变压器的损耗包括空载损耗和负荷损耗。正常情况下变压器运行电压基本不变，即空载损耗是一个恒量。而负荷损耗则随变压器运行负荷的变化而变化，且与负荷电流的平方成正比。当三相负荷不平衡运行时，变压器的负荷损耗可看成三只单相变压器的负荷损耗之和。
Transformer losses include no-load losses and load losses. Normally, the transformer operating voltage is basically unchanged, that is, no-load loss is a constant. The load loss varies with the change of transformer operating load and is proportional to the square of load current. When the three-phase load is unbalanced, the load loss of transformer can be regarded as the sum of the load loss of three single-phase transformers.