Air gap

In the technical sense, an air gap describes an area in the iron core of an electromagnet that is often filled with air. This not only ensures that more energy is generated and stored, as it prevents the material from becoming magnetically saturated. It also ensures that the electrical current that flows through the core made of iron, or a similar ferromagnetic material can be better metered and controlled. Air gaps are sometimes used in:

  • Transformers
  • Reactors
  • Magnetic heads
  • Speakers
  • DC relays

They are essential for the targeted use of electrically induced magnetism in the industrial, electrical engineering and medical sectors. In fact, an air gap does not have to be filled with air in every case. In sound engineering, solid diamagnetic materials are sometimes used as air gaps, which can prevent unwanted vibrations and thus disruptive mains hum.

What does an air gap in the magnetic circuit do?

A magnetic circuit - as it also occurs in transformers - usually consists of an iron core surrounded by a coil. The coil, in turn, is connected to a power source. Depending on the applied voltage and the inductivity of the coil, the magnetic flux density is generated, and a strong magnetic field is created. The permeability of the core body is also decisive. Ferromagnetic materials with high permeability, including iron, nickel, and cobalt, contain tiny elementary magnets that are initially disordered, but align themselves evenly when they meet another magnet or when they are wrapped in a current-carrying coil. This alignment makes the material itself magnetic and the field strength increases significantly. To demagnetize the ferromagnetic body again, it must be removed from the adjacent magnetic or electrical field. In addition, additional measures such as vibrations, heating or the application of a magnetic coercive field may be necessary.

If there is now a voltage that is slowly increased, sooner or later the moment of saturation occurs. This means that all elementary magnets in the ferromagnetic body are aligned in parallel and the body, which has become magnetic, reaches its maximum magnetization. From this point on, an assumed saturation curve would not continue to rise linearly, but rather flatten out. To avoid this effect and to be able to use the full potential of the magnetic circuit, a magnetic resistance in the form of an air gap is built in. This is located inside the core and breaks through it in the form of a narrow gap on one side.

The magnetic flux density is interrupted by the air gap. At the same time, the magnetic field in the air gap itself is particularly strong. The magnetic conductivity of the ferromagnetic core must therefore be increased to overcome this resistance. The conductivity is improved, a higher voltage can be applied, and the induced magnetic flux is consequently easier to control. The force of a magnet through which current flows is bundled and strengthened by air gaps.

Why should air gaps in the iron core of a transformer be as small as possible?

A transformer contains two coils, which in turn can be connected by an iron core. One of the two magnetic fields first induces a voltage in the opposite coil. The strength of this voltage depends on the existing turns ratio. If the transformer is also to be used for high power, the use of an air gap is necessary.

A magnetic circuit with an air gap in a transformer causes the inductance of the adjacent coil to decrease slightly. Accordingly, the efficiency of the transformer can easily decrease in lower power ranges. However, the air gap is necessary for higher powers to create a magnetic resistance that prevents saturation and makes higher currents usable. Air gaps are also small energy stores because they have a particularly strong magnetic field. To avoid an undesirable drop in energy density, the air gaps in magnetic circuits in transformers are designed to be as narrow as possible so as not to lose too much energy.


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