Several Problems in Sensor Design
Induction heating equipment includes induction heating furnace, power supply, water cooling system and machinery for loading and unloading materials, etc., but the main purpose is to design an inductor with high heating efficiency, low power consumption and long-term use.
The inductors used for induction heating of blanks are mainly multi-turn spiral inductors. According to the shape, size and process requirements of the blank, the structural form of the inductor and the furnace type for heating are selected. The second is to select the appropriate current frequency and determine the power required for the heating of the blank, which includes the effective power required for the heating of the blank itself and its various heat losses.
When the blank is inductively heated, the power and power density input to the surface of the blank due to induction are determined by various factors. The temperature difference between the surface and the center of the blank required by the process determines the maximum heating time and power density of the blank in the inductor, which also determines the length of the induction coil for sequential and continuous induction heating. The length of the induction coil used depends on the length of the blank.
In most cases, the terminal voltage of the inductor adopts a fixed voltage in design and actual use, and the voltage does not change during the whole process from the start of heating to the end of heating. Only in periodic induction heating, the voltage needs to be reduced when the blank heating needs to be uniform, or when the heating temperature exceeds the Curie point when the magnetic material is induction heated, the magnetism of the material disappears, and the heating rate is slowed down. In order to increase the heating rate And increase the terminal voltage of the inductor. In 24 hours a day, the voltage provided in the factory is fluctuating, and its range sometimes reaches 10% -15%. When using such a power supply voltage for power frequency induction heating, the heating temperature of the blank is very inconsistent in the same heating time. When the heating temperature requirements of the blank are relatively strict, a stable power supply voltage should be used. Therefore, a voltage stabilizing device needs to be added to the power supply system to ensure that the terminal voltage of the inductor fluctuates below 2%. It is very important to heat the workpiece by heating, otherwise the mechanical properties of the long workpiece will be inconsistent after heat treatment.
The power control during induction heating of the blank can be divided into two forms. The first form is based on the principle of controlling the heating time. According to the production takt time, the blank is sent into the induction heating furnace for heating and pushing out to obtain a fixed productivity. . In actual production, the control heating time is used more, and the temperature of the blank is measured when the equipment is debugged, and the heating time required to reach the specified heating temperature and the temperature difference between the surface and the center of the blank can be determined under a certain voltage condition. This method is ideal for forging and stamping processes with high productivity, which can ensure continuous forging and stamping processes. The second form is to control the power according to the temperature, which is actually based on the heating temperature. When the blank reaches the specified heating temperature, it will be discharged immediately.
furnace. This method is used for blanks with strict final heating temperature requirements, such as for hot forming of non-ferrous metals. Generally, in induction heating controlled by temperature, only a small number of blanks can be heated in one inductor, because there are many blanks heated at the same time, and the heating temperature is difficult to control.
When the power of the input blank, the heated area and the surface power density that meet the application requirements are obtained, the inductor can be designed and calculated. The key is to determine the number of turns of the induction coil, from which the current and electrical efficiency of the inductor can be calculated. , Power factor COS A and the cross-sectional size of the induction coil conductor.
The design and calculation of the inductor is more troublesome, and there are many calculation items. Because some assumptions are made in the derivation calculation formula, it is not completely consistent with the actual induction heating situation, so it is more difficult to calculate a very accurate result. . Sometimes there are too many turns of the induction coil, and the required heating temperature cannot be reached within the specified heating time; when the number of turns of the induction coil is small, the heating temperature has exceeded the required heating temperature within the specified heating time. Although a tap can be reserved on the induction coil and appropriate adjustments can be made, sometimes due to structural limitations, especially the power frequency inductor, it is not convenient to leave a tap. For such sensors that do not meet the technological requirements, they have to be scrapped and redesigned to manufacture new ones. According to our years of practice, some empirical data and charts are obtained, which not only simplifies the design and calculation process, saves calculation time, but also provides reliable calculation results.
Several principles that should be considered in the design of the sensor are introduced as follows.
