Ans: There are four types of coils e.g.
(a)Spiral type coil - This consists of layers wound in a continuous length from top to bottom of the coils ,the conductors of which consist of a number of square or rectangular strips in parallel. These coils are only used for low voltage winding carrying a heavy current of more than 100 amps .
(b)Cross over type coil - this type of consists of a number of layers having a number of turns per layer, the conductor being round wire or a strip insulated with a paper covering .Cross over coils are wound on formers . These coils are used for high voltage winding of distribution transformer and current up to about 20 amps .
(c) Helical type coil - This type of coil is wound in the form of a helix and consists of a number of rectangular strip wound in parallel radially so that each separate turn occupies the total radial depth of the winding . These coils are used for lower voltage winding of the larger transformer at voltages 11KV to 33KV . Multilayer helix can be used for higher voltage winding up to 500KV.
(d)Continuous disc-type coil – This coil consists of a number of discs wound continuously from a single wire or a number of strips in parallel . In each disc, a number of turns are wound radially over one another and the conductor passes from disc to disc uninterruptedly . These coils are used for both higher voltage winding from 33KV to 500KV and lower voltage winding from 11KV to 66KV above the rating of 1 MVA.