There are two types of losses in the transformer - losses in copper (in the wires with which it is wound) and losses in steel (in the core).
Losses in copper are caused by the presence of electrical resistance in the windings of the transformer. The current flowing in the winding creates a voltage drop on such a conductor. Some electric power develops on the winding and part of the energy is converted into heat that heats the winding.
Losses in steel
Steel losses consist of two types of losses:
loss due to eddy currents;
loss by cyclic re-magnetization.
The occurrence of eddy currents in the core can be explained as follows. A core made of steel is a metal conductor placed in a variable magnetic field. In the core, as in the turns of any winding, an induced E.D.S. will be created, and current will flow through the core. Since the cross section of the core is large, its electrical resistance is small. Therefore, the currents flowing in the core reach large values. At the same time, energy is actively consumed and converted into heat, which heats the core.
The magnitude of the second type of loss, i.e. the cyclic re-magnetization loss, is highly dependent on the core material. The core material can be represented as if consisting of a large number of elementary magnets (magnetic dipoles), which in the usual state are randomly located. When such material is introduced into the magnetic field, the magnetic dipoles begin to rotate in the direction of action of the magnetic field. If the magnetic field is variable, then the dipoles will periodically rotate first in one direction and then in the other direction with the frequency of change of this field. Friction forces arise and magnetic field energy is also transferred to heat that heats the core.
To increase the efficiency of the transformer, it is necessary to reduce all types of losses. Copper losses can be reduced by increasing the section of winding wires. However, the size, weight and cost of the transformer will increase significantly. Therefore, the section of the wires is increased only to such a value that no noticeable heating of the windings is observed. Losses on re-magnetization are significantly reduced if a special magnetic soft steel having a certain composition and structure is used as the material of the transformer core.
Finally, in order to reduce eddy current losses, the core is not assembled from monolithic steel bars, but from separate isolated plates with a thickness of several tenths of a millimeter. In addition, silicon is added as an additive to the core material. Both contribute to an increase in the electrical resistance of the core, which, in turn, entails a decrease in the amount of eddy currents.