Medium-frequency induction melting features fast heating speed, high efficiency, less burning loss, small heat loss, relatively low workshop temperature, reduced soot emission, energy conservation, improved productivity, better working conditions, lower labor intensity and remarkable effects in purifying workshop environment. Especially for cast iron, induction melting furnaces are superior to cupolas in that they help obtain low-sulfur molten iron, which is incomparable for cupolas. When selecting medium-frequency melting furnaces, foundry enterprises should take transformer capacity, output requirements, investment quota and other factors as selection criteria, and pay attention to the following matters when purchasing equipment.
1. Power supply requirements for medium-frequency induction melting
1.1 Transformer capacity for medium-frequency induction melting
For the currently commonly used SCR full-bridge parallel inverter medium-frequency power supply in the industry, the relational expression between transformer capacity and power supply power is: Transformer capacity value = Power supply power value × 1.2
For IGBT half-bridge series inverter medium-frequency power supply (commonly known as one dragging two, one for melting and one for heat preservation, both work at the same time), the relational expression between transformer capacity and power supply power is: Transformer capacity value = Power supply power value × 1.1
The transformer used here is a rectifier transformer. To reduce harmonic interference, it is recommended to use a dedicated transformer for one device as far as possible, that is, one medium-frequency power supply is equipped with one rectifier transformer.
1.2 Incoming Line Voltage of Medium Frequency Induction Melting
For medium-frequency power supplies below 1000KW, three-phase five-wire system 380V, 50HZ industrial power is generally adopted, and a 6-pulse single-rectifier medium-frequency power supply is configured; for medium-frequency power supplies above 1000KW, 660V incoming line voltage is preferred (some manufacturers use 575V or 750V. Since 575V and 750V are non-standard voltage grades, accessories are not easy to purchase, so it is not recommended to choose them), and a 12-pulse double-rectifier medium-frequency power supply is configured. There are two reasons for this: first, the rated working voltage is increased by increasing the incoming line voltage; second, the harmonics generated by high-power equipment will interfere with the power grid. Double rectification can obtain a relatively flat DC current, the load current is a rectangular wave, and the load voltage is close to a sine wave, which reduces the impact of power grid interference on other equipment.
Some users blindly pursue high voltage (for example, a 1000KW device uses a 900V incoming line voltage) and low current to achieve the purpose of energy saving. They do not know that this is at the cost of the service life of the electric furnace, and the gain does not make up for the loss. High voltage is easy to shorten the service life of electrical components, cause fatigue of copper busbars and cables, and greatly reduce the service life of the electric furnace. In addition, for electric furnace manufacturers, high voltage allows them to reduce raw material usage and cut costs. Electric furnace manufacturers are certainly willing to do this (high selling price with low cost). In the end, it is the users of the electric furnace that suffer losses.
2.Capacity Requirements
Generally speaking, the capacity of the medium-frequency induction melting furnace can be determined according to the weight of a single casting and the total weight of molten iron required per working day. Then the power and frequency of the medium-frequency power supply can be confirmed. Induction heating equipment is a non-standard product, and there is no unified national standard for it at present. The general configuration in the industry is shown in Table 1.
Table 1 Medium Frequency Induction Melting Furnace Selection Parameters
| No. | Melting Capacity/T | Power/KW | Frequency/HZ |
| 1 | 0.15 | 100 | 1000 |
It can be seen from Table 1 that the power density of domestic medium-frequency induction melting furnaces is basically around 500KW/ton, which is lower than the theoretical optimal value of 600-800KW. This is mainly due to considerations of furnace lining life and production management. Under high power density, electromagnetic stirring will cause strong scouring on the furnace lining, which imposes high requirements on furnace lining materials, furnace building methods, melting processes, metal materials, auxiliary materials, etc. According to the above configuration, the melting time per heat is 75 minutes (including charging, slag skimming, and tempering time). If it is necessary to shorten the melting time per heat, the power density of the power supply can be increased by 100KW/ton without changing the capacity of the furnace body.
| Item | Steel-shell Furnace | Aluminum-shell Furnace |
| Shell Material | Steel Structure | Aluminum Alloy |
| Furnace Tipping Mechanism | Hydraulic Cylinder | Reducer |
| Hydraulic Pump Station | Yes | No |
| Magnetic Yoke | Yes | No |
| Furnace Cover | Yes | No |
| Furnace Leakage Alarm | Yes | No |
| Energy Consumption | 580kw.h/t | 630kw.h/t |
| Service Life | 10 years | 4-5 years |
| Price | High | Low |
