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 need to use slip rings for excitation, which reduces rotor maintenance and wear and tear. Permanent magnet synchronous motors have high efficiency and are suitable for high-performance drive systems such as CNC machine tools, robots, and automatic production systems in the industry.

Generally, the design and construction of permanent magnet synchronous motors must consider both the stator and rotor structures to obtain high-performance motors.

Construction of permanent magnet synchronous motor

Air-gap flux density: Determined according to the design of asynchronous motors, etc., the design of the permanent magnet rotor, and the special requirements for using 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 tooth width, while the back of the stator determines the maximum total flux.

Also, the allowable saturation level depends on the application. Typically, a high-efficiency motor has a low flux density, while a motor designed for maximum torque density has a high flux density. The peak air gap flux density is typically in the range of 0.7–1.1 Tesla. Note that this is the total flux density, which is the sum of the rotor and stator fluxes. This means that if the armature reaction force is less, it means that the alignment torque is higher.

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 suitable for operation below base speed, as high inductance reduces the power factor.

Permanent magnet material:

Magnets play an important role in many devices, therefore, it is important to improve the properties of these materials, and currently, attention is focused on materials based on rare earth and transition metals to obtain permanent magnets with high magnetic properties. Depending on the technology, magnets have different magnetic and mechanical properties and exhibit 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 are a wide variety of grades. NdFeB magnets began to be commercialized in the early 1980s. They are widely used today in many different applications. The cost of this magnet material (calculated per energy product) is comparable to the cost of ferrite magnets, and on a per kilogram basis, NdFeB magnets cost about 10 to 20 times more than 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 ability of a material to resist demagnetization, Energy Product (BHmax), Density Magnetic Energy; Curie Temperature (TC), the temperature at which the material loses its magnetism. Neodymium magnets have higher remanence, higher coercive force, and energy product, but generally lower Curie temperature types, neodymium and terbium, and dysprosium in order to maintain their magnetic properties 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, and number of rotor slots. The design of the permanent magnet synchronous motor includes copper loss, back electromotive force, iron loss and self-inductance, mutual inductance, magnetic flux, stator resistance, etc.

Calculation of self and mutual inductance:

Inductance L can be defined as the ratio of the flux linkage to the current I that produces the flux in henries (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. An inductor usually consists of a coil, usually wound around a ferrite or ferromagnetic core, and its inductance value is only related to 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 that there is a current I in the conductor. 2. Use Biot-Savart’s law or Ampere’s loop law (if available) to determine that B is sufficiently symmetric. 3. Calculate the total flux connecting all loops. 4. Multiply the total magnetic flux by the number of circuits to obtain the flux linkage, and design the permanent magnet synchronous motor through the evaluation of the required parameters.

The study found that the design of using NdFeB as the AC permanent magnet rotor material increases the magnetic flux generated in the air gap, resulting in a decrease in the inner radius of the stator, while the inner radius of the stator using the samarium cobalt permanent magnet rotor material is larger. The results show that the effective copper loss in NdFeB is reduced by 8.124%. For samarium cobalt as a permanent magnet material, the magnetic flux will be a sinusoidal variation. Generally, the design and construction of permanent magnet synchronous motors must consider both the stator and rotor structures to obtain high-performance motors.

In conclusion

The permanent magnet synchronous motor (PMSM) is a synchronous motor that uses high magnetic materials for magnetization. It has the characteristics of high efficiency, simple structure, and easy control. This type of permanent magnet synchronous motor has applications in several fields such as traction, automotive, robotics, and aerospace technology. The power density of permanent magnet synchronous motors is higher than that of induction motors of the same rating because there is no stator power dedicated to generating magnetic fields.

At present, the design of the permanent magnet synchronous motor not only requires more power but also requires lower mass and smaller moments of inertia.