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The Fundamentals of

Induction Heating

What is INDUCTION HEATING ?

A non-contact method of heating a conductive material as a

result of the electric currents induced in it by an externally

applied magnetic field.

HOW DOES IT WORK ?

Work Piece

Induction Heating CoilInduction

Unit

Work Piece

Induction Heating Coil

HOW DOES IT WORK ?

Induction Heating Coil(Cross Section)

HOW DOES IT WORK ?

HOW DOES IT WORK ?

• When the Induction Unit is active….

HOW DOES IT WORK ?

• …. The Induction COIL is energised

HOW DOES IT WORK ?

• This generates an ALTERNATING CURRENT in coil circuit

HOW DOES IT WORK ?

• The current creates a MAGNETIC FIELD in proximity to the coil turns

HOW DOES IT WORK ?

• The current creates a MAGNETIC FIELD in proximity to the coil turns

HOW DOES IT WORK ?

• When the WORK PIECE is positioned within the coil….

HOW DOES IT WORK ?

• When the WORK PIECE is positioned within the coil….

HOW DOES IT WORK ?

• …. HEATING occurs within the work piece

HOW DOES IT WORK ?

HYSTERESIS HEATING

Hysteresis is the dependence of the state of a system on its

history.

HYSTERESIS HEATING

When heating components with induction, this situation can only occur:

(i) In magnetic materials (ii) Up to the Curie Point* of the material

*Curie Point being the temperature that a given material is no longer magnetic

HYSTERESIS HEATING

Fig #1

• Magnetic particles

• Room temperature

• Random direction

HYSTERESIS HEATING

Fig #2

• The current in the coilgenerates a magnetic field

• Particles begin to move andalign with the direction of thefield

HYSTERESIS HEATING

Fig #3

• The magnetic field changesdirection again

• More particles move and alignwith the direction of the field

HYSTERESIS HEATING

Fig #4

• Ultimately all particles are inalignment

• Curie Point achieved (720oC)

• Work piece no longer magnetic

• Molecular friction created

• Heat generated within the workpiece

EDDY CURRENT HEATING

Eddy currents are loops of electrical current induced within

conductors by a changing magnetic field in the conductor

due to Faraday’s law of induction.

EDDY CURRENT HEATING

When heating components with induction, this situation will occur:

(i) In Ferrous & Non-Ferrous materials (ii) At any temperature

The rate of heating is dependent of the electrical resistivity factor of the material being heated.

EDDY CURRENT HEATING

Electrical resistivity is the property that quantifies how

strongly a given material opposes the flow of electric

current.

A low resistivity factor indicates a material that readily

allows the flow of electric current.

EDDY CURRENT HEATING

Therefore

• Materials with a HIGH electrical resistivity factor heat very readily (with induction)

• Materials with a LOW electrical resistivity factor will require more time or power

SUMMARY OF HEATING RATES

Below Curie Point Above Curie Point

Magnetic Material Hysteresis + Eddy Current

Eddy Current Only

Non-Magnetic Material Eddy Current Only Eddy Current Only

FREQUENCY

The hertz (symbol: Hz) is the SI unit of frequency and is

defined as one cycle per second. It is named after Heinrich

Rudolf Hertz, as the first person to provide conclusive proof

of the existence of electromagnetic waves.

Hertz are commonly expressed in multiples such as kilohertz

(103 Hz, kHz)

OPERATING FREQUENCY

UK domestic power supply

50 cycles per second

50hz

OPERATING FREQUENCY

The nominal operating frequency of an induction heatingsystem is therefore the number of cycles of the AC current inthe heating coil.

Examples of typical operating frequencies can be (but notlimited to) the region of:

1kHz, 3kHz, 10khz, 25kHz, 100kHz, 200kHz, 400kHz

OPERATING FREQUENCY

Why is frequency important ?

• Induction is a SKIN HEATING process

• The eddy current travels over the surface of the work pieceat a “known” DEPTH of PENETRATION

• This depth is a function of the operating frequency

OPERATING FREQUENCY

The HIGHER the frequencyThe SHALLOWER the depth of penetration

The LOWER the frequencyThe DEEPER the depth of penetration

OPERATING FREQUENCY

HIGH Frequency MEDIUM Frequency

OPERATING FREQUENCY

Current Cancelation

kW & kHz – WORKING TOGETHER

A typical induction system is a consideration of power and frequency:

Examples:• 560kW / 10kHz – 16mm spring wire – pre-curie heating• 50kW / 400kHz - Case depth hardening of an automotive part• 1mW / 1kHz – Billet heating for extrusion – 24” round billet

APPLICATIONS

• Hardening

• Tempering

• Brazing

• Bonding

• Tube Welding

• Pre-heating

• Post-heating

• Forging

• Melting

• Annealing

• Low Relaxation

• Normalising

• Straightening

• Bolt expansion

• Thermal curing

• Shrink fitting

• Glass wool fiberizing

• + Many More

POWER CALCULATIONS

Basic Parameters required for application evaluation:

• Material• Size / diameter• Target temperature• Heating time / Cycle time / Production rate

POWER CALCULATIONS

Basic Parameters required for application evaluation:

• SAE 9254 Spring Steel• 16mm diameter• 960oC• 3,000kg/hr

POWER CALCULATIONS

POWER CALCULATIONS

POWER CALCULATIONS

WHY USE INDUCTION

• Repeatable process• Very accurate process control• Reduced cycle time / faster production rates• Highly efficient• No environmental emissions• Typically no “start-up time” for the equipment

SHOWCASE EXAMPLES

SHOWCASE EXAMPLES

SHOWCASE EXAMPLES

SHOWCASE EXAMPLES

SHOWCASE EXAMPLES

Weld Point

HF Current flows along Vee edges, VL, and

around the tube ID. The edges touch at the

apex, just ahead of the weld point.

Weld Rolls

Vee

2

Weld Coil

High Frequency Induction

Apex

VL

WIRE HEATING APPLICATIONS

• PRE-HEATING prior to FURNACE or GALVANISING LINE

• TIRE CORD / DIFFUSION LINES

• PC WIRE LOW RELAXATION – PC WIRE & STRAND

• COATING / CURING LINES

• HSS – HEATING prior to DRAWING

• AUTOMOTIVE SPRING STEEL PROCESSING

WIRE HEATING APPLICATIONS

WIRE HEATING APPLICATIONS

WIRE HEATING APPLICATIONS

WIRE HEATING APPLICATIONS

WIRE HEATING APPLICATIONS

Q & A

Everyday items – If there is time

• Bra underwire (Curing)

• Ice Lolly Moulds (Heat releasing)

• Fridge / Freezers (Brazing)

• Tyres (Tire Cord / Bead wire)

• Buildings (PC Wire)

• TV Speakers (plastic to metal bonding)

• Yogurt pots / lids (Cap sealing)