Abstract: A rectifier unit for 3-phase a.c. to d.c. conversion is of the type having a transformer with a star-connected and a delta-connected secondary the outputs of which are rectified and combined to give 12-pulse operation. To reduce the danger of current imbalance between the two secondaries, a compensating transformer unit is interposed between the secondaries and the rectifiers which modifies the signals such that any difference in the current drawn from one secondary compared with the other tends to equalize the current drawn from the secondaries. The compensating transformer unit can take the form of 3 individual transformer units but is preferably a single transformer with 3 limbs, each of which carries a main winding connected to one secondary and two opposed subsidiary windings which receive the phases from the other secondary which are closest to and on either side of the phase of the associated main winding.