What are the Functions and Features of Autotransformers?

An autotransformer is a special transformer with primary and secondary without insulation. In short, it means that its primary and secondary windings are on the same winding, i.e., a transformer with only one winding, which means it is a special transformer with a common set of coils for output and input.

When the autotransformer is used as a step-down transformer, part of the line turns are taken out from the winding as the secondary winding, and when the autotransformer is used as a step-up transformer, the applied voltage is only added to part of the line turns of the winding. Because DC power cannot be directly stepped up, it needs to go through electronic and other components.

Autotransformer functions

• The autotransformer is used in applications where primary and secondary isolation is not required. It has the advantages of small size, low consumables and high efficiency.
• Fundamentally autotransformer is designed to reduce cost by eliminating a coil and has the function of a normal transformer, which means it has the function of raising or lowering circuit voltage, isolating and lowering impedance.
  In addition, autotransformers can be made into AC regulators to flexibly change the output voltage for laboratory use.
• The autotransformer power supply method is very suitable for the power supply of large-capacity load, and the interference to the communication line is less, thus it is widely used by passenger lines as well as heavy-duty freight railroads.
• In the current power grid, there are almost no autotransformers below 220kV, and only from the 220kV voltage level do they start to have autotransformers, which are mostly used as liaison transformers between power grids.

Autotransformer features

• Autotransformer is small in size, light in weight, easy to transport and low in cost. Because the calculated capacity of the autotransformer is smaller than the rated capacity. Therefore, under the same rated capacity, the main size of the autotransformer is smaller, and the effective materials and structural materials are reduced accordingly, thus reducing the cost. The reduction of effective materials leads to a corresponding reduction in copper and iron consumption and higher efficiency of the autotransformer. At the same time, the reduction of the main dimensions and the mass allows the manufacture of transformers with larger capacities per unit under tolerable transport conditions. However, the above advantages are usually obvious only when only k≤2 in the autotransformer.
• Since the short-circuit impedance of autotransformers is smaller than that of the double-winding transformer, the voltage change rate is smaller, but the short-circuit current is larger.
• Its primary side and secondary side not only have a magnetic connection but also have an electric connection, while the ordinary transformer is only a magnetic connection.
• Because of the direct electrical connection between primary and secondary of an autotransformer, when the high voltage side overvoltage will cause the low voltage side serious overvoltage. In order to avoid this danger, one and two must be installed with a lightning arrester, do not think that one and two windings are in series, one has been installed, and the second can be omitted.
• In general transformers, the on-load regulator is often connected to the neutral point of grounding, so that the voltage level of the regulator can be lower than when the line end of the regulator. And autotransformer neutral regulating side will bring the so-called related regulating problem. Therefore, when autotransformer on-load regulation is required, only the line end regulation method can be used.
• Since the winding of an autotransformer is composed of two parts: primary winding and common winding, the number of turns of the primary winding is reduced accordingly compared with the number of turns and height of primary winding and common winding current and generated leakage resistance of common transformer.
• If the autotransformer has a third winding, the third winding will occupy the capacity of the common winding, which will affect the operation mode and exchange capacity of the autotransformer.