According to the number of phases, there are single-phase and three-phase transformers. It can be divided into power transformer, special power transformer, voltage regulating transformer, measuring transformer (voltage transformer and current transformer), small power transformer (for small power equipment) and safety transformer. According to the structure, it can be divided into core type and shell type. The coil has double winding and multi winding. According to the cooling mode, the autotransformer can be divided into oil immersed type and air cooling type.