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The line-side characteristics of ac traction drives are usually specified in terms of the power factor. By standards, this factor is defined for steady state conditions only. An average power factor throughout a load cycle according to the normal operation of a railway drive is presented. Different approaches to an average power factor have been reported in the literature. The approach presented here is based on a physical background. In contrast to previous definitions of an average power factor the new factor has the value of one if and only if constant active power is taken from the supply throughout the load cycle. As the power consumption as a function of time is rather a characteristic of the load cycle than of the power converter it is further suggested that the new average power factor be split into two factors, one specifying the load cycle, the other describing the converter.