Air gap
What is an air gap?
An 'air gap' has a special meaning in magnetic field technology. It refers to an (air-filled) gap in the ferromagnetic core of electromagnets. If it has been precisely calculated, the gap means that the electromagnet can be operated with very different currents and can store magnetic energy better than a magnet without an air gap. The air gap also prevents premature magnetic saturation, in which the magnetic field stops increasing at high currents and energy is lost as heat.Table of Contents
Generally speaking, an air gap is simply a gap that is filled with air.
In technology, this is usually the distance between opposite surfaces in components.
In the technology of electromagnets, an air gap is not simply a gap that arises by chance due to the design but instead has important functions.
What is the effect of an air gap?
In transformer iron cores, an air gap is present to prevent magnetic saturation of the iron core during normal operation. The magnetic field in the air gap is also significantly greater than that in the iron material, which means that magnetic energy is stored in the air gap. The transformer then acts as a short-term energy storage, which is advantageous for certain applications.A transformer consists of two opposing electromagnets. The magnetic field of one electromagnet induces a voltage in the coil of the second electromagnet. The magnitude of this voltage depends on the ratio of the number of turns of the two coils. The transformer, therefore, changes the magnitude of voltages and currents. A ferromagnetic core (usually iron) in the electromagnets supports this process.
When a transformer is to be used in a wide power range, i.e., if it is to operate at both low and high power levels and if its properties are to change as little as possible, iron cores with an air gap are installed in the electromagnets of the transformer.
An air gap in the iron core of an electromagnet fulfils a technically important purpose.
The air gap reduces magnetic saturation.
In addition, magnetic energy is stored in the air gap.
Air has a much lower magnetic permeability
than iron.
Thus, the air gap reduces the magnetic flux density
in the interrupted iron core compared to an iron core without an air gap.
In contrast, the magnetic field in the air gap is very strong.
This means that although the transformer works somewhat less efficiently at low power levels, the magnetic saturation of the iron core does not occur as quickly at higher power levels. The overall lower magnetic flux density in an electromagnet with an air gap is proportional to the magnetic field over a wider range than without an air gap.
Many ferromagnetic materials have a saturation magnetisation of 1-2 tesla. This is not much. In many technical applications, significantly higher magnetic flux densities do occur.
The physical reason for magnetic saturation is that the atomic spins of the ferromagnetic material (iron core) are fully aligned in a certain external magnetic field. Despite an increase in current, the magnetic flux density then no longer increases.
The electron spins
in all Weiss domains
of the ferromagnet are aligned in parallel.
In a transformer, the current then rises sharply in the so-called primary circuit of the transformer, i.e., in the coil whose voltage is to be converted.
This reduces the power of the transformer and a lot of energy is lost due to the transformer heating up.
Choosing the appropriate air gap thickness helps to "tailor" the transformer to the appropriate power range.
Author:
Dr Franz-Josef Schmitt
Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.
Dr Franz-Josef Schmitt
Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.
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