Why Transformer Rating in kVA?

Cu loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss depends on volt-ampere (VA) and not on phase angle between voltage and current i.e. it is dependent of load power factor. That is why rating of transformers is in kVA and not in Kw.

LOSSES IN A TRANSFORMER:

1. Core or iron Loss: It includes both hysteresis loss and eddy current loss. Because the core flux in a transformer remains practically constant for all loads (its variation being 1 to 3% from no-load to full-load), the core loss is practically the same at all loads. These losses are minimized by using steel of high silicon content for the core and by using very thin laminations. Iron or core loss is found from the O.C. test. The input of the transformer when on no-load measures the core loss.

2. Copper Loss: This loss is due to the ohmic resistance of the transformer windings. Total Cu loss=I₁²R₁+I₂²R₂=I₁²R₀₁=I₂²R₀₂. It is clear that Cu loss is proportional to (current)² or kVA². In other words, Cu loss at half the full-load is one-fourth of that at full-load. The value of Cu loss is found from the short-circuit test.

EFFICIENCY OF A TRANSFORMER:

Efficiency = Output/input

But a transformer being a highly efficient piece of equipment, has very small loss, hence it is impractical to try to measure transformer efficiency by measuring input and output. These quantities are nearly of the same size. A better method is to determine the losses and then to calculate the efficiency from;

Efficiency = Output / (Output + Losses) = Output / (Output + Cu loss + iron loss)

= (Input-losses) / Input = 1 – (Losses / input)

Efficiency can be computed by determining core loss from no-load or open-circuit test and Cu loss from the short-circuit test.

The ordinary or commercial efficiency of a transformer is given by the ratio

Output in watts / Input in watts

AUTO-TRANSFORMER:

It is a transformer with one winding only, part of this being common to both primary and secondary. Obviously, in this transformer the primary and secondary are not electrically isolated from each other as is the case in a 2-winding transformer. But its theory and operation similar to that of a two-winding transformer. Because of one winding, it uses less copper and hence is cheaper. It is used where transformation ratio differs little from unity.