Existing electric motors of higher power are optimized for driving stationary systems and are therefore generally too heavy, too large and too expensive for use in vehicles. New production processes are needed to ensure the cost efficient production of light-weight electric drives. This article presents an approach to reduce the rotor mass of permanently excited synchronous motors (PSM) by using a model-based optimized assembly procedure for rotor components. It aims to create savings in weight and winnings in dynamics by omitting the use of balancing discs that are usually needed to store mass for a costly balancing process. Investigations on two separate rotor designs are carried out to analyse whether the required balancing grade can be reached through an optimized assembly of the rotor components. For the first rotor design, an analysis of the unbalance state of all main rotor components (shaft, rotor discs and magnets) was carried out in order to validate the prediction of the resulting unbalance of the complete rotor. Improvement measures regarding the description of measuring and assembly deviations are listed and put into practice for the preparation of new investigations with a rotor design that sets higher demands to the desired residual rotor unbalance.