Why Transformers are rated In KVA and not in KW?
For any type of machine, rating means the output. Also, we know,
OUTPUT= INPUT – LOSSESS
As the transformer is connected to an Infinity system (Electrical Grid), the INPUT is unlimited. Hence, the output is limited by the Transformer losses. The losses produce heat in the transformer and if the heat is not within limits or not dissipated in time, the transformer cores, transformer winding and the transformer oil etc. may damage. This may cause failure of the transformer.
Reason 1:
Transformer has mainly two losses. Iron loss and Copper loss.
Iron loss (Pi)= hysteresis loss + eddy current loss
Hysteresis loss, Ph = khf Bxm
Eddy current loss , Pe = kef2 B2m
Both the Hysteresis and eddy Current loss depend on Max. Flux density (Bm). The Max. flux density depends on (Vm/f). Means, the iron loss depend on voltage.
For, more details if we put the value of Bm , Hysteresis loss depends only on Voltage and Eddy current loss depends on frequency and square of voltage. As the Voltage and frequency of a transformer is constant, the Iron loss is constant.
Copper loss= I2R.
The copper loss depends on the current.
Both the losses of transformer is depends on Voltage and Current. The power factor has no role in transformer losses. Hence, the output is depends on the product of Voltage and Current, i.e VA and dividing by 1000, we may say the losses depend on the KVA.
Reason 2:
At the time of design, the designer doesn’t know about the type of load on that transformer.
For Example, at Same Voltage and Current Load, A 100 KVA transformer at Unity Power factor can take 100 KVA x 1= 100 KW load.
And the same transformer can take load at 0.8 power factor is 100 KVA x 0.8 = 80 KW Load. But both are delivering the 100 KVA load.
Reason 3
The temperature rise of a transformer is depends on the losses of transformer and directly proportional to the apparent power (kVA) which flows through it. This is also a principle factor behind that the rating of the transformer is given in kVA and not in kW.
If a transformer has different mode of cooling, has different KVA rating.
Important – In actual practice, those electrical AC devices which act as the source (such as alternator, transformer) always rated in kVA to make their operation independent of the load power factor while the electrical devices which act as the load (e.g., motors) always rated in kW and draw the current from the source according to their power factor.