Abstract
Contents
Introduction
Reducing active power losses in electrical and electrical installations is an important technical and economic task. It is especially important to solve this problem for electrothermal, in particular, induction heating installations used in the metallurgical, machine-building, petrochemical and other energy-intensive industries. The power of induction installations at industrial enterprises of these industries reaches tens of megawatts, which predetermines rather large losses of active power.
Schematic diagram of induction heating
The schematic diagram is shown in (Fig. 1). The inductor creates a variable magnetic flux and works as the primary winding of a power transformer. The heated body is placed inside the inductor so that there is a gap between the inductor and the body. The heated body acts as a secondary winding of the transformer with one short-circuited turn. EMF arising in a heated body is proportional to the magnetic flux and ensures the occurrence of a current in the body that causes heating.
(1)
- Е – EMF, arising in the heated body;
- Ф – magnetic flux generated by the inductor, Wb;
- ω – the number of turns of the inductor;
- f – mains frequency, Hz.
The power released in the heated body is proportional to the square of the current and the resistance of the heated body:
(2)
- I – eddy current arising in the body, A;
- R – active resistance of the heated body, Ohm.
Fig. 1 – Schematic diagram of the IH (1 – inductor; 2 – magnetic flux in the heated body; 3 – heated body; 4 – induced current; 5 – air gap)
The use of capacitor units to increase the efficiency of induction installations
One of the way to increase the efficiency of induction installations is the use of capacitors. Due to the significant gap between the inductor and the load (metal immersed in the furnace), induction installations have a high inductance, which reduces the total cos (f). Since self-induction creates a positive phase shift (the current lags the voltage), and the capacitance negative (current is ahead of the voltage), then if the inductive and capacitive resistances of the phase shift do not occur, the curve of current variation coincides with the voltage curve and cos (f) = 1.
This condition is observed automatically in a circuit with free oscillations, therefore in installations with a tube generator cos (f) is always equal to one. In the installation, powered by machine generators, for the equality of inductive xL and capacitance xc, it is necessary to include capacitor banks. Capacitors are usually used in groups (batteries) made up of elements connected in series or in parallel.
As for power, serial and parallel connections are equivalent, but parallel connections of capacitors are preferable, as it provides a small generator current and no overvoltage on the inductor of the furnace and the capacitors.
In the installation of induction furnaces used paper-oil and ceramic capacitors. In paper-oil condensers, the plates are aluminum foil with a thickness of 0.007–0.015 mm, and the dielectric is waxed or oiled paper. Condensers are wound from tapes in the form of a roll, and then pressed into a flat package. Packages are immersed in a can of transformer oil, which is sealed to prevent contamination of the oil. The reactive power of a condenser can be increased if the heat released in it is removed by immersing a coil fed with running water or passing water between the double walls of a can. To increase heat transfer by radiation, the banks outside are painted black.
In high-frequency installations with lamp generators, ceramic capacitors are widely used. The plates in such capacitors are the thinnest layer of silver deposited on the inner and outer surfaces of the capacitor before firing ceramics. The advantage of ceramic dielectrics is their high dielectric constant (for some types of ceramics, it reaches 1000 versus 3-4 for oiled paper); The disadvantage is the difficulty of manufacturing thin-walled capacitors. The considerable thickness of the dielectric reduces the capacitance of the capacitor, so these capacitors are manufactured for small capacities.
In order to maintain cos (f) = 1, when L is changed, it is necessary to change the value of C. For this reason, in installations with machine generators, some of the capacitor banks are permanently included in the furnace circuit, and some can be turned on or off from the circuit during smelting.
Conclusion
- It is necessary to use capacitor batteries in order to avoid a decrease in cos (f) due to the presence of a gap between the inductor and the load (metal immersed in the furnace).
References
- Прецизионный нагрев цилиндрических заготовок в индукционном нагревателе периодического действия / В. Б. Демидович [и др.] // Изв. СПбГЭТУ
ЛЭТИ
. – 2001. – №2. – С. 53–59. - Энергоэффективный индукционный нагрев алюминиевых заготовок перед прессованием / В. С. Немков [и др.] // Индукционный нагрев. – 2012. – №3 (21). – C. 10–15.
- Инновационные технологии обработки титановых сплавов с применением индукционного нагрева / В. Б. Демидович [и др.] // Индукционный нагрев. – 2012. – С. 26–30.
- Влияние конструкции и режимов работы индукционных нагревателей на их энергетические показатели // В. С. Немков [и др.] // Электротехника. – 1986. – №3. – С. 23–25.
- Немков В. С. Теория и расчет цилиндрических электромагнитных систем индукционного нагрева : дис. … д-ра техн. наук : 05.09.10 / В. С. Немков ; ЛПИ – Л., 1979. – 326 с.
- Harvey J. G. The theory of multy–layed windings for induction heating and their application to a 1 MW 50 Hz longitudinal flux billet heater / J. G. Harvey // VIII Congress UIE. – Liege, 1976. – №4. – S.1la.
- Перспективы применения криогенной техники в электротермии / В. А. Григорьев [и др.] // Электротехн. пром-ть. – 1980. – №1. – С. 9–11.
- Ижикова А. Д. Индукторы с самокомпенсацией реактивной мощности систем электроснабжения электротехнологического назначения : автореф. дис. ... канд. техн. наук : 05.09.03 / А. Д. Ижикова ; ЮУГУ. – Челябинск, 2007. – 24 с.