This paper presents a modelling of a vibroflot dynamic behaviour. Vibroflots are used by ground improvement companies to perform vibro compaction. This technic consists in deeply densifying sandy soils by vibrations in order to make stable future infrastructures. A classical vibroflot is a slender structure composed of several rotors which are not rotating or driven by an asynchronous electrical motor that produces orbital vibration and therefore the soil compaction. The final objective of the investigation is to establish a global multi-physics model coupling electromagnetic motor, rotordynamics and soil models in order, for example, to adjust the speed of rotation that generates the soil resonance, making the soil compaction easier. The present paper presents only the electro-magnetic and mechanical models, in order to predict the vibroflot dynamic behaviour during the free hanging operation. Among the predicted results, the lateral displacement and motor amperage are of particular interest because they can be compared with the experimental measures collected on an in-situ full-scale instrumented vibroflot.