The power supply unit for induction furnaces, also known as a solid-state medium-frequency power supply (hereinafter referred to as the medium-frequency power supply), is a type of static frequency converter composed of power semiconductor devices. It operates in two modes: AC-DC-AC conversion and AC-AC conversion. This section covers only the former, which is the most commonly used type of medium-frequency power supply for melting furnaces.
Compared to traditional medium-frequency generator sets, medium-frequency power supplies offer advantages such as high efficiency, frequency conversion capability, low noise, small size, light weight, simple installation, and easy operation and maintenance.
Compared to power-frequency induction furnaces, medium-frequency induction furnaces demonstrate significant advantages in terms of operational flexibility and economics. These are mainly reflected in the following aspects:
High Power Density: High melting rate and thermal efficiency.
Pour-out Capability: The furnace can be emptied completely without requiring residual molten metal to facilitate the melting of new charges, making it easy to switch between different metal grades.
Direct Charging: Wet or oily charge materials can be loaded and melted directly without requiring prior drying treatment.
Automatic Frequency Tracking: It automatically adjusts frequency to adapt to changes in furnace parameters, eliminating the need for switching capacitors via contactors to compensate for power factor, as is required with power-frequency induction furnaces.
Compact Size: For the same productivity, the furnace body is smaller. Consequently, thermal stress on the lining is reduced, extending the service life of the refractory lining.
Reduced Maintenance: It does not require a tap-changer switch (which is prone to failure), reducing maintenance workload.
No Phase Balancer Required: It does not require a three-phase balancing device.
Space and Cost Savings: It has a small footprint, reducing civil engineering and installation costs.
Medium-frequency power supplies also have certain drawbacks. Large-capacity units can generate harmonics that interfere with the power grid and the environment, necessitating measures to resolve these issues.
Depending on the type of filter used, medium-frequency power supplies can be classified as current-source type or voltage-source type.
Current-source type: Uses a DC smoothing reactor for filtering. It produces a relatively straight DC current. The load current is a rectangular wave, while the load voltage approximates a sine wave.
Voltage-source type: Uses capacitors for filtering. It produces a relatively straight DC voltage. The load voltage is a rectangular wave, while the load current approximates a sine wave.
Depending on the load resonance method, medium-frequency power supplies can be classified into three types: parallel resonant, series resonant, and series-parallel resonant.
