Induction Heating – Diagram ,Working,  Application

Introduction to Induction Heating :

Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, through heat generated in the object by eddy currents. An induction heater consists of an electromagnet, and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flowing through the resistance of the material heat it by Joule heating. In ferromagnetic (and ferromagnetic) materials like iron, heat may also be generated by magnetic hysteresis losses. The frequency of current used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.

1) Electrical heating is superior to other methods of heating because of the following reasons:

  • Its maintenance is easy as there are no moving parts.
  • Electrical heating is clean and efficient
  • It offers compact and reliable heating system.

2) Electrical heating is done either at high frequency or at power frequency. At Power frequency heating, heat is transferred to the material to be heated by convection, conduction or radiation. At high-frequency heating, electrical energy is converted into heat within the material itself. High frequency heating is, therefore, more efficient.

3) Principle of Induction Heating: When a transformer primary is energized , iron loos occurs in the core of transformer. This core loss, made up of eddy-current loss and hysteresis loss, is undesirable and efforts are made to minimize this loss in transformer. This eddy current loss is, however, exploited in producing heat in the metals to be heated and this is what is called induction heating.

4) Factors Affecting Induction Heating δ

induction heating

In addition to eddy current loss, hysteresis loss also leads to heating of the metal object. But hysteresis loss is much less as compared to eddy current loss, hence it is usually neglected.

Factors governing induction heating are magnetic field intensity, relative permeability, frequency and resistivity. Higher the values the greater is the heat generated by induction heating.

5) Induction heating Circuit

For frequencies from 250Hz to 10KHz, rectifier – inverter circuits are used. The two basic circuit configurations are as under:

  • Voltage source series resonant inverter(i) Fig shown in (a) , diode rectifier converts utility voltage to direct voltage. The dc link voltage is kept level by the use of capacitor C1 . Inductor L1 limits the current pulsations.

    (ii) Load inductance L is large, therefore C should be suitably chosen so that RLC circuit is underdamped.

    (iii) For load commutation to occur, switching frequency should be below the circuit resonant or circuit ringing, frequency.

  • Current source parallel-resonant inverteri) Fig shown in b) , inductor L1 is quite large.

    ii) With the turning on of starting SCR Ts a constant current flows through L1 .

    ii) In this configuration, the switching frequency is kept somewhat more than the circuit resonant frequency. Load power is regulated by controlling the firing angle of thyristor rectifier.

Applications of induction heating

i. Induction hardening :

Induction hardening uses induced heat and rapid cooling (quenching) to increase the hardness and durability of steel. Induction is used to harden gears, crankshafts, camshafts, drive shafts, output shafts, torsion bars, rocker arms, CV joints, tulips, valves, rock drills, slewing rings, inner and outer races.

ii. Induction tempering :

Induction tempering is a heating process that optimizes mechanical properties such as toughness and ductility in workpieces that have already been hardened. Induction tempering is widely employed in the automotive industry to temper surface-hardened components such as shafts, bars and joints.

iii. Induction brazing

Brazing is a materials-joining process that uses a filler metal (and usually an anti-oxidizing solvent called flux) to join two pieces of close-fitting metal together without melting the base materials. The aeronautics sector uses induction to braze fan blades, blades for casings, and fuel and hydraulic systems.

iv. Induction bonding

Induction bonding uses induction heating to cure bonding adhesives. Induction is the main method for curing adhesives and sealants for car components such as doors, hoods, fenders, rearview mirrors and magnets.

v. Induction welding

With induction welding the heat is electromagnetically induced in the workpiece. Induction welding is used in the tube and pipe industry for the longitudinal welding of stainless steel (magnetic and non-magnetic), aluminum, low-carbon and high strength low-alloy (HSLA) steels and many other conductive materials.

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