Reasonable explanation of power frequency, electric furnace capacity, electric furnace power and power density of intermediate frequency furnace_Energy saving induction heating equipment manufacturers

When the coreless induction melting furnace is smelted, the induced current in the charge and the molten steel is obeyed by the skin effect, which is sequentially weakened from the surface to the center. Reducing the current intensity to 36.8 percent of thesurface of the current intensity to the point that the conductor surface is called from a current penetration depth.
The calculation formula of the current penetration depth Δt is:
△t = 5030 × ρ ^1/2 · μ _r ^-1/2 ·f ^-1/2
In the above formula: ρ - metal charge resistivity, Ω · cm;
μ _r - relative magnetic permeability of the metal charge;
f — power frequency , Hz.
The higher the power frequency, the smaller the current penetration depth and the more pronounced the skin effect.
The induced current in the charge is mainly concentrated in the penetration depth layer. The heat of the heating charge is mainly supplied by the surface layer. In order to raise the entire cross section of the charge to the same temperature, as the heating time is extended, the amount of heat lost from the charge to the surrounding medium increases, resulting in a decrease in thermal efficiency.

In order to achieve high electrical efficiency, a suitable frequency should be selected for furnaces of different capacities. From the theoretical analysis, it can be known that when the ratio of the diameter of the crucible to the penetration depth of the current is about 10, the electric furnace has the highest electrical efficiency. The frequency f at the highest electrical efficiency can be calculated by the following formula:
f = 2.5×10 ⁹ ρ · μ _r ^-1 · d ^-2
In the above formula: d — the inner diameter of the crucible, cm.
In general, the lower the power supply frequency, the lower the electrical efficiency. In the production, the power frequency should be reasonably selected according to the actual situation to reduce power consumption.
According to relevant information: When melting ferrous metals, the lowest frequency of the power supply is:
f _min = d ^-2 × 10 ⁵ (Hz)
Regardless of the power frequency furnace or the intermediate frequency furnace, as the furnace capacity increases, the ratio of surface area to volume of the furnace decreases, and the heat loss per ton capacity also decreases. The higher the rated melting rate of the coreless intermediate frequency furnace , the lower the average power consumption per ton of molten iron. Therefore, in order to reduce energy consumption, an electric furnace with a higher rated melting rate should be used as much as possible.
Usually the furnace capacity increases and the power increases. For a certain capacity of the furnace, when the power is increased, the power density of the furnace is also increased, the melting speed is increased, the melting time is shortened, the unit power consumption loss is correspondingly reduced, and the thermal efficiency is improved.
However, when the power is increased, the electromagnetic stirring force is also increased, the erosion of the lining is accelerated, the quality of the molten iron is affected, and the life of the lining is lowered. Therefore, the increase in the input power of the furnace is limited.
The calculation formula of electromagnetic stirring force is:
F _j = 31600 × ρ ^-1/2 ·f ^-1/2 · S ^-1 · P
In the above formula: S—the surface area of the metal charge surrounded by the inductor, cm ² ;
P — Part of the power converted to heat by heating the metal charge , kW.
The allowable values of the power density of the electric furnace at different frequencies are shown in the table below.
Table 2 Allowable values of power density of electric furnaces at different frequencies

frequency / Hz	1000	500	250	125	50
Electric furnace capacity / t	0.2~1.5	0.6~6	1.1~18	2.5~60	8~100
Power density / k W · t ^-1	1345	945	670	475	300

