Due to their compact size and high torque density, permanent magnet synchronous motors (PMSMs) are widely used in many industrial applications, particularly in high-performance drive systems such as submarine propulsion systems. PMSMs eliminate the need for slip rings for excitation, thus reducing rotor maintenance and wear. Their high efficiency makes them suitable for high-performance drive systems such as CNC machine tools, robots, and automated production systems in industry.
Typically, the design and construction of permanent magnet synchronous motors must take into account both the stator and rotor structures to achieve high-performance motors.
Construction of a permanent magnet synchronous motor
Air gap flux density: Determined based on asynchronous motor design, etc., and considering the special requirements and techniques of permanent magnet rotor design and use of switching stator windings. Furthermore, assuming a slotted stator, the air gap flux density is limited by stator core saturation. In particular, the peak flux density is limited by the tooth width, while the maximum total flux is determined by the back of the stator.
Furthermore, the permissible saturation level depends on the application. Typically, high-efficiency motors have lower flux densities, while motors designed for maximum torque density have higher flux densities. Peak air gap flux density is typically in the range of 0.7–1.1 Tesla. It should be noted that this is the total flux density, i.e., the sum of the rotor and stator flux. This means that lower armature reaction forces imply higher alignment torque.
However, to achieve a large reluctance torque contribution, the stator reaction force must be substantial. Machine parameters indicate that a large inductance (m) and a small inductance (L) are primarily required to obtain the alignment torque. This is generally suitable for operation below base speeds, as high inductance reduces the power factor.
Permanent magnet materials:
Magnets play a vital role in many devices, making it crucial to improve the performance of these materials. Currently, attention is focused on materials based on rare-earth and transition metals, which can yield permanent magnets with high magnetic properties. Depending on the technology used, magnets exhibit different magnetic and mechanical properties, as well as varying degrees of corrosion resistance.
Neodymium iron boron (Nd2Fe14B) and samarium cobalt (Sm1Co5 and Sm2Co17) magnets are among the most advanced commercially available permanent magnet materials today. A wide variety of grades are available within each class of rare-earth magnets. Neodymium iron boron magnets began commercialization in the early 1980s. They are widely available today and used in many different applications. The cost of this magnet material (per unit of energy) is comparable to that of ferrite magnets; per kilogram, neodymium iron boron magnets cost approximately 10 to 20 times more than ferrite magnets.
Some important properties used to compare permanent magnets are: remanence (Mr), which measures the strength of the permanent magnet's magnetic field; coercivity (Hcj), the material's resistance to demagnetization; energy product (BHmax), the density of magnetic energy; and Curie temperature (TC), the temperature at which the material loses its magnetism. Neodymium magnets have higher remanence, higher coercivity, and higher energy product, but generally lower Curie temperatures. Neodymium, along with terbium and dysprosium, retains its magnetism at high temperatures.
Permanent magnet synchronous motor design
In the design of permanent magnet synchronous motors (PMSMs), the permanent magnet rotor is constructed based on the stator frame of a three-phase induction motor, without altering the geometry of the stator and windings. Specifications and geometry include: motor speed, frequency, number of poles, stator length, inner and outer diameters, and number of rotor slots. The design of a PMSM involves considerations such as copper losses, back electromotive force, iron losses, self-inductance and mutual inductance, magnetic flux, and stator resistance.
Calculation of self-inductance and mutual inductance:
Inductance L can be defined as the ratio of magnetic flux linkage to the current I that generates magnetic flux, measured in Henry (H), equal to Weber per ampere. An inductor is a device used to store energy in a magnetic field, similar to how a capacitor stores energy in an electric field. Inductors typically consist of coils wound around a ferrite or ferromagnetic core, and their inductance value depends only on the physical structure of the conductor and the permeability of the material through which the magnetic flux passes.
The steps to find the inductance are as follows: 1. Assume there is a current I in the conductor. 2. Use the Biot-Savart law or Ampere's circuital law (if applicable) to determine that B is sufficiently symmetrical. 3. Calculate the total flux connecting all loops. 4. Multiply the total flux by the number of loops to obtain the flux linkage. Based on the evaluation of the required parameters, design the permanent magnet synchronous motor.
Research has found that using neodymium iron boron (NdFeB) as the AC permanent magnet rotor material increases the magnetic flux generated in the air gap, leading to a smaller stator inner radius, while using samarium cobalt (SCo) permanent magnet rotor material results in a larger stator inner radius. The results also show that the effective copper loss in NdFeB is reduced by 8.124%. For SCo as the permanent magnet material, the magnetic flux will be a sinusoidal variable. Typically, the design and construction of permanent magnet synchronous motors must consider both the stator and rotor structures to achieve high-performance motors.
in conclusion
A permanent magnet synchronous motor (PMSM) is a synchronous motor that uses highly magnetic materials for magnetization. It features high efficiency, simple structure, and ease of control. PMSMs are used in various fields, including traction, automotive, robotics, and aerospace. The power density of a PMSM is higher than that of an induction motor with the same rating because it does not have a dedicated stator power unit for generating the magnetic field.
Currently, the design of permanent magnet synchronous motors not only requires greater power, but also lower mass and smaller moment of inertia.
