If an alternating electric current is supplied to a coil, an alternating magnetic field is produced surround it. If another coil is brought inside this magnetic field, an alternating emf is induced across the second coil also as per Faraday's law of electromagnetic induction. This induced emf in the second coil can be utilized to feed a load. As in this example the flux is linked with both coils through air, this arrangement can be referred as air core transformer. Here, first coil serves as primary winding and second coil serves as secondary winding of the said transformer.
Whenever there is a need of changing voltage level from one level to another in power network we use an electrical transformer. Whenever there is a need of electrical isolation from one circuit to other, we use transformer. For receiving electric current and voltage signals of power system network, at low measurable level we use current transformer and potential transformer respectively. These are not only the applications of transformers. Electrical transformer also are used for many different purposes. In different electronics applications smaller sized transformers are used. Examples of these transformers are air core transformer, audio frequency transformer etc. These are referred as special purpose transformers and simply called special transformer. Construction wise they are different from general power transformer but basic working principle is same as all other transformer. That means in this case, the working principle of transformer is also based on mutual induction between two magnetically linked windings.
In an transformer primary winding and secondary windings are coupled with a low reluctance ferromagnetic core. The core is used in an electrical transformer for confining magnetic flux and facilitating maximum flux linkage between primary and secondary winding. Laminated steel sheets are used to construct the core of transformer. But, ferromagnetic materials like steel as core of transformer, suffers from hysteresis loss, eddy electric current losses. Also it faces problem of getting saturated after certain level of magnetization. But these can be avoided in air core transformer as ferromagnetic core is absent in this transformer.
In very high frequency applications, distortion in electromagnetic field should be avoided otherwise there may be a significant affect in output signal. So for maintaining quality of signal it is desired to avoid any noise or distortion in the signal. As ferromagnetic material causes noise or distortion in the signal, it should be avoided in high frequency application like signal transmission. Thus air core transformer is introduced, in the application of high frequency radio transmission. Here iron core of transformer is absent and the flux is linked with the windings through air. In addition to the noise-free operation, an air core transformer is quite light weight due to absence of heavy weight iron core. That is why this type of transformer is most suitable for portable, light weight electronic devices and high frequency devices. Air core transformers are are generally used in radio transmitter and communication devices etc.
Construction of Air Core Transformer
The coils of insulated copper wire, are wound on a plastic tube or cardboard etc. There are mainly two forms of air core transformer. One is cylindrical form and other is toroidal form. In cylindrical form of air transformer, insulated copper wires are wound on a non metallic cylindrical structure. This non metallic hollow structure gives the winding necessary mechanical support. The copper winding wounded around the non metallic cylinder is tapped at different points as necessary. The secondary supply is taken across these taps. Sometimes capacitor is connected to the winding to maintain the constant resonance of the tuning circuit. The flux flows through the air surrounding the winding and air inside hollow cylinder. For proper impedance matching, sometime a guard winding is wound surrounding the main copper winding. This guard winding is further connected with antenna receivers or grounded properly.
On the other hand in toroidal form of air core transformer, insulated copper wires are wound around a ring of hard plastic or other non metallic substance. Actually, the wire is wound in circular way around the periphery of the ring. This type of transformers are only used in very high frequency applications. In this case the winding is not tapped, here two separate windings are used for primary and secondary purpose. Therefore, the effect of stray coupling is minimized in toroidal air core transformer. The primary is to secondary ratio of windings varies and it depends upon the frequency range and the type of operation.