TRANSFORMER LOSSES
Although transformers are probably the most efficient machines known, they are not perfect. A transformer operating at 90% efficiency has a power loss of 10%. Some of these losses are I2R losses, eddy current losses, hysteresis losses, and magnetic flux leakage. Most of these losses result in heat production. Recall that I2R is one of the formulas for finding power or watts. In the case of a transformer, it describes the power loss associated with heat due to the resistance of the wire in both primary and secondary windings.
Eddy currents are currents that are induced into the metal core material by the changing magnetic field as alternating current produces a changing flux. Eddy currents are so named because they circulate around inside the metal in a similar manner as the swirling eddies in a river, Figure 19–27. These swirling currents produce heat, which is a power loss. Transformers are constructed with laminated cores to help reduce eddy currents. The surface of each lamination forms a layer of iron oxide, which acts as an insulator to help prevent the formation of eddy currents.
Hysteresis losses are losses due to molecular friction. As discussed previously, the reversal of the direction of current flow causes the molecules of iron in the core to realign themselves each time the current changes direction. The molecules of iron are continually rubbing against each other as they realign magnetically. The friction of the molecules rubbing together causes heat, which is a power loss. Hysteresis loss is proportional to frequency. The higher the frequency, the greater the loss. A special steel called silicon steel is often used in transformer cores to help reduce hysteresis loss. The power loss due to hysteresis and eddy currents is often called core loss.
Magnetic flux leakage does not produce heat, but does constitute a power loss. Flux leakage is caused by magnetic lines of flux radiating away from the transformer and not cutting the secondary windings. Flux leakage can be reduced by better core designs.
