Electrical Interview: RATING OF TRANSFORMER

 A typical name plate rating of a single phase electrical transformer is as follows: 20 KVA , 3300/220 V, 50 HZ. Here 20 KVA is the rated output at the secondary terminals. Note that the rated output is expressed in Kilo-volt-amperes (KVA) rather than in Kilowatts (KW). This is due to the fact that rated transformer output is limited by heating and hence by the losses in the transformer. 

                              

                                            

    

                                 The two types of losses in a transformer are core loss and ohmic losses. The core loss depends on  transformer voltage and ohmic loss on the transformer current. As these losses depend on transformer  voltage (V) and current (I) and are almost unaffected by the load power factor, the transformer rated output is expressed in VA or in KVA avd not in KW.