The design of ferrite-assisted synchronous reluctance machines is investigated, with particular attention to the pivotal aspect of avoiding irreversible de-magnetization. Geometric rules for obtaining a robust design are proposed and described analytically. The safe operating area is quantified in terms of the corresponding maximum electrical loading. Such demagnetization limit shows to be depending on the operating temperature and the machine size. Furthermore, the comparison between the continuous load and de-magnetization conditions shows that low and medium size machines can be stiffer against demagnetization, with respect to larger machines, and have room for transient overload. The analysis is validated by finite-elements and a design example is given, namely a twelve poles direct-drive machine, rated 910 Nm, 200 rpm.