If the secondary output voltage is less than the primary input voltage, the transformer is called a "step-down transformer". If the secondary output voltage is greater than the primary input voltage, it is called a "boost transformer".
The type of wire used as the main current-carrying conductor in the transformer winding is copper or aluminum. Although aluminum wire is lighter and generally cheaper than copper wire, it must use a larger conductor cross-sectional area to carry the same current as copper, so it is mainly used in large power transformer applications.
Small kVA power supplies and voltage transformers used in low-voltage electrical and electronic circuits tend to use copper conductors because they have higher mechanical strength and smaller conductor sizes than equivalent aluminum types. The downside is that these transformers will be heavier when their core is completed.
Transformer windings and coils can be roughly divided into concentric coils and sandwich coils. In the core transformer configuration, the windings are usually arranged concentrically around the core leg as shown above, where the higher voltage primary winding is wound on the lower voltage secondary winding.
Sandwich or "pancake" coils are composed of spirally wound flat conductors, named after the conductors are arranged in a disc. The alternating disks are spirally arranged in a staggered manner from the outside to the center, and the individual coils are stacked together and separated by insulating materials (such as plastic paper). Sandwich coils and windings are more common in shell core structures.
Helical winding, also known as spiral winding, is another very common cylindrical coil device used in low-voltage and high-current transformer applications. The winding is composed of a large cross-section rectangular conductor wound on its side. The insulated strands are continuously wound in parallel along the length of the cylinder, and suitable spacers are inserted between adjacent turns or discs to make the parallel strands The circulating current between is the smallest. The coil extends outward in a spiral shape, similar to the spiral of a corkscrew.
The insulating layer that prevents the conductor from short-circuiting in the transformer is usually a layer of varnish or enamel in an air-cooled transformer. Apply this thin varnish or enamel paint to the wire and then wrap it around the core.
In larger power supplies and distribution transformers, the conductors are insulated from each other using oil-impregnated paper or cloth. Immerse and seal the entire iron core and winding in a protective tank filled with transformer oil. Transformer oil is used both as an insulator and as a coolant.
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