At present, the medium-frequency electric furnace produced in China, regardless of its capacity (tonnage) or smelting speed, is far from the advanced foreign similar products. The reason is that the domestic intermediate frequency electric furnace is limited by the quality of the basic components, the key is that the power density is usually configured to 600.   Around kW/t , the operating frequency is 0.9 to 3 kHz, and the melting time is generally 90 to 120 minutes. The foreign power density is usually configured to 600 ~ 800  kW/t, small capacity melting furnace power density configuration up to 1000   kW/t , working frequency is 2.5~3kHz, melting time is 20~45min. Application of rapid furnace smelting, less burning of alloying elements, high quality molten steel, high output and low energy consumption.
As long as the power density of the ordinary medium frequency electric furnace is appropriately modified, rapid melting can be achieved. There are three different forms of transformation that can achieve the goal of rapid melting:
First, if the user transformer capacity is large and the overall load can be adjusted, but the capital is limited, the furnace capacity and the working frequency can be used unchanged, and the output power of the power control cabinet to the furnace body is increased, so that the power density is 800 to 1000.   kW/t . That is, the furnace body and the compensation capacitor are unchanged, and only the thyristor capacity is increased, and the investment is small. The smelting time after the transformation was shortened from the original 90-120 min to about 45 min.
Second, a user such as the power receiving load limit, and the funds are not comfortable, does not change the control cabinet can be used, changing only the furnace induction coil approach, power density of 800 to 1000   kW/t also gives the desired results.
Third, the output power of the power control cabinet is unchanged, the operating frequency is changed to 2.5 to 3 kHz, and the number of turns of the induction coil is changed to meet the power density of 800 to 1000.   kW/t and operating frequency requirements. The smelting time after the transformation is generally 25 to 30 minutes.
The smelting efficiency of the intermediate frequency electric furnace after the power capacity ratio transformation is shown in the following table.
Table 3 The efficiency of smelting after the power capacity ratio of the intermediate frequency electric furnace

project	Smelting per furnace Time/min	Steel per furnace Quality / kg	Power consumption per furnace /kW·h	Per ton of molten steel Power consumption / kW · h	Per ton of molten steel Time required / min
Transformation of the former	90～100	250	244~266	960~1064	360~400
After transformation	45~50	250	184～205	736-820	180~200

It can be seen from the above table that the power consumption of steel in each furnace can be saved by 56 to 61 kW·h , and the energy saving per ton of steel is 224 to 244 kW·h , and the smelting time per ton of molten steel can be shortened by 180 to 200 minutes .
Two sets of KGPS-1-0.5t integrated circuit thyristor medium frequency electric furnace purchased by a company of WISCO Group replaced the original KGPS control system circuit board with the KGPS-B type constant power controller produced by a company in Xiangfan , and carried out constant power transformation. Good results.
Before the transformation of the intermediate frequency electric furnace, there are problems such as difficulty in starting heavy load, less system protection function, low utilization rate of the transformer, and poor quality of molten steel. The original KGPSmedium frequency power supply output rated power is about 250 kW , the power density is only 500kW/t , and the workshop transformer capacity is 560kVA , and the average melting time per furnace is 90min . Obviously, the transformer capacity is not fully utilized. Due to the long smelting time (about 7 hours of smelting 2 t of molten steel per day ), the heating rate is slow, and the electric energy consumption is large ( average power consumption per ton of cast steel blanks is 375 kW·h) , and the burning of the active components in the molten steel is also Large, product quality is difficult to guarantee.
Compared with KGPS IF power supply, KGPS - B constant power controller has more stable and reliable system performance, low failure rate, good impedance matching performance, high power factor and relatively constant power, which solves the poor technical performance of the original equipment due to electronic control system. It is difficult to start, difficult to smelt, high power consumption, easy to burn inverter fast thyristor and other faults.
After the transformation with KGPS - B constant power controller, the constant power of the device reaches 400kW , the power density reaches 800kW/t , the average melting time per furnace is only 50 min , and the power consumption per ton of cast steel blanks is333kW·h , which is better than before the transformation. It is reduced by about 11% . Due to the shortening of the smelting cycle, the active ingredient of molten steel is burnt less, and the working time of workers is shortened. In the past, it worked for nearly 7hours every day , but now only 4 hours, the labor intensity of workers is reduced, and labor productivity is improved.
A company has made constant power conversion to its intermediate frequency power supply, increasing the intermediate frequency voltage from 650V to 750V , increasing the efficiency of the electric furnace by more than 10 %, and reducing the water consumption per ton of steel by 150kW·h .
When its configured power density is greater than 600 At kW/t , its melting capacity can basically achieve a melting time of less than 1 h per heat .