However, beyond a certain coercivity threshold, it becomes ineffective since other components of the motor cannot reliably operate at such temperatures. Coercivity should meet the requirements under recommended testing conditions without wasting resources.
| Impact of Performance Consistency: Inconsistencies in residual magnetism are undesirable; even if some magnets exhibit high performance, inconsistent magnetic flux across the unidirectional field sections can lead to torque asymmetry and vibration. Inconsistent coercivity, especially for products with particularly low coercivity, can easily result in reverse demagnetization, leading to uneven magnetic flux and motor vibration. This issue is especially pronounced in brushless motors.
Magnet Manufacturer
At Tengye, we’re thrilled to be early participants in your new projects. Our passion for magnets extends from material selection and design optimization to providing magnetic solutions. We hope to bring our knowledge and experience to the forefront by actively engaging in your upcoming projects. Work with our magnet experts to get the right magnet solution.
Image Source: Jdamagnet & Lesimpressionsnouvelles & Mdpi & Openpr & Web & Researchgate & Stanfordmagnets
