Transformer cores

If the ferromagnetic substance such as iron, powdered iron, or ferrite can be placed within pair of coils, extent of coupling can be increased far above that possible with an air core. However this improvement in coupling takes place with the price; some energy is invariably lost as heat in the core. The ferromagnetic transformer cores limit the frequency at which the transformer will work well.

The schematic symbol for an air-core transformer consists of two inductor symbols back to back. If a ferromagnetic core is used, two parallel lines are added to schematic symbol.

Figure--   Schematic symbols for air-core (A) and ferromagnetic-core (B) transformers.

Laminated  iron

In the transformers for 60-Hz utility alternating current, and also at the low audio frequencies, sheets of silicon steel, which are glued together in the layers, are employed as transformer cores. The silicon steel is called as transformer iron.

The reason layering can be used, instead of making the core from a single mass of metal, is that magnetic fields from the coils makes currents to flow in a solid core. These eddy currents go in circles, by heating up the core and wasting energy which would otherwise be transferred from the primary to the secondary. Eddy currents are choked off by the breaking up core into the layers, such that a current is not able flow well in circles.

Another type of loss, known as hysteresis loss, takes place in any ferromagnetic transformer core. Hysteresis is tendency for a core material to be sluggish in accepting a fluctuating magnetic field. Laminated cores have high hysteresis loss above audio frequencies, and are thus not good above a few kilohertz.

Ferrite and powdered iron

At the frequencies up to several megahertz, ferrite works well for the radio frequency (RF) transformers. This material contains high permeability and concentrates the flux efficiently. The high permeability reduces the number of turns required in the coils. But at the frequencies higher than a few megahertz, ferrite begins to show loss, and is no more effective.

For work well into the very high frequency range, or up to 100 MHz or more, powdered iron cores work well. The permeability of powdered iron is below that of ferrite, but at the high frequencies, it is not required to have high magnetic permeability. Actually, at radio frequencies above a few megahertz, air core coils are preferred, especially in the transmitting amplifiers.  At frequencies above several hundred megahertz, a ferromagnetic core is dispensed with the entirely.