This paper deals with the choice of the motor-transmission couple suitable to move a joint with reference to the dynamic operating range of the drive system, introducing the case in which its limit curve depends on the motor speed in a general way. The paper explains the problems arising in this case and how the design approach can be modified with respect to a rectangular operating range. A graphical method is proposed, which permits the designer to exclude some drive systems and, for each of the others, to find a range of transmission ratios that can certainly be coupled with the motor and/or a range of transmission ratios for which the analysis must be deepened