Why transformers are rated in kVA and not kW | why not transformer are connected to DC

(1) Why transformers are rated in kVA and not kW ?

Since copper loss depends on the current and core loss depends on the voltage, the total loss in the transformer depends on the Volt-Ampere product and not on the phase angle between  voltage and current i.e total loss in transformer is independent of the load power factor.

Hence, transformer rating is in Volt-Amperes and not in Watts.

(2) What happens if transformer is connected to dc supply ?

Consider the below single phase transformer.

When an ac is given to the transformer,an alternating flux is produced and an emf E₁ is induced in the primary.

But when a dc supply is given to the transformer, the primary draws a steady current and hence produce constant flux. Consequently, no emf is induced in the primary winding   i.e 

Since, primary winding has very low resistance R₁ , therefore, it draws excessive current given by   I₁ = ( V₁/R₁ ) . High current produces excessive heating in the primary winding. The result is that primary winding burns out.

Therefore, care must be taken not to connect primary of a transformer to dc.



Related concepts :-

   Transformer Loss | How to reduce transformer loss ?  

   Transformer construction- Core and Shell type transformer
    Open Circuit and Short Circuit test on Transformer

Transformer working Principle
Emf equation of transformer
Voltage regulation of transformer
Efficiency of a Transformer