Why transformer rating in kva

542 shares • Facebook • Twitter • LinkedIn • WhatsApp On ships, not only Transformers but also generators, protection devices, etc., are mostly rated in kVA. A motor does mechanical work and thus has mechanical output expressed in kW, and it has fixed Power factor, i.e., the motor has defined power factor, and the rating is written in KW on Motor nameplate data table. That’s why we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA. In case of an electric motor, it’s primary function is to convert the electrical power into mechanical power as the load it is connected to is not electrical, but mechanical and only active power is taken into account which has to be converted into the mechanical load. Moreover, the motor power factor does not depend on the load, and it works on any P.F because of its design. A transformer is a static device, which does not perform any mechanical work. But the primary functions are stepping down and stepping up of voltage ratings. Invariably, while stepping up/down the voltage, it also steps down/up the current inversely. While doing the work, the transformer bears two types of losses ; 1. Copper Losses : 2. Iron Losses or Core Losses (insulation Losses) Copper losses in the transformer ( I²R) depend on the current which is passing through transformer winding. On the other hand, the Iron Losses or Core Losses depend on the Voltage. It can be said that the copper loss depends on the rating current of the load the transformer is s...

Now the temperature rises because of the losses. The copper losses which vary with the load depend on I 2 and small core loss which depends on supply voltage which is practically constant is also present. The loss will be same as long as current magnitude is same irrespective of what the power factor (pf) of the load current. Just to clarify consider a single phase 110 V /220 V Transformer. Let its R eq = Equivalent Resistance be 3 ohm (referred to 220 V side). Let it be rated 2.2 kVA. The rated current on 110 V side is 20 A and on 220 V side is 10 A. First put a load on the 220 V side drawing 10 A at pf = 1.0. The total loss will be = 3(10 2) = 300 W + core loss of, say 30 W. The output will be = 220 (10) (1.0) = 2200 W = 2.2 kW. The kVA = VI/1000 = 220(10)/1000= 2.2 kVA Next put a purely inductive or capacitive load drawing the same 10 A but at a pf =0. Again the total losses will be = 3(10 2) = 300 W+ core loss of, say 30 W, same as before. But the output will be =220 (10) (0) = 0 kW. The kVA =VI/1000=220(10)/1000= 2.2 kVA So it can be seen that with the same kVA but different kW the losses are same in both cases and the temperature rise will be identical in both cases and therefore when we design a Transformer the rating of Transformer must be given in KVA or VA or MVA. Thank you! Categories Post navigation