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Hysteresis Heating and Eddy Current Heating
Impeders require cooling for several reasons. The most obvious one is that they operate
in close proximity to hot material however this only contributes a small fraction of the
total heat which must be removed by the cooling medium.
A theoretically perfect ferrite material would not require cooling, but unfortunately
this isn’t a perfect world, and such a material doesn’t exist yet. Heat is generated
within the ferrite by two main causes - hysteresis heating & eddy current heating.
Some additional heat may be produced as a result of magnetorestriction if the ferrite
is cycled through saturation.

Hysteresis Heating
When ferrite or any other magnetic material is within an alternating magnetic field, the
material reverses polarity at twice the frequency of the field. Since all magnetic materials
retain some magnetic “charge”, this residual magnetism has to be overcome by the
energizing field, and the energy required is converted to heat. At low frequencies, not
much heat is produced, but at 450 kHz, it is a major cause of ferrite heating. The ability of
the magnetic domains to align themselves with the applied field is affected by temperature.
As temperature is decreased, more energy must be applied to change the magnetic
polarization of the domains, so more heat is produced. Because of this, it is counterproductive
to cool ferrite excessively, as this actually produces more heat! Most ferrites
intended for use in impeders operate most efficiently at temperatures in the 80° to 120°C
range.

Eddy Current Heating (Resistance Heating)
Manganese/zinc ferrites conduct electricity to some extent, so eddy currents are induced
in them by the work coil. These currents flow around the outer circumference of the
ferrite, causing further heating due to the resistance of the material. Eddy current heating
can be reduced by using ferrite which has longitudinal slots, as this increases the length
of the current path. The slots also provide additional surface area for cooling. The
electrical resistivity of ferrite is dependant on formulation, processing parameters &
density. The higher the resistivity, the less eddy current heating will be produced.
For any given flux density, hysteresis heating is proportional to the rate of change (or
frequency) of the applied field, however eddy current heating increases with the square of
the frequency. In induction welding, higher frequencies require lower flux densities so the
square law does not apply fully, however it is still true that hysteresis heating predominates
at low frequencies, whereas eddy currents are the primary source of heat at higher
frequencies.

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