18 Questions and Answers on Transformer Knowledge

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1. What is a transformer? In AC circuits, a device that is used to increase or decrease voltage is called a transformer. A transformer can convert voltage of any value into the desired voltage level with the same frequency, in order to meet the requirements for the transmission, distribution, and use of electrical energy. For example, the electricity generated by power plants has a relatively low voltage level, and it must be stepped up in voltage before it can be transmitted to distant areas where it is needed. In those areas, the voltage must then be reduced to an appropriate level so that it can be used by power equipment and various household appliances. 2. How does a transformer change voltage? Transformers are made based on electromagnetic induction. It consists of an iron core made up of stacked silicon steel sheets, and two sets of coils wound around the core; the core and the coils are insulated from each other, with no electrical connection between them, as shown in the figure. We call the coil connected to the transformer and the power supply the primary coil (or primary side), while the coil connected to the electrical equipment is called the secondary coil (or secondary side). When the primary coil of the transformer is connected to an AC power source, changing magnetic flux lines are generated in the core. Since the secondary coil is wound around the same core, the magnetic flux lines cut through the secondary coil, which inevitably generates an induced electromotive force and results in a voltage across the ends of the coil. Since the magnetic flux lines are alternating, the voltage in the secondary coil is also alternating. Moreover, the frequency is exactly the same as the power supply frequency. Theoretically, it has been proven that the voltage ratio between the primary and secondary coils of a transformer is related to the ratio of the number of turns in these coils, and this can be expressed by the following formula: Primary coil voltage/Secondary coil voltage = Number of turns in primary coil/Number of turns in secondary coil. This shows that the greater the number of turns, the higher the voltage. Thus, it can be seen that the secondary coil is fewer in number than the primary coil, which is a step-down transformer. Otherwise, it is a step-up transformer. 3. What are the types of transformers? Based on the number of phases, there are single-phase and three-phase transformers. Classified by purpose, there are power transformers, special-purpose power transformers, voltage-regulating transformers, measurement transformers (voltage transformers, current transformers), and small power transformers (used for low-power devices); there are also safety transformers. Classified by structure, they come in core-type and shell-type versions. Coils can be of two-winding or multi-winding type, and autotransformers exist; based on the cooling method, they can be oil-immersed or air-cooled. 4. What components does a transformer consist of? A transformer is mainly composed of a core and coils, in addition to which it also includes an oil tank, oil conservator, insulating bushings, and tap changers. 5. What is the use of transformer oil? The function of transformer oil is: (1) insulation. (2) Heat dissipation. (3) Eliminate the arc effect. 6. What is an autotransformer? An autotransformer has only one set of coils, with the secondary coil being derived from taps on the primary coil. Its power transfer occurs not only through electromagnetic induction but also through direct electrical conduction. This type of transformer uses fewer silicon steel sheets and copper wires compared to ordinary transformers, and it is commonly used for voltage regulation. 7. How does a voltage regulator adjust the voltage? The structure of the voltage regulator is the same as that of an autotransformer; the only difference is that the core is made in the form of a ring, around which the windings are placed. The secondary coil taps are equipped with slidable brush contacts that move in an annular pattern along the surface of the coil, thereby enabling smooth voltage regulation. 8. What is the current relationship between the primary coil and the secondary coil of a transformer? When the transformer is operating under load, changes in the current in the secondary coil cause corresponding changes in the current in the primary coil. Based on the principle of magnetic potential balance, it can be deduced that the current in the primary and secondary coils is inversely proportional to the number of turns in those coils; the side with more turns has a lower current, while the side with fewer turns has a higher current. This can be expressed by the following formula: Primary coil current/Secondary coil current = Number of turns in secondary coil/Number of turns in primary coil. 9. What is the voltage variation rate of a transformer? The voltage change rate of a voltage regulator is one of the main performance indicators of a transformer. When a transformer supplies power to a load, the voltage at the load side of the transformer inevitably drops. By comparing this reduced voltage value with the rated voltage value and expressing it as a percentage, the voltage change rate is obtained, which can be represented by a formula ； Voltage change rate = [((Secondary rated voltage – Voltage at the load side) / Secondary rated voltage)] × 100%. In a normal power transformer, when connected to its rated load, the voltage variation rate is 4 to —