EuroWire January 2007

english The advantages of using induction heat technology in the treatment of wire products By Martin Wagstaff, Radyne, UK

Induction heating – the basic principles

An alternating magnetic field is then applied from the induction power source or generator, to the induction coil or element. By means of mutual conduction, magnetic flux lines are passed through the object in order to create a resistance to the flux path; as current subsequently flows, heat is generated. Penetration depth English physicist, Michael Faraday, in his development of the electrical transformer, initially noted the above phenomenon. Indeed, to eliminate this heating effect, transformers were subsequently designed with laminations to eliminate or reduce the effects of the electromagnetic field in heating the transformer. The reason why a laminated component built in the form of a transformer does not heat by electromagnetic induction is due to a phenomenon entitled ‘penetration depth’ or ‘reference depth’ and refers to the depth at which approximately 80% of current flows in a work piece. This depth is proportional to the electrical resistance of the material being heated and the operational output frequency (measured in Hertz) of the induction power source or generator developing the magnetic field. At high frequencies, the penetration or reference depth is thin when compared to low frequencies – and this is a key reason why induction heating is so widely used in selective heat treatments of steel where the case depths of heat treatment can be precisely controlled by careful selection of the output frequency of the induction system. A further determining factor on the effect of heating a metallic object in an electromagnetic field is power density, measured in kilowatts.

Accordingly, the higher the power density for a given frequency, the closer to the surface heating occurs. The lower the power density, the deeper the heating. The basic selection of induction heating for any specific process is, therefore, largely related to the choice of the correct induction power source output frequency and the correct power density for a given application. Calculating the frequency At frequencies used for induction heating, the current tends to flow in the surface of the conductor, to a depth dependant on the resistivity of the conductor, the frequency of the alternating current and the effective permeability of the conductor. The effective depth of current penetration, in metric form, is provided by the formulae: p = depth of current penetration r = resistivity in microhm centimetres µ = effective permeability (µ = 1 for non magnetic materials) Via selection of the correct frequency, we are able to control howmuchof thematerial is heated, with high frequencies resulting in low levels of effective penetration and deeper penetration resulting from lower frequencies. Taking our formulae, approximately 90% of the total heat is produced in the layer depth ’p’ with greater depths heated by conduction through the material. However, for the most efficient through heating, F μ π ρ 10 r 20 1 = In the above formulae:

In order to fully understand the many advantages associated with induction heating, it is first important to understand the very principles at the heart of the technology. Used for many different pro- cesses, since the inception of commercial induction heating in the early 1940s, typical applications included the melting of metals, heating prior to bending or forming, various heat treatments including hardening and tempering and the joining of metals by brazing or soldering. Earlier examples of induction heating also included the development of the radio frequency or tube-type oscillator equipment, which typically operated at high frequencies, and motor generator sets used to develop induction heating power at lower frequencies. If a textbook definition of the process of induction heating were required, it would invariably be: “Induction heating occurs when a metallic object is placed in an a varying electromagnetic field. Induction heating occurs due to the agitation of the molecular structure of the object via the electromagnetic field, when molecules are energised, collide and subsequently produce heat.” Consequently, induction heating may be compared to the electrical arrangement of a humble transformer, whereby the primary of the transformer comprises the induction power source or generator which provides power to the induction coil or element, with the object to be heated being placed in the magnetic field of that coil or element and being the transformer secondary.

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EuroWire – January 2007