Structure of small power transformer

The figure shows the appearance of some small power transformers. It is mainly composed of iron core, frame, winding, insulation and fasteners.

1. Iron core
Common iron cores of small power transformers are E type, EI type, C type, etc., as shown in the figure. The core is punched from silicon steel sheets, while the C type core is rolled from cold-rolled silicon steel strips.
The properties and uses of common silicon steel sheets are shown in Tables a and b.

E-type and EI-type cores are the most commonly used cores at present. Their main advantages are that the primary and secondary of the winding can share a skeleton, have a higher window space factor, and the core can form a protective shell for the winding, making the winding less susceptible to mechanical trauma. In addition, the heat dissipation area of ​​the core is also large, and the magnetic field divergence itself is also less. But it also has disadvantages, such as a large air gap in the magnetic circuit, which increases the magnetic resistance and reduces the performance of the magnetic circuit. In addition, it also has the disadvantages of more copper wires, large leakage inductance and large external magnetic field interference.
The manufacturing process of the C-type core is: after the cold-rolled silicon steel strip is wound and formed, it is made into a closed core through heat treatment, impregnation and other processes, and then the closed core is cut to form two C-type cores. After the wire is wrapped, a pair of C-type cores are put together and fastened together to form a transformer. The air gap of the C-type core can be made very small, and it also has the advantages of small size, light weight and high material utilization.

2. Skeleton
The skeleton is generally made of plastic pressing, and can also be made of plywood and bakelite fiberboard.

3. Windings
The windings of small power transformers are generally wound with enameled wire because it has good insulation, occupies a small volume and is cheap. For low-voltage and high-current coils, yarn-wrapped thick copper wire is sometimes used for winding.
In order to make the transformer have sufficient insulation strength, thin insulating materials such as capacitor paper and yellow wax silk are padded between each layer of the winding. In some occasions where high insulation is required, polyester film and polytetrafluoroethylene film can also be used.
The order of coil winding is usually to wind the primary coil inside the wire package, and then wind the secondary coil. In order to prevent the interference voltage from entering the radio equipment through the transformer, an electrostatic shielding layer is also added between the primary and secondary of the transformer to eliminate the interference voltage introduced by the distributed capacitance between the primary and secondary windings. In order to
facilitate heat dissipation, a certain gap should be left between the winding and the window, generally 1~3mm, but it should not be too large to avoid increasing the loss of the transformer. The lead wire of the winding generally uses a multi-strand insulated soft wire. For the winding wound with thick wire, the wire of the coil itself can be used as the lead wire, and an insulating sleeve can be added on the outside.

4. Iron
core After the iron core is installed in the winding, it must be clamped and fixed. Common methods include clamping strips and screws. For small power transformers of several watts, clamps can be used to fix them.