We already know what the no-load condition of a transformer is. Now we present the transformer load condition in greater detail.
Assuming that the transformer load is within the permissible limits, it can be stated that the magnetising current I0 remains unchanged, since it depends mainly on Xμ and RFe, (Xμ and RFe, >> XΦ1 and R1). However, an additional load current I’2 appears, which causes voltage drops across the winding resistances (the so-called copper losses) and the leakage reactances. These voltage drops mean that the voltage at the secondary side terminals is lower than in the no-load condition of the transformer, and it reaches its rated value at the rated secondary current of the transformer.
The conclusion is therefore that the rated secondary voltage is the lowest permissible voltage at the secondary terminals of the transformer, assuming that the primary side is supplied with the rated primary voltage and that rated currents are flowing in the transformer.
In other words: the transformer manufacturer must guarantee that under rated load conditions the voltage at the output terminals will not be lower than the rated value. Only then can the transformer deliver its rated power to the load.
