Brief Analysis on the Design and Construction of Permanent Magnet Synchronous Motor

Due to their compactness and high torque density, permanent magnet synchronous motors are widely used in many industrial applications, especially for high-performance drive systems such as submarine propulsion systems. Permanent magnet synchronous motors do not require the use of slip rings for excitation, which reduces rotor maintenance and losses. Permanent magnet synchronous motors are highly efficient and suitable for high-performance drive systems such as CNC machine tools, robots and automatic production systems in industry. In general, the design and construction of permanent magnet synchronous motors must take into account both the stator and rotor structures to obtain high-performance motors.

The structure of permanent magnet synchronous motor

Air gap flux density: Determined by asynchronous motor design, etc., the design of permanent magnet rotors and special requirements for the use of switched stator windings. In addition, the stator is assumed to be a slotted stator. The air gap flux density is limited by the saturation of the stator core. In particular, the peak flux density is limited by the width of the gear teeth, while the back of the stator determines the maximum total flux. In addition, the permissible saturation level depends on the application. Generally, high-efficiency motors have lower flux densities, while motors designed for maximum torque density have higher flux densities. The peak air gap flux density is usually in the range of 0.7–1.1 Tesla. It should be noted that this is the total flux density, that is, the sum of the rotor and stator fluxes. This means that if the armature reaction force is small, it means that the alignment torque is high. However, in order to achieve a large reluctance torque contribution, the stator reaction force must be large. The machine parameters show that a large m and a small inductance L are mainly required to obtain the alignment torque. This is generally applicable to operation below the base speed, because high inductance reduces the power factor.

Permanent magnet material:

Magnets play an important role in many devices, therefore, it is very important to improve the properties of these materials, and currently, attention is focused on materials based on rare earth metals and transition metals, which allow to obtain permanent magnets with high magnetic properties. Depending on the technology, magnets have different magnetic and mechanical properties and show different corrosion resistance. Neodymium iron boron (Nd2Fe14B) and samarium cobalt (Sm1Co5 and Sm2Co17) magnets are the most advanced commercial permanent magnet materials today. Within each class of rare earth magnets there is a wide range of various grades. NdFeB magnets became commercialized in the early 1980s. They are widely used today in many different applications. The cost of this magnet material (per energy product) is comparable to that of ferrite magnets, and on a per kilogram basis, the cost of NdFeB magnets is about 10 to 20 times that of ferrite magnets.

Some important properties used to compare permanent magnets are: remanence (Mr), which measures the strength of a permanent magnet's magnetic field, coercivity (Hcj), the material's ability to resist demagnetization, energy product (BHmax), the density of magnetic energy; Curie temperature (TC), the temperature at which a material loses its magnetism. Neodymium magnets have higher remanence, higher coercivity, and energy product, but the Curie temperature is usually lower. Neodymium is used with terbium and dysprosium in order to maintain its magnetism at high temperatures.

Permanent Magnet Synchronous Motor Design

In the design of a permanent magnet synchronous motor (PMSM), the construction of the permanent magnet rotor is based on the stator frame of a three-phase induction motor, without changing the geometry of the stator and windings. Specifications and geometry include: motor speed, frequency, number of poles, stator length, inner and outer diameters, number of rotor slots. The design of a permanent magnet synchronous motor includes copper losses, back EMF, iron losses and self and mutual inductance, flux, stator resistance, etc.

Calculation of Self-Inductance and Mutual Inductance

Inductance L can be defined as the ratio of magnetic flux linkage to the current I producing the flux, and is measured in henries (H), which is equal to webers 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. An inductor usually consists of a coil of wire, usually wound around a ferrite or ferromagnetic core, and its inductance value depends only on the physical structure of the conductor and the magnetic permeability of the material through which the flux passes.

The steps to find the inductance are as follows: 1. Assume that there is a current I in the conductor. 2. Use the Biot-Savart law or Ampere's loop law (if available) 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. By evaluating the required parameters, the permanent magnet synchronous motor is designed.

The study found that the design using NdFeB as the AC permanent magnet rotor material increased the magnetic flux generated in the air gap, resulting in a smaller stator inner radius, while the stator inner radius was larger when using SmCo permanent magnet rotor material. The results showed that the effective copper loss in NdFeB was reduced by 8.124%. For SmCo as a permanent magnet material, the magnetic flux will be a sinusoidal variation. In general, the design and construction of permanent magnet synchronous motors must consider both the stator and rotor structures to obtain high-performance motors.

in conclusion

A permanent magnet synchronous motor (PMSM) is a synchronous motor that uses highly magnetic materials for magnetization. It has the characteristics of high efficiency, simple structure, and easy control. This type of permanent magnet synchronous motor is used in many fields such as traction, automobiles, robotics, and aerospace technology. The power density of a permanent magnet synchronous motor is higher than that of an induction motor of the same rating because there is no stator power dedicated to generating a magnetic field. Currently, the design of a permanent magnet synchronous motor requires not only more power, but also lower mass and smaller moment of inertia.