The aim of this thesis is the analysis of the dynamic behaviour and performance of an innovative power transmission system, based on the technology of magnetic gears. The magnetic gear has the capability to transmit motion and torque in a contactless way between two coaxial rotors with permanent magnets through the interaction with a modulator element holding ferromagnetic poles. Several potential benefits can be achieved using this technology, as the reduction of noise, wearing and vibrations, and the absence of lubrication, which are typical of mechanical power transmission systems. This transmission device is called planetary magnetic gear (PMG), due to its topological and functional similarity with a planetary mechanical gearing device, from which the same working principles are derived.