An ideal transformer is loss-free. But within the practical transformer, there are following losses happening.
As shown in Fig. the total loss in a transformer can be divided into two types namely the copper loss and the iron loss.
The iron loss is further classified into two types namely the hysteresis loss and eddy current loss.
1) COPPER LOSS (Pcu) :
The total power loss taking place in the winding resistances of a transformer is known as the Copper loss.
Copper loss = Power loss within the primary resistance + Power loss within the secondary resistance.
The copper loss is denoted by Pcu.
where R1 and R2 are resistances of primary and secondary respectively.
Where I1^2 R1 = Power loss taking place in the primary resistance R1.
and I2^2 R2 = Power loss taking place in the secondary resistance R2.
The copper loss should be kept as low as possible to extend the efficiency of the transformer. To reduce the copper loss, it is essential to reduce the resistance R1 and R2 of the primary and secondary windings.
Copper losses are also called as “variable losses” as they are dependent on the square at load current. The relation between the copper loss at full load and that at half load is as follows:
Where Pcu(fl) =Copper loss at full load and Pcu(hl)= Copper loss at half load.
2) IRON LOSS (Pi)
Iron loss Pi is that the power loss happening within the iron core of the transformer.
It is equal to the sum of two components called hysteresis loss and eddy current loss.
Pi=Hysteresis Loss + Eddy current loss
i) Hysteresis losses :
We have already discussed the hysteresis loss taking place in a magnetic material.
The area enclosed by the hysteresis loop of a material represents the hysteresis loss. Hence special magnetic materials should be utilized in order to scale back the hysteresis loss. Materials like silicon steel have hysteresis loops with very small areas. Hence such materials are preferred for the development of the core. Commercially such steel is named Lohys, which means low hysteresis materials.
The hysteresis loss is frequency-dependent. As we increase the frequency of operation, the hysteresis loss will increase proportionally.
ii) Eddy current losses :
Due to the time-varying flux, there’s some induced emf within the transformer core.
This induced emf causes some currents to flow through the core body.
These currents are known as the eddy currents.
The core is made of steel and has some finite resistance. Hence due to the flow of eddy currents,
heat will be produced. The power loss due to the eddy currents is given by:
Eddy current loss= (Eddy current) X r Where r= Resistance of the core.
The eddy current losses are minimized by using the laminated core.
The core is manufactured as a stack of laminations instead of a cast-iron core.
These laminations are insulated from each other by means of a varnish coating on all the laminations.
Hence each lamination acts as a separate core with a small cross-sectional area, providing a large resistance to the flow of eddy currents. The eddy current loss also is frequency-dependent. It is directly proportional to the square of operating frequency.
Hence the iron loss Pi of the total loss is dependent on the frequency but the copper loss Pcu is constant irrespective of frequency. The iron loss is denoted by Pi. It is the sum of hysteresis and eddy current loss. Iron loss may be a constant loss that doesn’t depend upon the extent of the load.
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