At rated power, output power and input power ratio of the transformer, called the efficiency of the transformer, namely
Wherein η is the efficiency of the transformer; P1 is the input power, P2 is the output power.
When the transformer output power P2 equal to the input power P1, efficiency η is equal to 100%, the transformer will not have any loss. But in fact this transformer is not. The total time to produce transformers for power transmission loss, this loss mainly copper loss and iron loss. Copper loss refers to the loss caused by a transformer coil resistance. When the current through the coil resistance heating, some energy is transformed into heat energy loss. Usually by the coil is wound with insulated copper wire is made, so called copper loss.
Iron loss transformer includes two aspects. First, the hysteresis loss, when the AC current through the transformer, the transformer silicon steel sheet by the magnetic field lines change with its direction and size, so that the internal molecular friction between silicon and emit heat, so the loss of part of the electrical energy, which is the hysteresis loss . Another is the eddy current loss, when the transformer operation. There are magnetic field lines through the core, in a plane perpendicular to the magnetic field lines induced current is generated, since this current into a closed loop formed from circulation, and a spiral-shaped, it is called a vortex. The presence of the vortex core causes fever, consume energy, this loss is called an eddy current loss.
Power level is closely related to the efficiency of the transformer and the transformer, usually the greater the power loss and output power smaller than the efficiency is higher. Conversely, less power, efficiency is lower.