TRANSFORMER LOSSES

There are four types of losses occurs in the transformer and that is,     

      1) Copper loss

      2) Iron loss

      3) Stray loss

      4) Dielectric loss

Click here to know the difference between the power transformer and the distributed transformer.

Out of above mentioned losses, copper loss and iron loss can be considered as major losses and stray loss and dielectric loss can be considered as minor losses. Let’s explain this in details.

1) Copper loss: There are two winding in the transformer; primary winding and a secondary winding. The copper loss is nothing but, the losses occur due to the winding resistance. The copper loss is also known as I2R loss or cu loss. If primary winding resistance is R1 and secondary winding resistance is R2, then

Total copper loss in transformer = 

(I1^2) R1 + (I2^2) R2

Where I1 is full load primary current

I2 is full load secondary current

From the above equation, we can see that copper loss is directly proportional to the square of the load current. It means that, as the load current changes, the copper loss also changes. So, this loss is considered as a variable loss.

2) Iron loss: This loss takes place in the core of the transformer due to the time-varying nature of flux in the core. So, the iron loss is also known as core loss. The total iron loss is divided into two parts; hysteresis loss and eddy current loss.

          (A) Hysteresis loss:  Whenever it is subjected to alternating nature of magnetising force, the hysteresis loss occurs due to the reversal of magnetisation of the transformer core. In this case, after every half cycle, the domain present in the magnetic material will change their orientation. The power consumed by this change of orientation after every half cycle is known as hysteresis loss.

The magnetic reversal of the transformer core is plotted with the help of the BH curve and it represents as below figure.

The hysteresis loss occurs in one cycle is equal to the area enclosed with the one hysteresis loop.

The hysteresis loss can be determined with the Steinmetz's formula. Which is given as

Where n= Steinmetz coefficient = range 1.5 to 2.5

F = supply frequency

V = volume of core

X = Steinmetz exponent = 1.6 for silicon steel

          (B) Eddy current loss: The eddy current loss is nothing but, the I2R loss present in the core of the transformer due to the production of the eddy current in the core. The eddy current is produced because of the conductivity of the core.

The eddy current loss is directly proportional to the conductivity (σ) of the core.

Rse = the resistance offered by the core to flow of eddy current.

Rse is inversely proportional to the conductivity.

By reducing the conductivity, the eddy current can be reduced. It is possible to reduce conductivity without affecting magnetic properties by adding silica content and by using laminated core.

3) Stray loss: It can be divided into two parts; copper stray loss and iron stray loss.

          (A) Copper stray loss: This loss occurs only under full load condition due to the leakage flux. This loss is the additional I2R loss due to stray current within the conductor. This loss is considered as a variable loss. Instead of the solid conductor, the stranded conductors are used to reduce the copper stray loss and by this way, the skin effect of winding is also reduced.

          (B) Iron stray loss: The iron stray loss is the additional iron loss occurred due to the auxiliary iron parts like transformer tank, steel channels, and conservation tank. In this auxiliary iron parts, due to the leakage flux, this loss is produced. This loss is less in shell type transformer compared to the core type transformer.

The leakage flux is directly proportional to the load current. So, this loss is also considered as a variable loss. The iron stray loss is just 0.5% of full load output.

4) Dielectric loss: This loss is produced in the insulating material of the transformer. In insulating material, the free electrons are not available. When a voltage is applied, a small amount of current will flow through this due to the conversion of atoms. There is displacement of charges and the current produce due to this is known as displacement current.

The process of conversion of atoms into electric dipole is known as polarization. The dielectric loss is depending on applied voltage and it is independent of load current. So, this loss is considered a constant loss. This loss is 0.25% of full load output.