The interphase transformer inductance seriously affects the performance of 18 pulse rectifiers. Low inductance values cause non-characteristic harmonics whereas high inductance values increase the rectifier cost and size. Hence. determination of the interphase transformer inductance value is an important problem in the design of 18 pulse rectifiers. In this paper, an approach to determine the optimum inductance value of an interphase transformer is proposed and a practical formula is introduced. The proposed approach has been validated with simulation and experimental studies carried out with designed capacitive loaded autotransformer based 18 pulse rectifier for different IPT inductance values at different load levels. Experimental and simulation results show that cost effective interphase transformer inductance value can be determined with the proposed approach and this value reduces the line current harmonics and improves power factor drastically. (C) 2009 Elsevier Ltd. All rights reserved.