EP-3 Course: Types of Transformers According to Construction

In practical transformer design the manufacturer has the choice between two different basic constructions:

1. Core type Construction.
2. Shell type Construction. 

The one or the other of these Constructions has no influence on operational characteristics or the service reliability of the transformer, but there are essentially differences in the manufacturing process. Each manufacturer chooses the concept that he finds most convenient from a manufacturing point of view and tends to use this concept for the whole production volume.

1- Core-type transformers: see fig.(1) & (2)

The core-type transformer enclose the core i.e. the primary and secondary windings were wound on a separate lamps and around a closed iron ring so that the windings are well visible, but they hide the core limbs, Only the upper and lower youke of the core are visible. The axis of the core type windings is normally vertical.

fig (1): Core-Type Transformers

Core-type (or three limbs) is the most commonly used method of construction, the smaller core means less weight and expense. A wound type of core used in single phase distribution transformers is shown in fig. (3)

fig (2): Core-Type Transformers

fig (3): Wound Core-Type Transformers

2- Shell-Type Transformers: see fig. (4) & (5)

The shell-type transformer is considered the most efficient. Such transformers are used in transmitting commercial power. The core of the shell-type transformer is made of laminated silicon steel sheets placed on top of one another. The coils are wound around the central section of the core; the core of a shell-type encloses the windings and the core hides the major part of the windings. The axis of the shell-type windings can be horizontal or vertical in a shell-type.

fig (4): Shell-Type Transformers

fig (5): Shell-Type Transformers

Since the primary and secondary coils are wound close together around the core, the windings must be highly insulated. A special insulating material is coated on the wires of both coils.
Shell-type (or five limbs) is used for larger transformers because they can be made with a reduced height.


Core-type and shell-type transformers can also be constructed for three-phase transformers as in fig. (6) & (7).

fig (6): 3-Phase Core-Type Transformers

fig (7): 3-Phase Shell-Type Transformers

3- A comparison between the core-type and shell-types Transformers is given in below table as follows:

s/n	Core-type Transformers	Shell-type Transformers
1	enclose the core	Enclose the windings
2	the windings are well visible, but they hide the core limbs	the core hides the major part of the windings
3	The coils are wound around all core lamps	The coils are wound around the central section of the core
4	The axis of the core type windings is normally vertical.	The axis of the shell-type windings can be horizontal or vertical in a shell-type
5	Core-type (or three limbs) is the most commonly used method of construction, the smaller core means less weight and expense	The shell-type transformer is considered the most efficient and used for larger transformers because they can be made with a reduced height.
6	The cylindrical types of coils are used	Generally, multi-layer of disc type or sandwich coils are used
7	The coils can be easily removed for maintenance point of view	Large number of laminations must be removed for  making maintenance for any winding
8	As windings are distributed, the neutral cooling is more effective	As windings are surrounded by the core , there is no neutral cooling
9	It is preferred for low voltage transformers	It is preferred for high voltage transformer
10	It has a single magnetic circuit	It has double magnetic circuits
11	In a single phase type, the core has two limps	In a single phase type, the core has three limps
12	In a three phase type, the core has three limps	In a three phase type, the core has five limps

In the next topic, we will know more about different transformer types according to their applications.