Due to their compactness and high torque density, permanent magnet synchronous motors are widely used in many industrial applications, especially in high-performance drive systems such as submarine propulsion systems. Permanent magnet synchronous motors do not require slip rings for excitation, thus reducing 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. Typically, the design and construction of permanent magnet synchronous motors must consider both the stator and rotor structures to obtain a high-performance motor.
Structure of permanent magnet synchronous motor
Air gap flux density: Determined based on the asynchronous motor design, etc., the design of the permanent magnet rotor, and the special requirements for using switching stator windings. In addition, it is assumed that the stator is 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 stator back determines the maximum total flux. Furthermore, the allowed 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. 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, which is the sum of the rotor and stator fluxes. This means that if the armature reaction force is smaller, 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 needed to obtain the alignment torque. This generally applies to operations below base speed, as high inductance reduces power factor.
Permanent magnet material:
Magnets play an important role in many devices, so it is very important to improve the performance of these materials. Currently, attention is focused on materials based on rare earth metals and transition metals, which can 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 category of rare earth magnets, there is a wide variety of grades. NdFeB magnets became commercially available 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 the cost of ferrite magnets, which are approximately 10 to 20 times more expensive on a per-kilogram basis.
Some important properties used to compare permanent magnets are remanence (Mr), which measures the strength of the magnetic field of the permanent magnet, coercive force (Hcj), the material’s ability 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 coercivity, and energy product, but generally lower Curie temperature types. Neodymium competes 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 structure of the permanent magnet rotor is based on the stator frame of the 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 a permanent magnet synchronous motor includes copper loss, back electromotive force, iron loss, self-inductance and mutual inductance, magnetic flux, stator resistance, etc.
Calculation of self-inductance and mutual inductance: Inductance L can be defined as the ratio of the flux linkage to the current I that generates the magnetic flux. The unit is Henry (H), which is 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 of wire, usually wound around a ferrite or ferromagnetic core, and its inductance value is related only to the physical structure of the conductor and the permeability of the material through which the flux passes.
The steps to find the inductance are as follows:
- Assume that there is a current I in the conductor.
- Use Biot-Savart’s law or Ampere’s circuit law (if available) to determine that B is sufficiently symmetrical.
- Calculate the total flux connecting all loops.
- Multiply the total magnetic flux by the number of loops to obtain the flux linkage, and then design the permanent magnet synchronous motor by evaluating the required parameters.
The study found that the design using neodymium iron boron as the AC permanent magnet rotor material increased the magnetic flux generated in the air gap, resulting in a reduction in the inner radius of the stator, while the inner radius of the stator using samarium cobalt permanent magnet rotor material was 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. Typically, the design and construction of permanent magnet synchronous motors must consider both the stator and rotor structures to obtain a high-performance motor.
In conclusion
The 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 kind of permanent magnet synchronous motor has applications in various fields such as traction, automotive, 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 the magnetic field. At present, the design of permanent magnet synchronous motors not only requires greater power but also requires lower mass and smaller rotational inertia.
