Induction furnace working principle

Induction furnace working principle
All kinds of induction furnaces, whether it is a core induction furnace or a coreless induction furnace, and whether it is power frequency, intermediate frequency or high frequency, the basic circuit is composed of variable frequency power supply, capacitor, induction coil and metal charge in the crucible ( As shown in the figure.)
 
Basic circuit of induction furnace
(1) Principle of induction heating
The principle of induction heating is based on the following two basic laws of electricity: First, Faraday's law of electromagnetic induction
             E=B·L·v·sin∠(v·B)                                    (2-1)
 L—the length of the wire in the magnetic field;
(v·B)—the angle between the direction of the magnetic induction and the direction of the velocity.
Another basic law is Joule-Zhetz's law. This law is also known as the principle of current thermal effect. Joule - 楞tz's law can be written in the form of (2-3):   
Q   =   I   2   R t                                      (2-2)
Q — Joules, J;     I - current intensity, A;
       R — the resistance of the conductor, Ω;     t — when the conductor is energized, s;
     When an induction coil of a coreless induction furnace is connected with an alternating current of frequency f, an alternating magnetic field is generated in the space surrounded by the induction coil, and the polarity, magnetic induction and alternating of the alternating magnetic field. The frequency varies with the alternating current that produces the alternating magnetic field. If the induction coil is built with bismuth and filled with metal charge, a part of the magnetic field lines of the alternating magnetic field will pass through the metal charge, and the alternating magnetic field lines correspond to the relative movement of the cutting magnetic lines generated between the metal charge and the magnetic lines. Therefore, an induced electromotive force (E) will be generated in the metal charge, the size of which can be determined by:
               E = 4.44   Ф·   f   · n                                    (2-3)
 Ф—the magnetic flux of the alternating magnetic field in the induction coil, Wb;
       f   - the frequency of the alternating current, Hz;
       N—   The number of turns in the loop formed by the charge, usually n=1;
It can be seen from the above formula that if a large induced potential is generated in the charge, a method of increasing the magnetic flux, the frequency, and the number of turns can be theoretically used. However, the magnetic lines of force generated by the induction coil are forced to pass through the air (refer to Core induction furnace), and the air has a large magnetic resistance, so the magnetic flux is small, and it is difficult to increase the magnetic flux.   The number of turns of the charge is generally equal to 1, so in order to increase the induced potential,   Use more methods to increase the frequency.
    Since the metal charge itself forms a closed loop t, the induced current (I) generated in the metal charge is:
              I = 4.44Ф f /R    A                                     (2-4)
Where: R is the effective resistance of the metal charge, Ω;
The heating rate of the charge depends on the induced current, the effective resistance of the charge, and the energization time. The induced current is in turn dependent on the magnitude of the induced electromotive force, ie the amount of magnetic flux passing through the charge and the frequency of the alternating current. The magnitude of the induced current depends on the size of the metal charge block, the conductive nature of the charge, and the compactness of the charge. Different charges require different frequencies.
(2) Electromagnetic stirring
When an induced current flows through the charge, it is inevitably subjected to electromagnetic force. The metal charge is caused to produce directional motion; that is, "electromagnetic stirring", and its beneficial effects are as follows: 1) The temperature of the uniform molten metal. 2) Homogenize the molten metal. 3) Improve the kinetic conditions of the physicochemical reaction.

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