Hi!
Technically, PMSM and PMDD motors are the same. The main point of a direct drive motor is that already on the design stage you must know the load requirements and construct the motor with speed and torque matching these requirements. In large industrial applications, these requirements may affect the machine size, cooling scheme, or bearings type. But in general, there is no major difference. If you are a motor designer and your customer says you "I need a 500 rpm motor with 50 Nm torque", you may not even know, is it a direct drive motor or not? If the customer buys this motor and puts a simple fan to the shaft then this will be a direct drive motor. But if he takes the same motor and attaches some load to this via a gearbox then the motor simply turns to be a non-direct drive.
There is no "equation" to design a motor, but I would recommend you have a look at the book: Juha Pyrhonen, Tapani Jokinen, Valeria Hrabovcova - Design of Rotating Electrical Machines