Transformer core classification

Transformer core

When making any transformer, manufacturers try to achieve maximum magnetic coupling between the two inductors. The magnetic coupling can be increased many times by using ferromagnetic material or iron powder as the magnetic core. A pair of inductors wrapped around a ferromagnetic core has a better coupling coefficient than an air-core transformer. However, the use of ferromagnetic cores has its own limitations. Ferromagnetic cores have some energy losses due to hysteresis and eddy currents, and are also limited by their current carrying capacity. In addition to these limitations, the choice of core material also limits the frequency range of the transformer. Depending on the type of core used, transformers are classified as follows
Laminated iron transformers: These transformers have silicon steel as the core material. Silicon steel is also known as transformer iron or simply iron. Silicon steel is laminated in layers to avoid losses due to eddy currents and hysteresis. Eddy currents are circular currents that flow in magnetic materials when magnetized. Eddy currents cause the core to lose energy in the form of heat. Hysteresis is the tendency of a magnetic core to accept fluctuating magnetic flux. Due to hysteresis and eddy current losses, these transformers are only suitable for 60 Hz frequencies and other low frequencies in the audio range. As the frequency increases above a few kilohertz, the internal losses of the core increase beyond the feasible limit.
Ferrite Cores: Ferrite cores have high permeability and require fewer coil turns. However, at frequencies above a few megahertz, such cores start to show significant energy losses due to eddy currents and hysteresis. That's why these transformers are suitable for audio frequencies up to several megahertz.
Powdered Iron Cores: Powdered iron also has higher permeability and lower losses than ferrite cores due to hysteresis and eddy currents. As frequency increases, the need for high permeability decreases. Transformers with iron powder cores are suitable for very high frequencies up to 100 MHz. Since there is no need to achieve high permeability at very high frequencies above 100 MHz, air core transformers are more suitable due to their higher energy efficiency.
Air Core Transformer: In an air core transformer, both the primary and secondary coils are wrapped around a diamagnetic material. Magnetic coupling in such a transformer occurs through the air. In such a transformer, not only is the inductance of both coils low, but the mutual inductance is also very low, so the magnetic coupling between the coils is very small. These transformers do not lose energy due to hysteresis or eddy currents and are also capable of regulating large currents. This type of transformer is suitable for high voltage applications where energy efficiency is a primary concern, such as distribution transformers. These are also suitable for ultra-high RF applications above 100 MHz. At high radio frequencies, the required inductance value is low, which can be easily achieved with air core inductors.