Magnetic flux does not respect equipment boundaries. In an induction system, the field that leaves the coil also reaches machine frames, chambers, fixtures and sensors. Electromagnetic shielding in this context means giving that flux a harmless place to go, so those components stay cool and the coil does not have to work harder to compensate.
When shielding is needed
Magnetic shielding is used to:
- protect certain areas of the part from unintended heating
- protect furnace or installation components, such as frames, chambers and machine parts
- eliminate the influence of the field on sensors and control system components
- comply with electromagnetic safety requirements at workplaces
Conductive shields versus SMC shields
There are two families of shields, and they behave differently.
Conductive shields (Faraday or “robber” rings). Closed loops of a nonmagnetic, highly conductive metal, usually copper, oppose the original flux through Lenz’s law. They are inexpensive and give good shielding, but the induced current produces Joule losses. The reaction field also raises the reluctance of the circuit and the coil current needed for the same heat. The result is more coil current, lower power factor and lower efficiency.
Soft magnetic shields. Laminations, ferrites and SMCs provide a low-reluctance path that diverts flux. Soft magnetic materials improve coil parameters, whereas highly conductive materials have a negative effect. SMCs shield by concentrating the fields of reaction rather than reducing them, and they are segmented to limit eddy currents.
SMCs in the field
Crankshaft hardening. A bare clamshell coil causes significant unintended heating of the crankshaft web. Thin Fluxtrol 100 plates on the coil sides eliminated that heating and reduced energy waste and distortion. In one coil, four single-turn inductors separated by Fluxtrol rings shielded each other, eliminating mutual influence.
Vacuum and protective-atmosphere melting. Shielding improves furnace efficiency, reduces coil current and eliminates chamber losses, which lets designers shrink the chamber or increase the melting unit for the same chamber. One example is a ceramic-lined coil with Fluxtrol A shields on the side and bottom for melting radioactive materials in a glove box.
Furnaces in normal atmosphere. Shielding protects frame components from unintended heating and strongly reduces the magnetic field at workplaces without impairing coil efficiency.
Medium-size furnaces. For 1 to 10 kHz furnaces, lamination shunt packs become harder to build and their support structures heat more as frequency rises. SMC poles on top and bottom let the whole coil length work together, with no counterwinding or Faraday ring needed. Shielding also greatly reduces cross-talk when several melting pots sit close together.
Large low-frequency furnaces. Here silicon-steel shunts are less expensive and work well on the back of the coil, where the field is regular. An SMC ring on top backed by a Faraday ring gives much lower losses for the same shielding, at higher initial cost, so the capital-versus-operating cost trade should be weighed.
Cold-crucible furnaces. SMC controllers shield the chamber, and SMC poles at top and bottom reduce losses from Faraday rings. The ASM Handbook cites studies showing up to 50% better coil efficiency than a standard design with laminate shunts.
Reducing coil current while you shield
The distinguishing result of SMC shielding is that it often reduces coil current rather than raising it. Counterwound turns or Faraday rings control external fields effectively but reduce coil efficiency. SMC shielding gives comparable control with lower losses, which helps the power supply, busswork and matching network as well as the equipment being protected.
Cooling the shield itself
Shields themselves absorb heat from magnetic losses and, in some cases, from the hot part. They need cooling like any other flux controller: conduction to water-cooled copper through a thermally conductive adhesive for medium loading, and water channels inside the SMC for very heavy loading. The ASM Handbook also advises choosing a material whose losses and saturation suit the local flux density.
Discuss your shielding problem
If a frame is heating, a chamber is too large or a sensor is disturbed by stray field, contact Fluxtrol with your coil and furnace details. Materials are available through the SMC order form .