Operation at no-load and under load

 

Without load the transformer responds like a coil of large inductance. The current consumed by the input winding is the no-load current. The no-load voltage induced at the output is computed according to the primary transformer equation. When operating at no-load almost all of the magnetic flux is concentrated in the iron core. When a load is connected, the load current weakens the input current in accordance with Lenz's law. Some of the lines of flux are then distributed outside the core and this is called leakage flux.

Transformation ratios

 

In an ideal transformer which achieves 100% power coupling (loss-free) the same magnetic flux permeates both windings. Without load the voltages respond in line with the number of winding turns. The higher to lower voltage ratio is called the transformation ratio. When the transformer is coupled the currents under load respond in inverse ratio to the number of turns. The impedances are carried forward in the square of the transformation ratio. Due to the power losses arising in real transformers these transformation ratios are only approximately correct.