Currently, high-efficiency motors have become the mainstream in the development and application of small and medium-sized motors, especially in industrialized countries such as Europe and the United States, which pay close attention to energy conservation and environmental protection, actively promoting energy efficiency labeling for motors and mandating the development and application of high-efficiency motors through relevant government financial incentives. To expand its share of the international motor market, adopting new technologies, processes, and materials to maximize motor efficiency has become an urgent task for Chinese motor manufacturers.
Since the loss distribution of an electric motor varies with the power and number of poles, measures should be taken to address the relevant losses at different power and number of poles. Among these measures, the selection of magnetic materials is particularly important, as it has a significant impact on stator copper losses, iron losses, and other performance characteristics. At the same time, the cost of the core material is a major part of the cost of the electric motor. Therefore, the selection of silicon steel sheets suitable for high-efficiency motors is crucial for the design and manufacture of high-efficiency motors and should be carefully considered.
By comparing and analyzing the electromagnetic properties of silicon steel sheets for motors, this paper studies and discusses the selection and application of silicon steel sheets for high-efficiency motors. Through experiments and data analysis, the paper proposes selection principles for silicon steel sheets for high-efficiency motors, providing a reference for the selection of core materials for high-efficiency motors.
Silicon steel sheets, also known as electrical steel sheets, are made by adding a small amount of silicon to iron. This increases the resistivity of the material and significantly improves magnetic aging, but also increases its brittleness and reduces its magnetic flux density. Therefore, the limit for silicon content is 4.5%. Silicon steel sheets are mainly used in power frequency AC electromagnetic devices, such as the cores of transformers, motors, instrument transformers, switches, and relays. In my country, silicon steel sheets are conventionally classified into hot-rolled and cold-rolled silicon steel sheets, while internationally, they are mostly classified into two main categories based on grain orientation: non-oriented and grain-oriented silicon steel sheets.
Hot-rolled silicon steel sheets are magnetic, non-oriented silicon steel sheets, belonging to the ferrosilicon alloy category. Compared to cold-rolled sheets, they have advantages such as better material stability, significantly lower punching and shear stress, lower specific gravity, and greater thickness variation within the same sheet. Cold-rolled sheets, on the other hand, are low-silicon, low-carbon alloys, characterized by low iron loss, high magnetic flux density, smooth surface, uniform thickness, and minimal thickness variation within the same sheet. The use of silicon steel sheets instead of carbon steel plates and pure iron in motor cores represents a significant historical advancement, as the low-loss silicon steel sheets improve motor performance and reduce motor size.
The reason why silicon-free steel sheets are now used instead of silicon steel sheets in the cores of small motors is that modern silicon-free steel sheets, unlike the original low-carbon steel sheets, not only have high magnetic flux density but also iron loss similar to that of silicon steel sheets. Whether it's cold-rolled silicon steel sheets or silicon-free steel sheets, their magnetic flux density and losses are very sensitive to mechanical stress. Therefore, how to restore their original magnetic permeability and iron loss after stamping is a problem that must be considered when selecting silicon steel sheets.
When selecting silicon steel sheets for high-efficiency motors, one should not only pursue high grades and low iron losses. In fact, a comprehensive consideration is necessary. Although higher grades result in lower iron losses, their magnetic permeability will be relatively poor, which is especially important for small motors. Figure 1 shows the ratio of no-load current to full-load current for three 4-pole motors of 1.5kW, 15kW, and 150kW.
As can be seen from Figure 1, in a 1.5kW motor, because the reactive current accounts for a large proportion of the stator current, the no-load current (mainly the magnetizing current) accounts for a considerable proportion of the full-load current, reaching about 70%. As the power increases, the proportion of the no-load current in the full-load current gradually decreases.
Therefore, for motors with lower power, stator copper loss accounts for a larger proportion of the total loss. It is advisable to use electrical steel sheets with good magnetic permeability as the stator core, which can greatly reduce the excitation current and significantly improve iron loss and stator copper loss.
For motors with higher power, since the no-load current accounts for a small proportion of the full-load current, iron loss already accounts for a considerable proportion of the total loss. Using high-permeability silicon steel sheets does not significantly improve efficiency. Therefore, reducing the unit loss of the core material will help reduce iron loss.
Due to design and manufacturing reasons, the iron loss in the motor test was much higher than the value calculated based on the unit iron loss value provided by the steel mill. The main reason for the increase in iron loss is that the steel mill's unit iron loss value is tested on the strip sample according to the Epstein method. However, after the material is punched, sheared and stacked, it is subjected to great stress, which deteriorates the magnetic permeability of the lamination and increases the iron loss. In addition, the presence of tooth slots causes the air gap tooth harmonic magnetic field to cause no-load high-frequency loss on the iron core surface. All of these will lead to a significant increase in iron loss after the motor is manufactured. Therefore, in addition to selecting magnetic materials with lower unit iron loss, it is also necessary to control the lamination pressure and take necessary process measures to reduce iron loss.
Although the government proposed promoting cold-rolled steel sheets (i.e., replacing hot-rolled steel with cold-rolled steel) as early as the end of 2002, the various drawbacks of cold-rolled steel sheets (such as their long lifespan and sensitivity to mechanical stress) have remained an obstacle to the development of high-efficiency motors. Currently, major domestic high-efficiency motor manufacturers use hot-rolled steel sheets for their silicon steel sheets, and their grades and related electromagnetic properties are shown in Table 1 (at a frequency of 50Hz).
This grade of silicon steel sheet is high in silicon and carbon, with low density, light weight, good material stability, and virtually no failure period. Its punching and shearing stress is much lower than that of cold-rolled sheets. As shown in Table 1, its low-field magnetic flux density is not very good, but it is sufficient for high-efficiency motors, as the stator magnetic flux density of high-efficiency motors is between 1.0T and 1.5T, and the rotor magnetic flux density is between 1.0T and 1.6T.
To fully utilize the advantages of cold-rolled sheets, it is essential to address the mechanical stress issues following punching and shearing. Theoretically, there are many methods to resolve this stress, with the most effective being the selection of a suitable annealing process. However, due to limitations in equipment and process parameters, this process has not been widely adopted in China and has only been used in small motors.
European small motor manufacturers used silicon-free steel sheets, which, although having high magnetic induction intensity and iron loss similar to silicon steel sheets, are very sensitive to mechanical stress and require appropriate processes to restore their original performance.
Based on theoretical introductions and test parameters from other motor manufacturers, and considering export products of home appliances, we also conducted a simulation experiment of this process. Due to equipment limitations, we simply placed the punched and sheared silicon steel sheets in a heating furnace and held them at 500℃ for 2 hours. The results showed that the magnetic permeability of the silicon steel sheets recovered well after annealing, and the iron loss was significantly improved. The test results are shown in Table 2.
The degree to which high-efficiency motors are achieved is closely related to the selection of raw materials, especially magnetic materials. High-efficiency motors primarily focus on reducing various losses, and the deviation between the design and experimental values for iron loss, stator copper loss, rotor copper loss, and miscellaneous losses depends entirely on the appropriateness of the silicon steel sheets selected and the quality of the core manufacturing.
