Load circuit and equivalent electrical analysis of medium frequency induction heating equipment

Load circuit analysis of medium frequency induction heating equipment

(1) Load characteristics and calculation of load parameters. Physical phenomena such as electromagnetic induction, heating, heat transfer and thermal radiation are involved in the load inductor. Here, induction melting is used as an example to illustrate the electromagnetic induction phenomenon. The smelting furnace is composed of induction coil, resistant material, magnetic yoke, etc. The induction coil is wound by a hollow copper tube, and water is passed through the tube for cooling. The cross-section of the copper tube can be round or square. Square ones have higher electrical efficiency. Inside the inductor is a crucible made of refractory material. The smelted metal, such as steel, is placed in the crucible.

①Magnetic field. When the induction coil is supplied with an intermediate frequency current, an intermediate frequency magnetic field is generated. Most of the magnetic flux passes through the steel, which is called the main magnetic flux; the other part of the magnetic flux does not pass through the steel, which is called the leakage flux. The main magnetic flux forms the main inductance, which is mutual inductance. Only the main magnetic flux can generate induced electric potential in the steel, form eddy currents and heat the steel. The leakage magnetic flux forms a leakage inductance, which only produces a voltage drop on the circuit and cannot be heated. The relative magnitudes of main magnetic flux and leakage flux are affected by many factors, as described below.

a. The thicker the crucible and the smaller the furnace, the greater the leakage magnetic flux and the less the main magnetic flux. Therefore, the newly knotted crucible has more magnetic flux leakage. After smelting for several furnaces, the crucible becomes thinner and the magnetic flux leakage decreases.

b. When there is less steel in the furnace, the main magnetic flux decreases and the leakage magnetic flux increases.

c. The relative position of the induction coil of the crucible furnace also affects the magnetic flux. The larger the gap between the induction coil and the steel, the greater the leakage flux and the less the main magnetic flux. If the crucible furnace is too high or too low, the leakage flux will increase. , the main magnetic flux decreases, so the relative positions of the newly knotted crucible furnace and induction coil will affect the distribution of main magnetic flux and leakage flux.

d. When the steel melts into a liquid state, the molten steel fills the furnace, causing all the magnetic flux entering the furnace to pass through the molten steel, so the main magnetic flux increases and the leakage magnetic flux decreases.

e. When the temperature of steel exceeds the Curie point, the steel loses its magnetism, and the main magnetic flux decreases due to the decrease in magnetic permeability.

f. Different materials to be smelted also affect the magnetic field distribution. When smelting metals such as copper and aluminum, due to the low resistivity and greater penetration depth than steel, the eddy current loop induced in copper and aluminum has a smaller resistance, so the main medium-frequency magnetic flux is induced immediately as soon as it enters the copper and aluminum. The eddy currents are offset, thus reducing the main magnetic flux.

②Resistance. Since the steel is placed in a relatively messy manner in the furnace during smelting, steel pipes are used as equivalents for the convenience of calculation. The actual electromagnetic field distribution and current distribution in conductors under various load conditions are relatively complex. When various loads are introduced later, different correction methods are used for parameter calculation.

(2) Equivalent circuit of medium frequency induction heating equipment load. When the current frequency is high (medium frequency and high frequency), or the diameter of the heated steel pipe is large, in order to simplify the calculation, it can be considered that the current flows uniformly within the penetration depth layer of the heated steel pipe. The relationship between the induction coil and the heated steel pipe is like a transformer. The induction coil is the primary side, and the steel pipe itself is both the core and the secondary side of the transformer. The secondary side is single turn and short circuited. When the Curie point temperature is exceeded, the ferromagnetic material loses its magnetism and becomes a hollow transformer. For a smelting furnace, the size of the furnace, the thickness of the crucible, the relative position of the induction coil and the furnace, the properties of the metal material, and the size of the charge and shape, etc., all have an impact on the equivalent circuit parameters of the sensor. During the heating process, the temperature of the metal charge continues to rise, and its relative magnetic permeability and resistivity continue to change. Adding cold materials to the furnace, melting the furnace materials, and laying down the furnace materials. These changes in the furnace materials will be reflected in the equivalent circuit parameters. Therefore, the equivalent circuit parameters of the induction furnace are also constantly changing during the heating process. Especially near the Curie point, the circuit parameters change the most. Therefore, it is difficult to accurately calculate these parameters, but satisfactory results can be obtained through certain assumptions and approximations during engineering design calculations.