According to different methods of electrical energy conversion, electric heating is usually divided into resistance heating, induction heating, arc heating, electron beam heating, infrared heating, and dielectric heating.
Fold Resistance Heating Coil
Using the Joule effect of current to convert electrical energy into thermal energy to heat objects. It is usually divided into direct resistance heating and indirect resistance heating. In the former, the power supply voltage is directly applied to the object to be heated. When current flows, the object itself Electric Heating Ironing Machine
generates heat. Objects that can be directly resistance heated must be conductors but have high resistivity. Since heat is generated within the object itself, it is internal heating with high thermal efficiency. Indirect resistance heating requires heating elements made of special alloy materials or non-metallic materials, which generate heat and transfer it to the object through radiation, convection, and conduction. Since the object and the heating element are separate, the type of object is generally not restricted, and operation is simple.
The materials used for heating elements in indirect resistance heating generally require high resistivity, low temperature coefficient of resistance, minimal deformation at high temperatures, and resistance to embrittlement. Commonly used materials include iron-chromium-aluminum alloys, nickel-chromium alloys, and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum operating temperature of metal heating elements can reach 1000~1500°C depending on the material; non-metallic heating elements can reach 1500~1700°C. The latter are easy to install and can be replaced while hot, but they require a voltage regulating device during operation and have a shorter lifespan than alloy heating elements. They are generally used in high-temperature furnaces, places where the temperature exceeds the maximum allowable operating temperature of metal heating elements, and certain special occasions.
Fold Induction Heating Coil
Using the heat effect generated by induced currents (eddy currents) when a conductor is placed in an alternating electromagnetic field, causing the conductor itself to heat up. According to different heating process requirements, the frequency of the AC power used for induction heating includes power frequency (50~60 Hz), intermediate frequency (60~10000 Hz), and high frequency (above 10000 Hz). Power frequency power is the AC power commonly used in industry, with a frequency of 50 Hz in most countries. The voltage applied to the induction device from the power frequency power must be adjustable. Depending on the power of the heating equipment and the capacity of the power supply grid, high-voltage power (6~10 kV) can be supplied through a transformer, or the heating equipment can be directly connected to a 380 V low-voltage grid.
For a long time, intermediate frequency power was provided by intermediate frequency generator sets, consisting of an intermediate frequency generator and a driving asynchronous motor. The output power of such sets is generally in the range of 50~1000 kW. With the development of power electronics technology, thyristor inverter intermediate frequency power supplies are now used. This type of power supply uses thyristors to first convert power frequency AC into DC, and then convert DC into AC of the required frequency. Due to their small size, light weight, no noise, and reliable operation, they have gradually replaced intermediate frequency generator sets.
High-frequency power supplies typically use a transformer to step up the three-phase 380 V voltage to about 20,000 V, then use thyratrons or high-voltage silicon rectifiers to rectify the power frequency AC into DC, and then use electronic oscillator tubes to convert DC into high-frequency, high-voltage AC. The output power of high-frequency power supply equipment ranges from tens of kW to hundreds of kW.
Objects for induction heating must be conductors. When high-frequency AC current passes through a conductor, the skin effect occurs, meaning the current density is higher on the surface and lower in the center.
Induction heating can achieve uniform heating of the entire metal object or surface heating; it can be used for metal melting, brazing, various heat treatments (quenching, tempering, normalizing), and forging heating; in the high-frequency range, by changing the shape of the heating coil (also called inductor), localized heating can be performed arbitrarily.
