Transformer Rating

The maximum current that transformer can deliver to loads as known as the current rating of the transformer. The voltage, maximum current delivery of the transformer and the product of the voltage and maximum current known as VA rating are engraved in the nameplate of the transformer.

The rating of the transformer is adversely affected with abnormal temperature rise caused by the losses. The no load losses increases with an increase in the frequency and or voltage and the copper loss increase if the load power factor is more lagging which demands more current from the transformer and the copper loss gets increased. The losses can be kept within the limit by maintaining the rated voltage ,frequency and load current. 

The losses cause temperature rise of the transformer and if transformer is operated above its rated safe temperature limit, the winding insulation is apt to fail. The temperature can be held within the safe limit by proper cooling.

The losses depends on the V and I. The rating of the transformer depends on the losses. The losses  are independent of the power factor that is why the rating of the transformer does not depend on the load and it depends on the V x I which is known as VA rating of the transformer. The designer designs the transformer taking into account the maximum VA rating of the transformer. The transformer is specified for apparent power rating- VA rating.

The transformer primary VA is equal to the secondary VA plus the losses in the transformer. If the losses increase, the secondary VA delivery will get affected because of the temperature rise of the transformer. Generally the losses of the transformer is negligible the primary VA is equal to the secondary VA.

The VA rating marked on the nameplate is applicable for both the winding and VA rating of both the winding of transformer is the same. For a 440/220 Volts, 100 KVA transformer, the primary winding VA is 100 kVA and the secondary winding kVA is also 100 kVA.