Two methods have been evolved to determine the commutation reactance of a transformer operating in a rectifier or invertor circuit. One of the methods, called the commutation-overlap method, gives an accurate value. The other, designated the area-loss method, is simpler in evaluation and gives an approximate value, the approximation being closer as the d.c. waveform approaches smoothness.Using a particular built-in probe-coil arrangement on a 3-phase convertor-transformer (with cylindrical windings of equal height) to probe the leakage flux, it was found that the probed leakage inductance remained invariant in the three conventional short-circuit tests for a 3-phase rectifier-transformer as well as in rectifier and invertor operation of the transformer. It was concluded for the type of transformer used that the short-circuit reactance is independent of the method of test, and that the commutation and short-circuit reactances are identical.