A flux controller is not simply an accessory added to an induction coil. It changes the magnetic circuit – and therefore where energy is delivered to the workpiece.
In an induction heating system, the copper inductor carries alternating current and creates an alternating magnetic field.
That field induces current in the workpiece. The resulting electrical losses heat the part.
A magnetic flux controller changes the path of that magnetic field.
That sounds simple, but it is the reason a relatively small piece of magnetic material can have a large effect on heat pattern, efficiency, stray heating and coil behaviour.
Robert Goldstein’s ASM Handbook article defines magnetic flux controllers as materials other than the copper coil that are used in induction systems to alter the flow of the magnetic field. [1]
They may be called concentrators, controllers, diverters, cores, impeders, yokes, shunts or screens depending on what they are doing.
The name changes. The physics does not.
Think of the induction system as a magnetic circuit
The magnetic field produced by the coil does not automatically travel only where you want it.
It spreads through the available magnetic paths around the inductor, workpiece, fixtures and surrounding structure.
A flux controller introduces a material with magnetic properties that change those paths.
Fluxtrol’s technical literature describes three broad functions:
- concentration
- shielding
- redistribution of the magnetic field [2]
Those functions can be used separately or together.
Concentration: putting more field where it is useful
A concentrator is positioned so that magnetic flux is encouraged toward the region of the workpiece that needs heating.
The effect can increase coupling between coil and part and reshape the heat pattern.
This can be useful where a coil otherwise produces a broad or weak heating zone, or where energy needs to be focused near a feature.
Goldstein’s ASM article lists precise control of the magnetic field and resulting heat pattern among the principal benefits of correctly applied magnetic flux controllers. [1]
The word “correctly” matters.
A concentrator cannot fix every poor coil design, and adding more magnetic material is not automatically better.
Shielding: keeping the field away from something
Sometimes the problem is not where the field needs to go, but where it should not go.
Nearby shafts, fixtures, machine components or sections of the workpiece can experience unwanted induced heating.
A magnetic controller can redirect or shield field from those regions.
Fluxtrol’s literature identifies reduction of external magnetic fields close to the coil as another major benefit. [1]
This can improve thermal control and reduce unwanted heating of adjacent structures.
Redistribution: reshaping the field rather than simply increasing it
Many applications need something between concentration and shielding.
The goal may be to balance heating around a geometry, sharpen a transition, change the relative intensity between two areas or compensate for an asymmetric electromagnetic condition.
In those cases, the controller is redistributing flux.
That is why “flux concentrator” is not always the most accurate general term. The material may be doing more than concentrating.
Why heat pattern changes
The workpiece heats where induced current density and electrical losses are highest.
Change the magnetic field, and you change the induced-current distribution.
That is the connection between a magnetic controller and the final thermal pattern.
Fluxtrol’s paper by V. Nemkov states that soft magnetic composite controllers can provide accurate heat-pattern control and improve inductor and installation performance. [2]
This is particularly important in induction heat treatment, where case depth and hardened pattern are often tightly specified.
Efficiency can improve too
If more of the useful magnetic field couples to the workpiece and less escapes into surrounding space, the induction system may use power more effectively.
Goldstein’s ASM article lists improvement in efficiency of high-frequency power-supplying circuitry as another possible benefit of magnetic flux control. [1]
That does not mean every application will show the same energy saving.
The result depends on geometry, frequency, material, coil design and the condition of the existing system.
Efficiency should be treated as an engineering outcome to be measured, not a guaranteed percentage.
Why soft magnetic composites are useful
Traditional magnetic-flux control in induction systems has included laminated magnetic materials.
Modern soft magnetic composites, or SMCs, allow magnetic material to be machined into three-dimensional shapes and used in high-frequency induction environments.
Nemkov’s technical paper notes that controller material selection requires consideration of magnetic, electrical, thermal and mechanical properties because service conditions vary widely. [2]
The material is simultaneously being asked to influence a strong alternating magnetic field while living close to a hot copper coil, coolant passages, mechanical clamping and a heated workpiece.
Permeability is not the only property that matters
It is tempting to compare magnetic materials by a single headline number such as permeability. That can be misleading.
The useful material depends on operating frequency, field strength, geometry, losses, temperature, thermal conductivity, machinability and mechanical constraints.
Fluxtrol’s more recent technical summary of the ASM work explicitly notes that higher permeability is not automatically better and that material selection involves finding an appropriate operating range for the application. [3]
A magnetic material can also approach saturation if the magnetic loading is too high, reducing the benefit expected from the controller.
Geometry matters as much as material
Two pieces of the same magnetic material can behave very differently if shaped and positioned differently.
Thickness, air gaps, distance from the workpiece, coverage around the coil and the direction of the magnetic path all affect performance.
This is why successful controller design often combines:
- electromagnetic simulation
- practical coil-design experience
- thermal understanding
- experimentation on the actual part
Fluxtrol’s technical library includes work on computer-assisted design, crankshaft hardening, axle scan hardening and other applications where field control is part of the inductor design rather than an afterthought. [4]
What can go wrong?
Incorrectly applied magnetic material can create its own problems.
Depending on the application, possible failure modes include:
- local overheating
- mechanical cracking
- water ingress or degradation
- excessive magnetic losses
- saturation
- poor heat-pattern control
- interference with cooling
- reduced coil accessibility or maintainability
This is why selecting a controller from a catalogue only by colour or “frequency range” is not enough for demanding applications.
The actual electromagnetic and thermal loading matters.
Where flux controllers are commonly used
Applications can include:
- induction hardening
- brazing
- shrink fitting
- tube and pipe welding
- melting
- internal-diameter heating
- scan heating
- single-shot heating
- coils near sensitive adjacent features
In tube welding, the magnetic component placed inside the tube is commonly called an impeder. Its geometry and material perform a specialised field-control role in the welding process.
When should you consider a flux controller?
Look at magnetic field control when you have symptoms such as:
- insufficient heating in a target area
- excessive heating nearby
- a broad heat pattern that needs sharpening
- poor electrical efficiency
- uneven case pattern
- strong stray-field effects
- recurring coil hot spots
- a coil geometry that is difficult to make efficient without magnetic assistance
The right first step is not always “add material.” It is to understand what the present field is doing.
Information an applications engineer will need
A useful induction-heating enquiry should include:
- part drawing
- part material
- target heat pattern or case depth
- process type
- power
- frequency
- cycle or scan speed
- existing coil drawing or photographs
- cooling arrangement
- current concentrator material, if any
- what is currently going wrong
Photographs are valuable. A sectioned part showing the actual hardening pattern can be even more useful.
The practical conclusion
A magnetic flux controller is part of the magnetic circuit.
It can concentrate, shield or redistribute field, which can change heat pattern, coupling, efficiency and stray heating.
But it is not a universal bolt-on improvement.
Material, geometry, frequency, magnetic loading, thermal conditions and coil design all interact.
The best results come when flux control is designed as part of the induction process, not added after the process has already gone wrong.
Sources and references
[1] R. Goldstein, ‘Magnetic Flux Controllers in Induction Heating and Melting’, ASM International / Fluxtrol – Source
[2] V. Nemkov, ‘Magnetic Flux Control in Induction Systems’, Fluxtrol – Source
[3] Fluxtrol – The Industry Standard for Magnetic Flux Control – Source
[4] Fluxtrol – Technical Library Articles – Source