2 reasons; first, motors and drive electronics are both heat producing systems. In order to combine them you have to derate the assembly to prevent damage, and second, motors are designed by mechanical engineers, motor drive circuits are designed by electronics engineers. The two disciplines and attending manufacturing processes are completely different.
Chuck, A primary reason to separate motors and drives in the past has been packaging the two together, and especially the ability of the drive to handle heat. There is significant technology involved in mounting the drive to the top of the motor, but it is still an issue as motor/drive combos go up in torque (with resulting increases in heat).
Researchers working with additive manufacturing have said multimaterial techniques will allow industry “to fabricate materials with combinations of density, strength, and thermal expansion that do not exist [yet].”
The term "multiphysics" is used to describe the simulation of multiple types of physics and their influence on one another -- for example, the investigation of the behavior of a chemical in liquid form will involve both chemistry and fluid dynamics.
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