Manufacturer designs transformer based on required voltage and current and specified them on the nameplate of the transformer in terms of VA called the rating. It can be also said that the maximum voltage and current that can be safely applied to transformer called the rating. The rating of transformer depends upon temperature rise which depends upon losses taking place in the transformer. Although temperature can be maintained within permissible limits by using the proper cooling system. If the effectiveness of the cooling system is more then the rating of the transformer will be more and vice versa. For a given cooling system rating of an electrical machine is indirectly determined by the losses present in the machine.
In a transformer, losses are of two types
- Constant losses or core losses – These depend on V
- Variable losses or ohmic (I2R) losses – These depend on I
Hence total losses depend on V and I. Since rating of transformer depends upon losses and losses depends upon V and I, hence the rating of transformer depends upon V×I, which is also termed as VI rating. As losses are independent of the power factor of load, the rating of the transformer is also independent of load and can be only decided based on losses. That’s why transformer is generally specified with apparent power rating (VA or KVA) and not in KW.
Lets us take an example, a transformer working on rated voltage and rated current with load power factor equal to zero, as load power factor is zero it will deliver zero power to load, but it has rated KVA output, hence rating must be expressed in KVA.
For any transformer rated input in KVA at the primary = rated output in KVA at secondary + losses.
As transformer runs on very high efficiency its losses can be neglected and hence rated input in KVA at the primary = rated output in KVA at secondary. From here we can say that rated KVA marked on the nameplate of transformer refers to both the windings. i.e KVA rating for both the primary and secondary winding is same.
The KVA rating mentioned on the nameplate of the transformer is load side KVA but only at full load condition.