Principle and Application of Autotransformer

Autotransformer refers to a transformer whose primary and secondary windings are on the same winding, and the primary and secondary windings are directly connected in series and self-coupled. According to the structure, it can be subdivided into adjustable pressure type and fixed type. The coupling of an autotransformer means electromagnetic coupling. An ordinary transformer transmits energy through the electromagnetic coupling of the primary and secondary coils. The primary and secondary sides have no direct electrical connection. The primary and secondary sides of the autotransformer have direct electrical connection. The low-voltage coil is part of the high-voltage coil. Protective equipment such as autotransformers are also used in protective equipment for communication lines.

Ⅰ. Working principle of autotransformer:

Autotransformer is to wind two or more coils on a closed iron core. When one coil is connected to the AC power supply (that is, the primary coil), alternating current flows through the coil, and this alternating current flows in the iron core. Alternating magnetic field is generated in the middle, the alternating main magnetic flux generates its own induced electromotive force in the primary coil, and at the same time, the mutual induced electromotive force is induced in the other coil (the secondary coil). By changing the relationship between the primary and secondary coil turns ratio to change the primary and secondary coil terminal voltage to achieve voltage conversion, the general turns ratio is 1.5:1~2:1. Because the primary and secondary coils are directly connected, there is a risk of inter-level leakage. Therefore, it cannot be used as a running light transformer.

1. The autotransformer is a special transformer that shares a set of coils for output and input. The step-up and step-down are realized by different taps. The voltage of the taps that are less than the common coil will decrease, and the voltage of the taps that are more than the common coil will increase. high.

⒉. In fact, the principle is the same as that of an ordinary transformer, except that its primary coil is its secondary coil. In general transformers, the primary coil on the left uses electromagnetic induction to make the secondary coil on the right generate voltage, and the autotransformer influences itself.

⒊. The autotransformer is a transformer with only one winding. When used as a step-down transformer, a part of the turns are drawn from the winding as the secondary winding; when used as a step-up transformer, the applied voltage is only applied to part of the winding Turn up. Usually the part of the winding that belongs to both the primary and the secondary is called the common winding, and the rest of the autotransformer is called the series winding. Compared with the ordinary transformer, the autotransformer of the same capacity is not only small in size, but also more efficient, and the transformer The larger the capacity, the higher the voltage. This advantage becomes more prominent. Therefore, with the development of power systems, the improvement of voltage levels and the increase of transmission capacity, autotransformers have been widely used due to their large capacity, low loss, and low cost. 

Ⅱ, the application of autotransformer:

Autotransformers are used in occasions where primary and secondary isolation are not required, and have the advantages of small size, less consumables, and high efficiency. Common AC (manual rotating) voltage regulators, transformers in small household AC voltage regulators, transformers in three-phase motor auto-decompression starter boxes, etc., are all application examples of auto-transformers.

With the rapid development of power systems in the direction of large capacity and high voltage, autotransformers have been widely used in high-voltage power networks due to their low cost and high efficiency, becoming voltage conversion equipment that transfers important electrical energy. As one of the most important equipment in the high-voltage power grid, the autotransformer is of great significance to ensure the safe and reliable operation of the power grid and the flexible distribution of electrical energy.

With the rapid development of high-speed railways, the reliability of autotransformers is critical to the safe operation of high-speed railways. Direct lightning strikes and foreign objects in the catenary cause frequent high-speed rail short-circuit trip accidents. The short-circuit impulse current generated can easily cause autotransformer winding failure, greatly reducing the reliability of transformer operation and seriously affecting the safe operation of high-speed rail.

Autotransformer step-down start editing voice:

Autotransformer step-down start refers to the use of autotransformer to reduce the starting voltage added to the stator winding of the motor when the motor starts. After the motor is started, the motor is separated from the autotransformer, so that it can move normally under full pressure. With autotransformer step-down starting, the starting current and starting torque of the motor are reduced in proportion to the square of its terminal voltage. With the same starting current, a larger starting torque can be obtained. For example, the current 8000kW mainstream rescue ship side thrust starter chooses the transformer tap voltage to drop to 45% of the rated voltage, its starting current is 20.25% of the full voltage starting current, and the starting torque is 20.25% of the full voltage starting torque.

The auto-coupling step-down starting circuit cannot be operated frequently. If the start is unsuccessful, the second start should be separated by more than a few minutes. After two consecutive starts, it should be started again at least half an hour later. This is to prevent the start of the autotransformer windings. The current is too large and the heat will damage the insulation of the autotransformer.

Related transformer editing voice:

Neutralizing transformer:

Neutralizing Transformer: A device that reduces the impact of strong wires on communication lines. The number of its secondary coils is the same as the number of communication wires, and it is directly connected to the communication wires; its primary coil is connected in series to the pilot wire grounded at both ends. In this way, the current in the strong wire and the pilot wire will generate a corresponding potential to the ground on the wire line. It changes the potential distribution of the communication wires and ensures that the ground potential along the communication lines does not exceed the limit value. This method of series connection does not change the insulation of the communication line to ground, and at the same time plays a role in protecting the communication line. Its disadvantage is that it needs to add a pilot line.

Shielded transformer:

Shielding transformer (Reduction Transformer): Shielding transformer is also called reduction transformer. Its working principle is the same as that of a neutralizing transformer, and it is used for communication cable protection. Its secondary money loop is a whole body, which is wound by a section of cable core or the cable core of the cable itself, and there is no need to set a separate secondary coil for the wire; and its primary coil is wound by insulated copper wire and is directly connected in series to the cable In the outer skin.

Separating transformer:

Isolating Transformer: A protective device that prevents strong wires from affecting communication lines. Also known as insulating transformer. Its working principle is to plug the primary and secondary coils with a ratio of 1:1 to a pair of communication wires respectively, which separates the wires into multiple sections, reduces the induced longitudinal potential on the wires, and protects the communication lines. . It is suitable for audio communication lines, but it is not possible to carry out DC test and transmit DC signals on the communication line using separation transformer.

Suction transformer:

Booster Transformer: It is used in the AC electrified railway power supply system and is a device used to reduce the impact on adjacent communication lines. It will forcibly absorb the current that originally escaped into the earth to a specially erected return line, and introduce the current into the traction substation. It not only saves energy and reduces consumption, but also protects the communication line.