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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)
