Engineers choosing a magnetic flux controller for an induction coil have three material families to pick from: ferrites, soft magnetic composites (SMCs) and electrical steel laminations. No one of them wins everywhere, and the right choice depends on frequency, flux density, geometry and cooling. Fluxtrol makes SMCs, so this comparison is drawn from the published ASM Handbook Volume 4C chapter and Fluxtrol’s materials paper, and it names where ferrites and laminations are the better choice.
Comparison at a glance
| Ferrites | Electrical steel laminations | Soft magnetic composites | |
|---|---|---|---|
| Typical frequency | Above about 100 kHz, up to 13.56 MHz | Up to about 30 kHz (50 kHz in special cases) | 50 Hz to 13.56 MHz, by grade |
| Saturation flux density | Low, below about 0.3 to 0.4 T | High, about 1.7 T | Intermediate (see products ) |
| Permeability | High in weak fields | Very high | Lower, sufficient for open circuits |
| 3D magnetic fields | Not a typical use | Intense eddy-current heating | Perform well |
| Temperature limit | Low Curie point, thermal shock | High, limited by coating | Limited by polymer binder, usually sufficient |
| Machinability | Brittle, very hard, diamond tools | Cut or stamped sheets, laborious assembly | Machinable with ordinary sharp or carbide tools |
| Size | Standard shapes | Very large simple parts | Plates up to 220 mm today |
| Cooling | Poor thermal shock tolerance | Complicated | Conduction or internal water channels |
| Price | Low | Lowest for large parts | Higher, partly offset by labor and performance |
Ferrites
Ferrites are used for flux control in high-frequency applications, usually above 100 kHz: impeders for HF tube welding, inductors for sealing and plastic welding, and small brazing coils. They offer high permeability in weak fields, high electrical resistivity (not for all grades), chemical resistance, low cost and operation up to 13.56 MHz.
The drawbacks are low saturation flux density, a low Curie point, sensitivity to thermal shock, brittleness and hardness. Induction coil designs vary widely and few coils of any one type are built, so it is not economical to make net-shape ferrite controllers. Designers adapt coils to standard shapes such as plates, rods, C and E forms. Where volume justifies it, as with ferrite rods for tube welding impeders, special shapes are made.
Laminations
Laminations are the main material for low and middle frequencies: matching transformers, shunts and cores for melting furnaces, forge heating furnaces and large heat treating coils. Very large components of simple geometry are possible, such as furnace shunts several meters long. Saturation is high (about 1.7 T), losses are low at low frequency and temperature resistance is good.
Limits are poor performance in 3D magnetic fields, limited machinability, laborious assembly, a frequency ceiling and complicated cooling. Individual laminations are typically 0.5 to 1 mm thick at line frequency and can be as thin as 0.002 in. at higher frequency, which makes them harder to work with as frequency rises. Laminations suit channel furnaces and large crucible shunts, where field geometry is simple, volume is large and cost matters.
Soft magnetic composites
SMCs are iron or iron-alloy particles, each insulated and held in an organic binder. Because eddy currents are limited to individual particles, SMCs work in 3D fields and across the whole induction range. Losses are comparable to laminations at low frequency and to ferrites at high frequency, thermal conductivity reaches up to 0.2 W/cm K, and the material can be cooled by conduction to copper or by internal water channels. Machining is easier than for laminations, and parts with walls under 1 mm have been produced.
Drawbacks: temperature resistance is lower than laminations, plate dimensions are limited, and the purchase price is higher. With labor and performance included, SMCs are often cheaper than laminations in total.
SMC versus ferrite in practice
Brazing and soldering coils are a good example of the machinability gap. SMCs can be machined into custom shapes with standard tools, whereas hard, brittle ferrites require diamond-bladed tools. In HF tube welding the gap is saturation: ferrite impeder cores in small tubes saturate near 0.4 to 0.5 T, current then flows more freely around the tube ID, and efficiency drops. SMC cores designed for the loading carry more than twice that flux density. Fluxtrol trials on production tube mills report 20 to 50% energy savings and longer impeder life, with the caveats of higher cost and greater cooling needs. See the SMC impeder cores page for the evidence.
Mixing materials on one coil
Laminations and SMCs are often used together on a single coil, with SMC in areas of complex geometry and 3D field, such as the end zones of tube seam annealers. For coils already built with laminations, Fluxtrol LRM is a direct replacement, and the case studies include coils whose life rose after lamination replacement.
Which should you choose
- Above 100 kHz with a simple geometry and a modest flux density: ferrite is usually adequate.
- Large, simple, low-frequency parts: laminations.
- 3D field, complex geometry, high flux density or medium frequency: SMC.
- Not sure: contact Fluxtrol with your coil, frequency and power, or see the grade comparison .