The main raw material for motor cores is silicon steel sheets. Currently, the most commonly used ones are 470, 600 and 800 in cold-rolled sheets, among which 470 and 600 sheets are more frequently applied in high-efficiency motors.
1.Low loss.
Core loss at a certain frequency and magnetic induction intensity is a major indicator of electrical steel sheets. Core loss consists of two parts: hysteresis loss and eddy current loss. Hysteresis loss is the energy consumption caused by the alternating magnetization of the core, which is related to the material composition and grain size, and can be represented by the area of the hysteresis loop. Eddy current loss is the resistance loss caused by the eddy current generated during the alternating magnetization of the core, which is related to the material’s own resistivity and thickness. Therefore, to reduce core loss, electrical steel sheets have a smaller thickness and a higher resistivity.
2.High magnetic conductivity.
The higher the magnetic conductivity, the smaller the cross-sectional area of the magnetic circuit can be reduced when the flux remains constant, saving the copper used in the excitation winding and reducing the size of the motor.
3.Good laminating properties.
Electrical steel plates should have an appropriate hardness, neither too brittle nor too soft. The surface should be smooth, flat, and uniform in thickness (with the requirement for plate difference control), which is conducive to mold punching and improving the stacking coefficient. The same mold can be used for cold-rolled steel plates, and its service life can be significantly extended compared to hot-rolled steel plates. Some cold-rolled electrical steel plates with inorganic or organic coatings can increase the number of punching strokes per pass of the mold by nearly ten times after one-time grinding. ●Low cost and easy to use. In addition to the above requirements, some motors often have higher requirements for magnetic conductive materials. For example, small magnetic failure and small magnetic expansion. These requirements are various and should be comprehensively considered.
●Silicon steel sheet
An alloy steel containing silicon, which is rolled into thin sheets. It is generally referred to as silicon steel sheet. Depending on the manufacturing process, it is classified into hot-rolled silicon steel sheet (which has largely been phased out) and cold-rolled silicon steel sheet. The cold-rolled silicon steel sheet can be further divided into oriented and non-oriented types. Currently, silicon steel sheets are mostly supplied in sheet form. To improve the magnetic properties of the silicon steel sheet and reduce its shear strength, domestic silicon steel sheets have undergone annealing treatment in the rolling mill.
●Silicon steel sheet-free
The motor core uses silicon steel sheets instead of low-carbon steel plates and pure iron. This was a significant advancement in history. The low-loss silicon steel sheets improved the performance of the motor and reduced its size. Now, instead of using silicon steel sheets, low-silicon steel sheets (also known as low-carbon electrical steel strips or pure iron electrical steel strips) are used to make the cores of small motors because the modern technology-produced low-silicon steel sheets are different from the original low-carbon steel plates. They not only have a high magnetic induction strength but also have iron loss similar to that of silicon steel sheets. The small AC motors designed and manufactured with low-silicon steel sheets can further reduce the size, lighten the weight, and lower the cost. Moreover, because the low-silicon steel sheets are softer, they can increase the punching speed and extend the lifespan of the molds. Now, low-silicon steel sheets are widely used as the core material for small motors in foreign countries. In industrialized countries, their usage accounts for approximately 50-60% of the total output of electrical steel sheets.
At present, there are two situations where the motor factory uses non-silicon steel sheets. One is that the non-silicon steel sheets after cold rolling are directly punched into sheets, and then annealing treatment is carried out at the motor factory; the other is that the annealed steel sheets provided by the steel mill are punched and used directly by the motor factory. Non-silicon steel sheets are high-magnetic-conductivity materials, and their magnetic induction intensity and loss are very sensitive to mechanical stress. Therefore, after punching and before using, eliminating stress annealing is an important measure to improve magnetic performance. The heat treatment of non-silicon steel sheets requires specialized heat treatment equipment, but most motor factories in our country do not have such conditions yet. This is a problem that needs to be solved when using non-silicon steel sheets.
● The silicon content and impurity silicon have a decisive influence on the performance of silicon steel sheets. After adding silicon to the iron, the resistivity increases, and it also helps to separate harmful impurity carbon. Generally, when pure iron is added with silicon, the magnetic induction intensity decreases slightly, but the iron loss is significantly reduced. As the silicon content increases, the hardness and brittleness increase, which brings difficulties to rolling, stamping, shearing, and mechanical processing. Currently, the silicon content of silicon steel sheets is generally not more than 4.5%. If the silicon content is higher, it is difficult to perform rolling and processing.
● Thickness. Considering that the eddy current loss in the iron core is proportional to the square of the thickness of the steel sheet, for the same type of silicon steel sheet, the thinner the thickness, the smaller the iron core loss, but the manufacturing time of the iron core increases and the stacking coefficient decreases. Generally, motors use silicon steel sheets with a thickness of 0.5 millimeters, and when the iron core loss requirements of large steam turbine generators are very strict, 0.35 millimeters-thick silicon steel sheets are used.
● Stress. During the processes of shearing, stacking, or winding the iron core, stress will be generated, which deteriorates the magnetic performance and increases the iron loss. In the range of about 1 millimeter on both sides of the cutting (breaking) section line, a visible black strip residual stress zone is formed. Generally, annealing treatment can be adopted to eliminate the stress and restore the original magnetic performance; the magnetic performance of high-performance cold-rolled silicon steel sheets is more sensitive to stress.
Post time: Mar-04-2026