In rotor dynamics, the auto-balancing can be an effective solution to handle the case of variable unbalance. For the LoWash EU project, which aims to build a high performance washer-dryer, such a balancing system has to be prototyped for high spin-drying speeds. The system is based on two trolleys rolling in a ring linked to the drum. A specific solution is developed for deleting the disadvantages at low speed inherent to this kind of system. First design and parametric studies are performed with analytical model and multi-body model. For the final design, the best compromise between performance and industrial requirements is calculated with a complete multi-body model. Two prototypes have been manufactured and implemented in a washer-dryer, and their performance assessed by tests: while the balancing masses behaviour is observed by a swift camera, the residual unbalance is deduced from the signals provided by a set of accelerometers. The results highlight the efficiency of such a device for balancing the fluctuating unbalance of the rotor, but also the relevance of a multi-body model for an optimized design and a good evaluation of the performance.