There are the following differences between an electric furnace transformer and an ordinary transformer:
Overload capacity requirements are different.
Different short-circuit resistance.
During operation, the power supply transformer is also required to adjust the voltage.
Industrial electric arc furnace transformers can be roughly divided into three categories: resistance furnace transformers, electric furnace transformers and induction furnace transformers.
A resistance furnace transformer is used for resistance furnaces and salt bath furnaces of mechanical parts heating, heat treatment, powder metallurgy sintering, non-ferrous metal smelting, etc. Because the resistance of the heater is too small or the resistance of the heater changes too much during the heating process, a resistance furnace transformer needs to be equipped between the electric arc furnace and the power grid to reduce and adjust the input voltage of the electric furnace.
Resistance furnace transformers and salt bath furnace transformers with small capacity and low voltage are mostly dry-type transformers with box shells and natural cooling; Resistance furnace transformers with medium capacity (hundreds to thousands of volt-ampere) are mostly oil-immersed self-cooling transformers; The large capacity is the forced oil circulation water-cooled transformer.
An electric arc furnace transformer is a special transformer for arc furnace power supply of iron and steel smelting. Large capacity, complex structure and high technical requirements. Its secondary side voltage is low, generally from tens of volts to hundreds of volts, and it is required to be adjusted in a large range; The secondary side current often reaches thousands to tens of thousands of amps. In addition, in iron and steel smelting, high power is required in the melting period, and the transformer is required to have 20% overload capacity within 2 hours. In the process of steelmaking, because the collapse of furnace charge is easy to cause electrode short circuits, the original side of the electric arc furnace transformer should be connected in series with the current limiting reactor or make it have a large impedance to limit short-circuit current.
During the operation of electric arc furnaces, the transformer supplying power is also required to adjust the voltage. There are two voltage regulation modes of electric furnace transformers.
① Direct voltage regulation method: on the original winding of the transformer, lead out the tap and connect the tap changer. The voltage regulation of the secondary side can also be realized by △ - y exchange between the three phases on the primary side. Delta-y switching is a very economical voltage regulation method. Without changing the number of turns on the primary side, the secondary side voltage can be reduced to 1 / 10 of the original. Since each phase winding has 9 taps, plus △ - y commutation, a total of 2 can be obtained × 9, i.e. level 18 voltage.
② Indirect voltage regulation method: Main Transformer 1 is a three-winding transformer (see multi-winding transformer), and winding 1 is a high-voltage winding connected to the power grid; Winding 2 is winding with the tap, which is connected in series with the original winding 4 of series transformer II through tap changer; Winding 3 is connected in series with auxiliary winding 5 of series transformer II to supply power to a load of electric furnace. By changing the primary side voltage of series transformer II through a tap changer, the output voltage of winding 3 and 5 can be changed. The 3 and 5 windings connected to the load generally have only 1 ~ 2 turns, and the 8-shaped coil is usually made of copper or aluminum plate.
An induction furnace transformer is an induction furnace used to melt black and non-ferrous metals. It is essentially a special electric furnace transformer.
The induction furnaces can be divided into iron core and non-iron core. An iron core induction furnace is a transformer with an iron core and short-circuit secondary winding. The primary winding of the transformer is connected to the power supply, and the secondary winding actually has only one turn, which is the molten metal installed in the melting tank. When there is current in the primary winding, the secondary winding will generate induced current and circulate in the slot, so as to emit heat and melt the metal.