The most important requirements of transformer winding are:
- The winding should be economical both as regards initial cost, with a view to the market availability of copper, and the efficiency of the transformer in service.
- The heating conditions of the windings should meet standard requirements, since departure from these requirements towards allowing higher temperature will drastically shorten the service life of the transformer.
- The winding should be mechanically stable in respect to the forces appearing when sudden short circuit of the transformer occur.
- The winding should have the necessary electrical strength in respect to over voltages.
The different types of windings are classified and briefly discussed below:
1. Concentric windings:
2. Sandwich windings
I. Concentric windings. Refer Fig. 10. These windings are used for core type transformers. Each limb is wound with a group of coils consisting of both primary and secondary turns which may be concentric cylinders. The l.v. winding is placed next to the core and h.v. winding on the outside. But the two windings can be sub-divided, and interlaced with high tension and low tension section alternately to reduce leakage reactance. These windings can be further divided as follows:
i. Cross-over windings. Cross-over windings are used for currents up to 20 A so they are suitable for h.v. winding of small transformers. The conductors are either cotton covered round wires or strips insulated with paper. Cross-over coils are wound over formers and each coil consists of a number of layers with a number of turns per layer. The complete winding consists of a number of coils connected in series. Two ends of each coil are brought out, one from inside and one from outside. The inside end of a coil is connected to the outside end of the adjacent coil.
ii. Helical winding. A helical winding consists of rectangular strips wound in the form of a helix. The strips are wound in parallel radially and each turn occupies the total radial depth of winding.
Helical coils are well suited for l.v. windings of large transformers. They can also be used for h.v. windings by putting extra insulation between layers in addition to insulation of conductors.
iii. Continuous disc winding. This type of winding consists of a number of flat strips wound spirally from inside (radially) outwards. The conductor is used in such lengths as are sufficient for complete winding or section of winding between tappings. The conductor can either be a single strip or a number of strips in parallel, wound on the flat. This gives a robust construction for each disc. The discs are wound on insulating cylinders spaced from it by strips along the length of cylinder. The discs are separated from each other with press board sectors attached to the vertical strips. The vertical and horizontal spacers provide ducts for free circulation of oil which is in contact with every turn.
iv. Sandwich coils. Sandwich coils (Fig.11) are employed in transformers of shell type. Both high and low voltage windings are split into a number of sections. Each high voltage section lies between the voltage sections.
The advantages of sandwich coils is that their leakage can be easily controlled and so any desired value of leakage reactance can be had by the division of windings.