The efficiency of an induction heating coil is limited by how much of its current does useful work. In a bare coil, much of the current flows where it heats nothing. Soft magnetic composites (SMCs) correct that by controlling the magnetic flux, so the same power supply delivers more heat to the part, or the same heat takes less power.
Why a bare induction heating coil wastes current
An induction system resembles a transformer with a short-circuited secondary: the coil is the primary and the workpiece is the secondary. Unlike a transformer, the magnetic circuit is open. The path includes the workpiece surface, the air in the coupling gap and the back path for flux, and only the back path can be improved by adding a magnetic material.
Take a single-turn OD coil without a flux controller. A significant portion of its current flows up the sides of the turn and around the back. That current does not heat the part. It draws extra power from the supply and adds losses in the coil, busswork and matching components.
The concentrator effect
An SMC placed on the back and sides of the coil lowers the reluctance of the back path. Less current is then needed there to drive the flux, and nearly all of the coil current flows on the heating face. The field occupies a smaller area, and the current in the part concentrates under the heat face. The ASM Handbook calls this the concentrator effect.
The practical results for the process:
- Higher coil efficiency and better use of power in the workpiece
- Lower coil current, and lower losses in the power-supplying circuitry
- Improved power factor
- Narrower, closer to rectangular power distribution under the coil face
What the numbers look like
Sourced figures give a range, not a promise:
| Coil or process | Reported effect with soft magnetic material |
|---|---|
| Single-turn channel coils (continuous heating, e.g. fasteners) | 20 to 50% less power to heat the desired area |
| Multi-turn channel coils (e.g. bar-end heaters) | Typically 5 to 20% energy savings |
| Axle scan hardening, where fillet heating drives the design | 15 to 50% energy savings, with an improved pattern |
| ID coil, simulated 55 mm ID stainless tube, 15 kHz, 10 kW in part | Coil current 1,850 A to 875 A; efficiency 70% to 84%; coil kVA 81 to 40 |
The ID coil case shows why internal inductors respond so strongly. Flux must flow in a closed loop through a narrow space inside the coil, so a core “magnetically expands” that space and cuts the current needed to push flux around the turns. Smaller parts show even larger effects.
Fluxtrol case studies report the same pattern in production. A camshaft static hardening coil with no previous concentrator cut cycle time 26% and power 30% after adding Fluxtrol A, and a fastener channel coil doubled its production rate. See the case studies for each setup.
More permeability is not the goal
For the same coil current, workpiece power rises quickly with concentrator permeability and then approaches a threshold. In most induction heating applications the threshold is reached below a permeability of about 100, and lower at high frequency. Fluxtrol testing found that a permeability of 20 to 40 is sufficient for good performance in most systems. Beyond that, extra permeability can concentrate power in the copper corners and reduce coil life with no benefit. That is why SMC grades are matched to frequency and loading rather than chosen on permeability alone.
Which coils gain most
The aspect ratio of the coil decides the size of the benefit. For OD coils, benefit is greatest when the length-to-diameter ratio and the coupling gap-to-length ratio are small, and it falls as they rise. For very large gaps relative to coil length, a controller can even lower electrical efficiency. For ID coils the effect is larger overall, and the best aspect ratios are inverted. In linear coils such as hairpin and split-and-return, the region between opposite-direction turns holds most of the magnetic resistance, so a controller there is very beneficial.
Plan the upgrade with simulation
Because the result depends on coil style, process and part, simulation is the most reliable way to forecast the improvement. It also predicts coil temperature, which indicates expected coil life. Fluxtrol’s simulation services and the grade comparison support that selection.
Get a coil evaluation
If your process is power-limited, running hot copper or slow, contact Fluxtrol to evaluate your coil, or request pricing on a specific grade through the SMC order form .