Conventional high torque low speed drive systems commonly have a mechanical transmission between the induction motor and the load consisting of gears, gear heads, belt/pulleys or camshafts. The main drawbacks of such setups, are the deficiency of the drive system, its high cost and the maintenance needs. An alternative to this, is the replacement of the induction motor and its mechanical transmission elements with a permanent magnet (PM) synchronous motor (PMSM) directly coupled to the load running at low speed. In this context, the paper deals with the design evaluation of a 5kW 50rpm motor and concentrates on two radial flux promising topologies i.e. with surface-mounted permanent magnets with inner and outer rotor. Since the goal is mainly the minimization of the machine's active weight (with respect to constraints related to outer dimensions, efficiency and cost), the designs of the PM machines are conducted by solving an optimization problem. Three optimization methods are adopted and three weighted cost functions are presented. The effectiveness of the methods in finding competitive alternative PMSM designs is then evaluated. The presented results are compared with other found in literature and reveal satisfactorily enhanced design solutions and performance of the proposed methods.