The amount of electrically powered equipment in aircraft has increased in recent years in the move towards the More Electric Aircraft (MEA), which is expected to offer superior characteristics. To meet the stringent power quality specifications imposed by the air-frame manufacturers over the wide range of supply frequencies, many manufacturers are adopting 18-pulse Auto-Transfomer Rectifier Units (ATRU) for the ac-dc front end converter in actuators and motor drives. However, 18-pulse ATRUs often fail to meet the performance, size and weight expectations. A literature survey on converters with low effects on the supply is presented, focusing on multi-pulse rectifiers and on harmonic injection or pulse multiplication techniques. Subsequently, two novel, low VA rating, passive harmonic injection circuits, referred to as current injection and voltage injection, are analysed, simulated and experimentally evaluated. They achieve 24-pulse performance when added to a 12-pulse converter based on a 0.5 p.u. series-connected open-star/delta transformer arrangement. The Total Harmonic Distortion (THO) of the line currents was 2.6% at 400 Hz when 0.1 p.u.line inductors were used. These two injection circuits can be combined together to obtain a passive 36-pulse converter, or can be combined with a low VA-rated, bi-directional switch that operates at six times the supply frequency in two different ways, to obtain 36 and 48-pulse characteristics. Analysis and simulation results are presented, and experimental results for the two 36-pulse converters are shown. The THD of the line currents at 400 Hz, using 0.1 p.u.line inductors was 1.5% and 1.2 % respectively. Finally, a harmonic injection technique is presented that achieves 24-pulse performance when added to a parallel-connected, 12-pulse, voltage-sourced converter. This technique, which can be implemented by passive or active means, was analysed and simulated. Experimental results on a boost-type, 24-pulse converter, with a 0.5 p.u. star/delta transformer connection are presented, where the THD of the line currents remained below 3.5% at 400 Hz with 0.12 p.u. line inductors. The technique is also valid for converters using Line Interphase Transformers (LIT).