1. Use diagrams to simplify calculations
Some calculation results have been listed in the chart for direct selection, such as the blank diameter, current frequency, heating temperature, temperature difference between the surface and the center of the blank and heating time in Table 3-15. Some empirical data can be used for the conduction and radiation heat loss during induction heating of the blank. The heat loss of the solid cylindrical blank is 10% -15% of the effective power of the blank heating, and the heat loss of the hollow cylindrical blank is the effective power of the blank heating. 15% -25%, this calculation will not affect the accuracy of the calculation.
2. Select the lower limit of current frequency
When the blank is induction heated, two current frequencies can be selected for the same blank diameter (see Table 3-15). The lower current frequency should be selected, because the current frequency is high and the cost of the power supply is high.
3. Select the rated voltage
The terminal voltage of the inductor selects the rated voltage to make full use of the capacity of the power supply, especially in the case of power frequency induction heating, if the terminal voltage of the inductor is lower than the rated voltage of the power supply, the number of capacitors used to improve the power factor cos<p increases greatly. If it is large, it will increase the equipment cost of the power supply.
4. Average heating power and equipment installation power
The blank is heated continuously or sequentially. When the terminal voltage supplied to the inductor is “=constant, the power consumed by the inductor remains unchanged. Calculated by the average power, the installation power of the equipment only needs to be greater than the average power. The magnetic material blank is used as a cycle. Type induction heating, the power consumed by the inductor changes with the heating time, and the heating power before the Curie point is 1.5-2 times the average power, so the installation power of the equipment should be greater than the blank heating before the Curie point. power.
5. Control the power per unit area
When the blank is induction heated, due to the requirements of the temperature difference between the surface and the center of the blank and the heating time, the power per unit area of the blank is selected to be 0.2-0. 05kW/cm2o when designing the inductor.
6. Selection of blank resistivity
When the blank adopts sequential and continuous induction heating, the heating temperature of the blank in the sensor changes continuously from low to high along the axial direction. When calculating the sensor, the resistance of the blank should be selected according to 100 ~ 200°C lower than the heating temperature. rate, the calculation result will be more accurate.
7. Selection of phase number of power frequency sensor
Power frequency inductors can be designed as single-phase, two-phase and three-phase. The single-phase power frequency inductor has a better heating effect, and the three-phase power frequency inductor has a large electromagnetic force, which sometimes pushes the blank out of the inductor. If the single-phase power frequency inductor needs a large power, a three-phase balancer needs to be added to the power supply system to balance the load of the three-phase power supply. The three-phase power frequency inductor can be connected to the three-phase power supply. The load of the three-phase power supply cannot be completely balanced, and the three-phase power supply voltage itself provided by the factory workshop is not the same. When designing a power frequency inductor, single-phase or three-phase should be selected according to the size of the blank, the type of induction heating furnace used, the level of heating temperature and the size of the productivity.
8. Selection of sensor calculation method
Due to the different structures of the inductors, the inductors used for intermediate frequency induction heating are not equipped with magnetic conductors (large-capacity intermediate frequency induction melting furnaces are equipped with magnetic conductors), while the inductors for power frequency induction heating are equipped with magnetic conductors, so In the design and calculation of the inductor, it is considered that the inductor without a magnetic conductor adopts the inductance calculation method, and the inductor with a magnetic conductor adopts the magnetic circuit calculation method, and the calculation results are more accurate.
9. Make full use of the cooling water of the inductor to save energy
The water used to cool the sensor is for cooling only and is not contaminated. Generally, the inlet water temperature is less than 30Y, and the outlet water temperature after cooling is 50Y. At present, most manufacturers use the cooling water in circulation. If the water temperature is high, they will add room temperature water to reduce the water temperature, but the heat of the cooling water is not used. The power frequency induction heating furnace of a factory has a power of 700kW. If the efficiency of the inductor is 70%, 210kW of heat will be taken away by the water, and the water consumption will be 9t/h. In order to make full use of the hot water after cooling the inductor, the cooled hot water can be introduced into the production workshop as domestic water. Since the induction heating furnace operates continuously in three shifts a day, hot water is available for people to use 24 hours a day in the bathroom, which makes full use of cooling water and thermal energy.