Losses in a Transformer | How to reduce transformer Losses

There are 4 types of losses in a transformer i.e Core loss,Ohmic loss,  Stray load loss and dielectric loss. Lets understand each one by one in a easy way.

(1)Core losses in transformer :- Core losses have two components. The hysteresis loss P_h and the eddy current loss P_e .These together are called no-load losses of a transformer and are calculated by open circuit test.

i.e Core loss, P_c = P_h + P_e

(a)Hysteresis loss (P_h) in a transformer :- The hysteresis loss expression is given by

            hysteresis loss, P_h =K_h f B_m^x

         where,

K_h = a proportionality constant whose value depends upon the volume and quality of the core material

f    = frequency of the power supply and hence frequency of the alternating flux

B_m= maximum flux density in the core

x   = called steinmetz’s constant and its varies from 1.5 to 2.5 and depends on the magnetic properties of the core material
By default, x=2 is taken as per new IEEE standards.

Another expression for hysteresis loss can be obtain by putting voltage equation,

     V= √2 π f N B_mA_i

Or

B_m= V / √2πfNA_i

    where,

V=voltage induced in the primary=supply voltage   for an ideal transformer

N=number of turns in primary, constant for a given transformer

A_i= net core area,constant for a given transformer

Putting B_mvalue in hysteresis loss, P_h =K_h f B_m^x  expression. We get,

P_h =C_h f^1-xV^x  

where, C_h = K_h/(√2πNA_i)^x       , is a new constant 

Thus you can see, hysteresis loss depends on supply voltage and supply frequency.

How to reduce hysteresis losses in a transformer?

As  proportionality constant K_h depends upon the quality of the core material therefore we must choose a core material that has low K_h and high permeability. usually silicon steel and Cold Rolled Grain Oriented steel are used.   

(b)Eddy current loss (P_e) in a transformer :- The eddy current loss formula is given by

eddy current loss, P_e =K_e f² B_m² t²

where,

K_e = proportionality constant whose value depends upon the volume and resistivity of the core material

t  = thickness of the laminations

Another expression for eddy current loss can be obtain by putting voltage equation,  B_m= V / √2πfNA_i    into eddy current loss expression

We get,

    P_e =k_e V²

where,

    k_e = a constant = K_e / (√2πNA_i)²

Hence, we see that eddy current loss of a transformer is independent of frequency for a given supply voltage.

How to reduce eddy current losses in a transformer ?

As you saw, eddy current expression depends on K_e and thickness of the laminations t. Therefore, we must choose a core material with high resistivity and also the thickness of the laminations must be as small as possible.

(2)Ohmic losses in transformer :- The ohmic losses in transformer is due to the winding resistance. So, whenever the transformer is loaded these losses occur as these losses depend on the current through the windings.

       Ohmic loss = I²R

     Where,

       I=current through windings

       R=winding resistance

    That is why when transformer is not loaded ohmic losses are negligible as the current through the windings is negligibly small and this principle is used in open circuit test of a transformer which is used to measure core losses in transformer.

As resistances vary with temperature therefore these losses also vary with temperature and must be calculated at standard operating temperature.

How to reduce ohmic losses in transformer ?

Simple, use winding conductor of  low resistivity.

(3)Stray losses in transformers :- Always there exists some leakage flux in a transformer. This leakage flux induces eddy currents in transformer tank, bolts, channels  etc. . This eddy current loss is called stray load loss of a transformer.

 Leakage flux must be minimised to reduce it.


(4)Dielectric loss :- This loss occurs in the insulating materials i.e in the transformer oil and the solid insulation of high voltage transformers.

Its worth mentioning that transformers don’t have any mechanical losses as it is a static device and this makes transformer efficiency as high as 99% , highest among all electrical machines. 

This explains various losses in a transformer and how to reduce losses in transformer. 

Closely related concepts :-
(1) Open circuit and Short Circuit test of transformer
(2) Transformer construction - shell type and core type transformer 
(3) Transformer working principle
(4) Voltage regulation of transformer
(5)  Efficiency of a Transformer
(6) Polarity test on Transformer