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11/16/2013 Induction Heating Theory 1

INDUCTION HEATING THEORY

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HISTORY

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DiscoveredBy Michael Faraday in 1831

Induction heating effectWas an undesirable side effect of motors, generators andtransformers

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BASICS

When an alternating current flows through a coil amagnetic field is generated

Coil

This field is concentrated on the inside of the coil and itsmagnitude depends on the strength of the current and thenumber of turns in the coil

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PENETRATION DEPTH

The induced current also generates its own magnetic field,which is in opposition to the field generated by the coil andwhich prevents the latter field from penetrating to thecenter of the metal object.

Field

Penetrationdepth

Number of arrows represents frequencyCoil

Workpiece

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PENETRATION DEPTH

The distance from the surface to the depth where currentdensity has dropped to 37% is referred to as thePENETRATION DEPTH . The depth of penetrationincreases with decreasing frequency .

Field

Penetrationdepth

Number of arrows represents frequency

Coil

Workpiece

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COIL DESIGN

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The design of coils (inductors) is largely based onexperience, however there are some basic rules thatshould be kept in mind

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COIL DESIGN

Rule No.1The coil should be as close as possible to the workpiece at the area to

be heatedA distance between the workpiece and the coil will reduce the flux inthe workpiece and consequently the current and heat generation

0

20

40

60

80

100

0 2 4 6 8 10mm

Relative effeciency %

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COIL DESIGN

Rule No.2The area of the coil against the workpiece determines the heated areaThe widest side of a rectangular coil tube should be against theworkpiece to achieve the highest input

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COIL DESIGN

Rule No.3The coil should be designed so that the field from the opposite sides ofthe coil do not cancel each other

Cancellation of fieldGOOD BAD

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COIL DESIGN

Rule No.3On the other hand this effect should be used to minimize losses in theareas where heating is not required


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COIL DESIGN

Rule No.3That means that the connecting tubes from the transformer to theworking coil should be as close together as possible without risk ofshort circuiting


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COIL DESIGN

Rule No.4Coils surrounding the workpiece like a round bar placed inside a roundcoil gives the highest coupling efficiency

Type of coil Magneticsteel

Other metals

Helical around workpiecePancakeHairpinOne turn around wor kpieceChannelInternal

0,750,350,450,600,650,40

0,500,250,300,400,450,20

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COIL DESIGN

Rule No.5 Ends and edges are heated more easily than areas where heat candissipate in all directionsDiffering material thickness may require only the “thick” part to be

heated, where possible allow conduction to heat the thin parts

To achieve the desired heating pattern often bothexperience and tests are needed

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COIL DESIGN

Flux concentratorsAre magnetic materials which are utilized to gather the fields of fluxCan be used to increase heating or decrease heating in unwanted areas

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COIL DESIGN

Flux concentratorsIt can be seen that flux concentrators can be used to avoid unwantedheating effects in adjacent material

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COIL DESIGN

Flux concentrators Are normally available in two types

Iron laminations Composite ferrite material

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COIL DESIGN

Efficiency with flux concentrators

0

0,2

0,4

0,6

0,8

1

0 1mm 2mm 3mm 4mm 5mm 6mm 7mm 8mm 9mm 10mm

Coil with flux concentrators Coil without flux concentrators

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COIL DESIGN

• IMPEDANCE MATCHING V ARIABLES – Coil Width – Coil Length –

Tubing Geometry – The Distance – Field Concentrators – Working Material (Magnetic / Non-

Magnetic)

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