Classification and difference of electromagnetic induction heating

Classification and difference of electromagnetic induction heating
The British physicist Faraday's law of electromagnetic induction heating  tells us that magnetism can generate electricity. The right hand rule of Denmark's natural philosopher Oster (Ampere's rule) tells us that electricity can be magnetized. However, whether it is magnetoelectric or electromagnetism, the object to be targeted must have good electrical, magnetic or both conductive and magnetic properties. Since only metal materials are usually eligible, in the case of non-metallic materials, only a very small number of materials such as graphite are eligible. Therefore, electromagnetic induction heating technology is mainly used for heating metal materials and graphite.

So how is heat generated? The alternating current outputted by the device is converted into a magnetic field by an inductive coil (induction coil) and then applied to a metal workpiece in an electromagnetic field. At this time, a lot of closed rotating currents (eddy currents) are naturally generated in the workpiece, which is extremely large (corresponding to a short-circuit current). Since the current has a thermal effect (Q=I*I*R*T), it naturally produces a lot of heat. In addition, there is a hysteresis loss inside the workpiece, which also causes a certain amount of heat inside the workpiece. As a result, the workpiece heats up sharply in a very short time (in seconds). If necessary, any metal material can reach the melting point and sublimate. According to the frequency of the alternating current output by the device, the induction heating technology can be divided into five categories according to the operating frequency: low frequency induction heating, medium frequency induction heating, super audio induction heating, high frequency induction heating and ultra high frequency induction heating.
There is a skin effect due to the alternating current flowing in the conductor, that is, as the frequency of the current increases, the current tends to flow through the surface of the conductor. Therefore, these five induction heating methods have different characteristics.
Comparison of characteristics: Low frequency induction heating mode has the lowest frequency, frequency range: power frequency 50HZ to 1KHZ or so, common frequency is mostly power frequency. The relative heating depth is the deepest, and the heating thickness is the largest, about 10-20mm; Mainly used for the overall heating, annealing, tempering and surface quenching of large workpieces.
Medium frequency induction heating mode frequency range:
Generally, it is about 1KHZ to 20KHZ, and the typical value is about 8KHZ. The heating depth and thickness are about 3-10 mm. It is mainly used for heating, annealing, tempering, quenching and quenching and surface quenching of large workpieces, large diameter shafts, large diameter thick wall pipes, large modulus gears, etc., and red punching and calcining of smaller diameter bars.
Super audio induction heating mode frequency range:
Generally 20KHZ to 40KHZ (because the audio frequency is 20HZ to 20KHZ, it is called super audio). Heating depth, thickness, about 2-3mm. It is mostly used for deep heating, annealing, tempering, quenching and tempering of medium-diameter workpieces, heating, welding, thermal assembly of medium-diameter thin-walled pipes, medium gear quenching, etc.
 
High frequency induction heating mode frequency range:
Generally 40KHZ to 200KHZ, commonly used 40KHZ to 80KHZ. Heating depth, thickness, about 1-2mm. It is mainly used for deep heating, red punching, calcining, annealing, tempering, quenching and tempering, surface quenching, medium-diameter pipe heating and welding, heat assembly, pinion quenching, etc. for small workpieces. Ultra-high frequency induction heating mode has the highest frequency, and the frequency range is generally above 200KHZ, which can be as high as tens of MHZ. The heating depth and thickness are the smallest, about 0.1-1 mm. It is mainly used for local miniaturization or extremely thin bar quenching, welding, surface quenching of small workpieces, etc.
The main advantages and disadvantages of inductive heating:
1) No external heating is required for the workpiece, and local heating can be selectively performed, so that the electric energy consumption is small and the workpiece deformation is small.
2) The heating speed is fast, and the workpiece can reach the required temperature in a very short time, even within 1 second. As a result, the surface of the workpiece is oxidized and decarburized, and most workpieces do not require gas protection.
3) The surface hardened layer can be adjusted by adjusting the operating frequency and power of the device as needed. Thereby, the martensite structure of the hardened layer is finer, and the hardness, strength and toughness are relatively high.
4) The workpiece after heat treatment by induction heating has a thick tough region under the hard layer of the surface, and has good compressive internal stress, so that the workpiece has higher fatigue resistance and breaking ability.
5) The heating equipment is easy to install on the production line, easy to realize mechanization and automation, easy to manage, can effectively reduce transportation, save manpower and improve production efficiency.
6) A multi-purpose machine. The heat treatment processes such as quenching, annealing, tempering, normalizing, quenching and tempering can be completed, and welding, smelting, hot assembly, thermal disassembly and diathermy forming can be completed.
7) Easy to use, easy to operate, can be turned on or off at any time. There is no need to warm up.
8) It can be operated manually or semi-automatically and fully automatically; it can be used continuously for a long time, or it can be used at random. Conducive to the use of equipment in the power supply low price period.
9) High power utilization rate, environmental protection and energy saving, safe and reliable, workers working conditions are good, the state advocates. Etc. Although, it also has some shortcomings.
For example, the equipment is relatively complicated, the cost of one input is relatively high, and the inductive components (induction coils) are poorly interchangeable and adaptable, and are not suitable for application on some complicated shapes. However, its comprehensive index is good, and its advantages are obviously more than its shortcomings. Therefore, inductive heating is currently a major process in metal processing. It is an ideal choice to replace heating methods such as coal heating, oil heating, gas heating, electric furnace heating, and electric oven heating.
How to choose induction heating equipment? Mainly from several aspects:
1) Shape and size of the workpiece to be heated: large workpiece, bar material, solid material, and induction heating equipment with high relative power and low frequency;
2) For small workpieces, pipes, plates, gears, etc., use induction heating equipment with low relative power and high frequency.
3) Depth and area required for heating: deep heating depth, large area, overall heating, should use high-intensity, low-frequency induction heating equipment; shallow heating depth, small area, local heating, low relative power, high frequency induction Heating equipment. The required heating rate requires a fast heating rate, and an induction heating device with relatively high power and relatively high frequency should be used.
4) Continuous working time of equipment: continuous working time is long, relatively inductive heating equipment with slightly higher power.
5) Connection distance between the sensing component and the device: The connection length is long, and even the water-cooled cable connection is needed. The induction heating device with larger power should be used relatively.
6) Process requirements: Generally speaking, quenching, welding and other processes can be relatively small in power selection, higher in frequency selection; annealing, tempering and other processes, with higher relative power selection and lower frequency selection; red punching and hot calcination , smelting, etc., need a process with good heat transfer effect, then the power should be selected larger and the frequency selected lower.
7) Material of the workpiece: The relative power of the high melting point of the metal material is larger, the relative power of the lower melting point is smaller; the power of the smaller resistivity is larger, and the power of the larger resistivity is smaller. and many more.
The above basic knowledge must be comprehensively analyzed and applied in order to be used well, skillfully used, and freely used. This is not only necessary for the professional technicians of each induction heating device, but also for the user and the user to understand and master.

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