I. Single-phase transformer and three-phase transformer
Simply speaking, the transformer used for single-phase alternating current is called single-phase transformer, and the transformer used for three-phase alternating current is called three-phase transformer. Looking from the shape, there are two high-voltage ceramic bushing, two or four low-voltage ceramic bushing are single-phase transformers; there are three high-voltage ceramic bushing, three low-voltage ceramic bushing (and a shorter ceramic bushing beside some) are three-phase transformers. Their internal coils are also connected differently. Single-phase transformer has only one set of high and low voltage coils. Its high voltage coils have two terminals. Most of the low voltage coils have four terminals. If they are connected in series or in parallel, two voltages can be obtained. For example, in some small distribution transformers, 220 volt voltage can be obtained by series connection of low voltage coils and 110 volt voltage can be obtained by parallel connection.
The primary coil and secondary coil of three-phase transformer are usually connected by three groups of coils inside the transformer, and then lead to the outside of the tank through the ceramic bushing. These three coils have different connection methods. A connection method is called star connection, and the symbol is Y. One end of each set of coils is pulled out of each terminal (three terminals), and the other end is connected together, called neutral point (or neutral point). The neutral point terminal is called neutral point terminal (some transformers do not have neutral point terminals outside) which leads to the outside of the tank through a shorter ceramic bushing. Another method of connection is called triangular connection, and the symbol is delta. It is to connect one end of each set of coils with the other, like a triangle, and then draw a terminal from each connection point.
The primary and secondary coils of three-phase transformer can be connected in different ways according to the need. For example, the nameplate of some transformers is marked with the symbol "Y-Delta", which indicates that the primary coil of the three-phase transformer is a star connection and the secondary coil is a triangle connection. The voltage of three-phase transformer is divided into phase voltage and line voltage. When transformer adopts triangular connection, the line voltage is equal to the phase voltage; when star connection is adopted, the line voltage is 1.732 times of the phase voltage, and the voltage between each terminal and the neutral terminal is the phase voltage. Don't make mistakes when using it.
2. Double-winding, Three-winding and Single-winding Transformers
Transformers with high and low voltage coils are called double-winding transformers. For example, the nameplate of the transformer says 35KV/10KV, which means that it is a double-winding transformer. Its high-voltage coil is 35KV, and its low-voltage coil is 10KV. When connected to a 35KV power supply, 10 KV can be obtained.
Some transformers are equipped with three kinds of coils: high voltage, medium voltage and low voltage. They are called three-winding transformers. For example, the nameplate of the transformer says 110 kilovolts/35 kilovolts/10 kilovolts, which means that it is a three-winding transformer. Its high-voltage coil is 110 kilovolts, its medium-voltage coil is 35 kilovolts, and its low-voltage coil is 10 kilovolts. When connected to 110 kilovolts power supply, it can get 35 kilovolts and 10 kilovolts.
There are a few special transformers, which have one high voltage and two identical low voltage coils, also known as three-winding transformers; some have one high voltage and three low voltage coils, called Four-winding transformers. In some transformers, two terminals are drawn from the high-voltage coil, and a connector is drawn from the half waist of the coil. A part of the primary coil is used as a secondary coil. In fact, it has only one coil, so it is called a single-winding transformer, also known as an autotransformer. This kind of transformer is economical in some cases because of its less material and low cost. However, because its high-voltage coil and low-voltage coil are directly connected, some safety measures are needed, so the use of occasions are limited.
