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Classification of transformers

Updated on May 4, 2014

The Transformer is basically a very simple device. It mainly consists of a core, windings and insulations. The windings are wound over the laminated magnetic core. These windings are insulated from the core and from each other.

With regards to the application transformers can be classified as follows

1. Step up transformer

These transformers raise the input voltage to a higher voltage level.

2. Step down transformer

These transformers reduce the input voltage to a lower voltage level.

Transformer is a reversible device. Therefore each transformer can be used as both step up and step down transformer. For example, if we supply a low voltage (within the LV rating) to the low voltage terminal, we will be getting the stepped up voltage at the high voltage terminals and if we supply a high voltage (within the HV rating) to the high voltage terminal, we will be getting the stepped down voltage at the low voltage terminals.

Transformer can also be classified based on the type of construction used. There are two types of cores namely core type and shell type. Depending upon the type of core used transformers can be classified into two categories as:

  1. core type transformers
  2. shell type transformers

Core type

1. Core type transformers

In this type of transformers the core is in the form of rectangular frame with the windings wound on the limbs of the core. A single phase transformer may be designed with the primary windings over on limb and the secondary windings over another limb. But on doing so, the leakage reactance will be large leading to a heavy power loss. In actual practice both the limbs of the frame contains half of secondary and primary windings. By doing, so the leakage reactance can be reduced.

The low voltage windings are placed inside the high voltage windings. This is to reduce the insulation requirement of high voltage windings.

Shell type

2. Shell type transformer

In shell type transformers, the core has three limbs: a central limb and two side limbs. The windings are made around the central limb. The width of the central limb is twice that of the side limb. Both the HV and LV windings are divided into number of coils and are arranged alternatively. The HV windings are sandwiched between the LV windings.

• Construction

Core type
Shell type
Simpler in design
Little complex than core type
Permits easy assembly and insulation of windings
Sandwiched structure make the process difficult.
Easy to dismantle.
Difficult to dismantle

• Fault withstand capacity

Whenever any two conductors carrying current is brought together they experiences a force. This force will be proportional to the product of currents carried by them and the direction in which the force acts depends upon the direction of current flowing though them. In case of transformers, during fault condition the current through the windings will be very high. When the secondary winding is short circuited with the primary, very high electro motive forces are created. Since the windings carry currents in opposite directions the inner windings experiences a force crushing it on the core and the outer windings experience a tensile force pushing it outwards.

Core type
Shell type
Core type construction has poor mechanical strength since the windings are not supported externally.
Shell type windings withstand the forces produced during short circuit conditions.
More susceptible to damage under short circuit conditions.
Less susceptible to damage under short circuit conditions.

• Leakage reactance

Core type
Shell type
Leakage reactance will be more due to more spacing between the windings
Leakage reactance will be less

• Repair

Core type
Shell type
The windings of a core type transformer are completely accessible except for small portion of window. Hence the coil can be made easily inspected
Since windings are surrounded by the core it is difficult to dismantle and repair the coils.

• Cooling

Core type
Shell type
In core type transformers, the windings surround the core and have better cooling than the core.
In shell type transformer, the core surrounds the windings and has better cooling than the windings.

Single phase core type and Shell type transformer

Since the insulation of the windings should not undergo any damage due to temperature rise core type of construction is universally followed.

Three phase core type and shell type transformer

Classification based on the number of phases.

1. Single phase

2. Three phase

Classification based on the method of cooling

a. Self-air–cooled (dry type)

b. Air-blast–cooled (dry type)

c. Liquid-immersed, self-cooled

d. Oil-immersed, combination self-cooled and air-blast

e. Oil-immersed, water-cooled

f. Oil-immersed, forced-oil–cooled

g. Oil-immersed, combination self-cooled and water-cooled

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