Transformer (electrical engineering)

An electrical transformer transfers energy between windings through electromagnetic induction. Its operation depends on changing magnetic flux, rather than a static magnetic field alone.

For an ideal electrical transformer with turns ratio $n=N_p/N_s$, the secondary load $R_L$ appears at the primary as $n^2{R}_L$, while a source resistance $R_g$ appears at the secondary as $R_g/n^2$. A finite magnetizing inductance $L_m$ contributes a parallel branch $i\omega L_m$ at the primary; its magnitude must greatly exceed the reflected load for the loaded input to be approximately resistive. The corresponding unloaded secondary self-reactance is $\omega L_m/n^2$. That reactance is distinct from the source's transformed output resistance. A specification that simultaneously calls the loaded input matched to a small resistance and calls the primary impedance very large must distinguish these two branches.