The independent electromagnetic stirring function serves two main purposes: first, to create an overall circulation of the steel liquid, and second, to provide sufficient stirring force without significantly raising the steel liquid temperature. The main types of electromagnetic stirring used in vacuum induction furnaces today are three-phase power frequency stirring and single-phase electromagnetic stirring (UDS).
In vacuum induction melting, the electromagnetic field generated by the furnace coil not only melts the steel but also causes electromagnetic stirring. Under the influence of this alternating electromagnetic field, the steel liquid forms a two-stage four-zone stirring. This stirring can create stagnant areas within the steel liquid, which affects the alloying process. Since the melting frequency is high (medium frequency), the stirring force is relatively weak. Therefore, this built-in stirring function from induction melting is typically suitable only for small-capacity vacuum induction furnaces. If the furnace capacity exceeds 1t or if stronger stirring is required, an independent electromagnetic stirring system is necessary to meet the process requirements.
Three-Phase Power Frequency Stirring
Three-phase power frequency stirring involves connecting the coil to a three-phase power frequency supply to achieve overall stirring of the steel liquid. This method is the most commonly used for electromagnetic stirring. It offers a large stirring force and has minimal impact on the temperature of the steel liquid. However, three-phase power frequency stirring requires an independent power source (typically a power frequency transformer) and switching switchgear, making it more expensive.
Single-Phase Electromagnetic Stirring (UDS)
Single-phase electromagnetic stirring works by adjusting the phase difference of the power supply (typically 90°) to achieve overall stirring of the steel liquid. This method transforms the two-stage four-zone stirring into a two-zone overall stirring, effectively solving the problem of uneven stirring. While the stirring force is limited due to the small change in frequency compared to the melting frequency, increasing the power to boost the stirring force would cause a significant rise in the steel liquid temperature, which is not allowed. As a result, the power for single-phase electromagnetic stirring is typically limited.
The main advantage of single-phase electromagnetic stirring is its relatively low cost. It provides some stirring functionality without significantly increasing the power supply cost. In the 1980s, AJAX, an American company, developed a medium-frequency power supply based on this principle. Later, the American company Emerson also promoted this technology. Currently, only a few domestic companies, such as Yingjie Electric, have developed medium-frequency melting power supplies with UDS functionality.
Application and Cost-Effectiveness
For vacuum induction furnaces below 3t, single-phase electromagnetic stirring can meet the requirements of most melting processes. This technology offers significant cost-effectiveness in this capacity range. In contrast, three-phase power frequency stirring, while offering superior performance, is generally used in larger vacuum induction furnaces due to its higher cost.
Conclusion
For vacuum induction furnaces with a capacity below 3t, single-phase electromagnetic stirring (UDS) provides sufficient stirring performance and offers a clear cost advantage. This technology meets the needs of most small-scale melting processes while keeping power supply costs relatively low. For larger furnaces, three-phase power frequency stirring is a better choice, although it comes at a higher cost.
