Transformer losses

When the primary winding of the transformer is energized, the magnetic flux generated by the coil flows in the iron core. Because the iron core itself is also a conductor, an electric potential will be induced on the plane perpendicular to the magnetic field lines. This electric potential forms a closed loop on the section of the iron core and generates a current. It looks like a vortex so it is called "vortex". This "eddy current" increases the loss of the transformer and increases the temperature rise of the transformer's iron core heating transformer.
The losses caused by "eddy currents" are called "iron losses". In addition, a large amount of copper wire is needed to wind the transformer. These copper wires have resistance. When the current flows, the resistance will consume a certain amount of power. This part of the loss is often consumed as heat. We call this loss "copper loss". . Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss.  
Due to the iron loss and copper loss of the transformer, its output power is always less than the input power. For this reason, we introduce an efficiency parameter to describe this, η=output power/input power.