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Power transformers play a crucial role in the field of power transmission and equipment power supply. Observant users may notice that power transformers are always "paired" with alternating current （AC） and rarely interact with direct current （DC）. What technical logic lies behind this phenomenon?
The core operating principle of power transformers is based on electromagnetic induction. They mainly consist of an iron core （or magnetic core） and primary and secondary coils. When AC passes through the primary coil, the periodic changes in the magnitude and direction of the current generate a similarly periodic magnetic field around the coil. According to Faraday's law of electromagnetic induction, the changing magnetic field induces an electromotive force in the secondary coil, thus achieving voltage transformation. For instance, in urban power transmission, the AC generated by power plants is stepped up to ultra-high voltage through step-up transformers to reduce power losses during long-distance transmission. When the electricity reaches areas near end-users, step-down transformers are used to lower the voltage to levels suitable for residential and industrial applications.
DC, on the other hand, maintains a constant current direction and magnitude. When DC is applied to the primary coil of a power transformer, it can only generate a stable, unchanging magnetic field. However, a stable magnetic field cannot induce an electromotive force in the secondary coil, making voltage conversion impossible. Moreover, constant DC may cause the transformer's iron core to saturate. Once the core saturates, the inductance of the transformer drops sharply, the magnetizing current increases significantly, and ultimately, the transformer overheats severely, potentially burning out the coils and damaging the equipment. There was a case where a factory mistakenly connected a DC power source to a transformer. Within just a few minutes, the transformer smoked due to overheating and had to be replaced urgently, resulting in high maintenance costs and disrupting normal production.
Of course, in some special applications, although it may seem that the transformer is handling DC, in fact, an inverter circuit is used to convert the DC into AC first, and then the transformer is employed for voltage transformation. For example, in solar photovoltaic power generation systems, the DC generated by solar panels needs to be converted into AC by an inverter before it can be stepped up or down by a transformer and integrated into the AC power grid.
With the continuous development of power technology, although power transformers currently remain predominantly compatible with AC, scientists are exploring new technologies and materials to break through traditional limitations and enable transformers to operate efficiently in DC environments. However, at present, a deep understanding of the close relationship between power transformers and AC not only helps engineers optimize power system designs but also assists ordinary users in using electrical equipment correctly, avoiding potential safety hazards and economic losses caused by incorrect operation.
