How to find the Efficiency of Transformer from OC and SC Test

From OC Test, we can obtain no-load current and hence no-load power, P_oc.

This no-load power consumed is equal to no-load iron-loss P_i.

(i.e.)

Poc = P_i.

From SC test, we get P_sc, which is power consumed only at the transformer windings due to the winding resistance R. Therefore, the Power consumed is I_sc²R (Full Load Copper loss).

 So, P_sc = I_sc²R = P_cu (Full Load Copper loss)

Now, Efficiency,

E=(O/P Power) * 100/(I/P Power

  =(O/P Power) * 100/((O/P Power)+P_i+P_cu)

Here, Full load O/P Power is nothing but (VA rating of Transformer) * (Power Factor).

This is applicable on both primary and secondary side.

Power Factor = cosθ

Therefore,

E= (VA rating x cosθ) * 100/((VA rating x cosθ) + Poc + P_sc)     %

This is the efficiency of the transformer at Full load.

To obtain the efficiency at any load, VA rating can be multiplied with load fraction (n). Where load fraction is the ratio of required load to full load.
(In other words, if full-load current is I, then 50% full-load will take a current of I/2 and so on. This is indirectly depicted using load fraction 'n'.)

Therefore, Efficiency at any load can be obtained as follows,

E= (n * VA rating * cosθ) x 100/((n * VA rating * cosθ) + Poc + n² x P_sc)     %

 where, n=(given load)/(Rated load)