TRANSFORMER

Transformer construction

• There are two types of transformer

1. The core type transformer
2. The shell type transformer

• These two types Transformer are differ from each other in windings.
• The magnetic core is a stack of thin Silicon Steel lamination about 0.35 mm thick for 50 Hz transformer.
• The thin layer are varnish to reduce the Eddy current losses
•  For core losses (which are made from C.R.G.O.  (cold rolled grain oriented sheet Steel.)). This material has low core loss and high permeability

1.Core type transformer

• The windings surround a considerable part of Steel core.
• For a given output and voltage rating it requires less iron but more conductor material.
• Most of the flux is confined to high permeability core  but some leakage flux flows through the core legs and nonmagnetic materials surrounding the core. For avoiding it half of the low voltage (L.V.) winding is placed over one leg and other half over the second limb. 
• Low voltage windings are placed adjacent to steal core and high voltage winding outside in order to minimise the amount of insulation required.
•  it performs better for high voltage high power levels

2.Shell type transformer

• In this type of Transformer the LV and HV winding are wound over the central Limb and are inter leaved or sandwiched. it performs better for low voltage low power levels where as cool type to construction is used for a high voltage high power transformer
• Lower power transformer are air cooled where as large power transformer are immersed in oil for better cooling
• In iron core Transformers most of the flux is confined to high permeability Core. there is however some flux that leaks through the core legs and non-magnetic materials surrounding the core. this flux called leakage flux links one winding and not the other.

Note:- there are two types of windings employed for Transformers. the concentric coils are used for core type Transformers as shown in the above figure. and interleaved coils  for shell type transformer as shown in the above figure.

• Low power transformers are air cooled where as large power transformer are immersed in oil for better cooling in oil cooled Transformer the oil serves as a coolant and also as an insulating medium
• For power frequency range of 25 to 400 Hz transformers are constructed with 0.35 mm thick Silicon Steel laminations
• For audio frequency range of 20 to 20000 Hz iron core with suitable refinement is used
• For high frequency employed in communication circuits who is made up of powered ferromagnetic alloy.
• In special cases the magnetic circuit of a transformer may be made of non magnetic material and in such a case the transformer is referred to as an air core transformer
• The Air core transformer is Primarily used in radio devices and in certain types of measuring and testing instruments.
• Core made of soft ferrites are also used for pulse Transformers as well as for a high frequency electronics Transformers.