Why Transformer Rating In KVA, Not in KW ?
As we know, There are two type of losses in a transformer;
1. Copper Losses
2. Iron Losses or Core Losses or Insulation Losses
- Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage.
- When a manufacturer makes a transformer, UPS etc, they have no idea of the type of load that will be used & consequently they can only rate the device according to its maximum current output that the conductors can safely carry (at unity Power Factor) & the insulation rating of the conductors (voltage & temperature).
That’s why the Transformer Rating may be expressed in kVA, Not in kW.
Why only transformers, even alternators are rated in VA, kVA or MVA etc.
- In this case the rating of a transformer depends on the winding or oil temperature reaching the specified limiting value (decided by the type of insulation) minus a safety margin, something like 170 – 20 =150 deg C.
- The temperature rises because of the losses. The copper losses which vary with the load depends on I^2 (square of current magnitude)– of course the constant and small core loss (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 is.
- Just to clarify consider a single phase 110 V /220 V transformer . Let its Req=eqivalent resistane 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 (2.2) but different kW (2.2 and 0 ) the losses are same in both cases and the temperature rise will be identical and have to be rated same.
- Thus the rating is decided by the magnitude of the current, or as voltage is constant , by the VA or kVA or MVA and not by W or kW or MW.