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How are laboratory heating plates classified according to the mode of electrical energy conversion
Laboratory, the heating plate can be widely used in areas such as production, scientific research and test, compared with the general fuel heating, electric heating can obtain higher temperature (such as electric arc heating, the temperature can reach more than 3000 ℃), easy to realize the automatic control of temperature and remote control, according to the different methods of electric energy conversion, electric heating is usually divided into resistance heating, induction heating, electric heating, electron beam heating, infrared heating and dielectric heating, etc. Next small series to introduce two forms:
1. Resistance heating: use the joule effect of current to convert electrical energy into heat energy to heat an object. Usually divided into direct resistance heating and indirect resistance heating. The supply voltage of the former is applied directly to the heated object, and the heated object itself heats up when there is a current flowing through it. An object that can be heated directly by resistance must be a conductor with a high resistivity. Since the heat is generated from the heated object itself, it is internal heating, and the thermal efficiency is very high. Indirect resistance heating requires heating elements made of special alloy or non-metallic materials, which generate heat energy that is transmitted to the heated object by means of radiation, convection and conduction. As the heated object and heating element are divided into two parts, the type of heated object is generally unlimited and easy to operate. The material used for heating element of indirect resistance heating is generally required to have large resistivity, small resistance temperature coefficient, small deformation and not easy embrittlement at high temperature. There are commonly used iron aluminum alloy, nickel chromium alloy and other metal materials and silicon carbide, molybdenum disilicide and other non-metallic materials. The zui high operating temperature of metal heating element can reach 1000 ~ 1500℃ according to the material type. The high operating temperature of zui can reach 1500 ~ 1700℃. The latter is easy to install and can be replaced by hot furnace, but it needs pressure regulating device when working, and its service life is shorter than that of alloy heating element. It is generally used in high temperature furnace, where the temperature exceeds the high working temperature allowed by metal heating element zui and some special occasions.
2. Induction heating: the conductor itself is heated by the thermal effect of induced current (eddy current) generated by the conductor in the alternating electromagnetic field. According to different heating process requirements, the ac power supply frequency used for induction heating includes power frequency (50 ~ 60 hz), intermediate frequency (60 ~ 10000 hz) and high frequency (higher than 10000 hz). Power frequency power supply is the ac power supply commonly used in industry, the power frequency of most countries in the world is 50 hz. The voltage applied to the induction unit by the power supply for induction heating must be adjustable. According to the power of the heating equipment and the capacity of the power supply network, high-voltage power supply (6 ~ 10 kv) can be used to supply power through the transformer. The heating equipment can also be directly connected to the 380-volt low-voltage power grid.
If power supply used if generator set for a long time. It consists of if generator and drive asynchronous motor. The output power of such units is generally in the range of 50 ~ 1000 kw. With the development of power electronics technology, thyristor inverter is now used as if power supply. The if power supply USES a thyristor to convert the power-frequency alternating current (ac) to direct current (dc) and then to the desired frequency (ac). Due to its small size, light weight, noiseless and reliable operation, the frequency conversion equipment has gradually replaced the if generator set.
The high frequency power supply usually raises the three-phase 380 volts voltage to about 20,000 volts high voltage first with the transformer, then USES the thyratron or the high voltage silicon rectifier element to rectify the power frequency alternating current to the direct current, then USES the electronic oscillation tube to transform the direct current to the high frequency, the high voltage alternating current. The output power of high-frequency power supply equipment ranges from tens to hundreds of kilowatts.
An object heated by induction must be a conductor. When high-frequency ac current passes through the conductor, the conductor produces skin effect, that is, the conductor surface current density is large, the conductor center current density is small.
Induction heating can uniform heating and surface heating. Smelting metal; At high frequencies, changing the shape of the coil, or sensor, allows arbitrary local heating.
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