There are in general two types of transformer
Shell type and core type transformer These two types differ from each other in the manner in which winding wound on the magnetic core.
Core type Transformer
In core type, the magnetic core is built of laminations to form a rectangular frame and the windings are arranged concentrically with each other around the legs or limbs.
The top and bottom horizontal portions of the core are called a yoke. The yokes connect the two limbs and have a cross-sectional area equal to or greater than that of limbs.
Each limb carries one-half of primary and secondary windings. The two windings are closely coupled together to reduce the leakage reactance.
The low voltage winding is wound near the core and high voltage winding is wound over low voltage winding away from the core in order to reduce the amount of insulating materials required.
Shell Type Type Transformer
In shell-type, the windings are wound around the central limb and the flux travels through two side limbs and completes its circuit.
The central limb carries total mutual flux while the side limbs forming a part of a magnetic circuit carry half the total flux. The cross-sectional area of the central limb is twice that of each side limbs
A transformer that changes voltage from a lower voltage to a higher voltage is called a step-up transformer.
Step-up X’mer has more turns in secondary winding than primary hence voltage per turn increases, it increases the voltage level, mostly used at generating station to transmit electrical power in the transmission line at a higher voltage.
Step Down type Transformer
A transformer that lowers down voltage to utilization voltage is called a step-down transformer, it reduced the voltage from a higher to a lower voltage level. It has less turn on the secondary winding compared to the primary winding.
It is used in a distribution network, where the transmission line end. mostly delta – star-connected windings, the primary is connected in delta and the secondary is in star for deriving neutral point.
Note that a step-down transformer can be used as a step-up transformer, in which the secondary of a step-down transformer becomes primary and the primary of the step-down becomes secondary winding.
Step-up or step-down operations of the transformer are decided after installation on site.