Fault Tolerance Analysis of a Ironless PM Machine for Energy Storage

Energy storage is an emerging technology that can enable the transition towards renewable-energy-based distributed generation, reducing peak power demand and the time difference between production and use.Energy storage can be implemented both at grid level or at user level and nowadays different technologies are available for this purpose. Chemical batteries represent the de facto standard of storage systems for performance and maturity, however, chemical batteries feature a quite large environmental footprint and use precious raw materials.Mechanical storage technologies could be a viable alternative, because of their reduced impacts on the environment and on the exploitation of raw materials. Mechanical storage technologies require a motor/generator as an interface between the flywheel, the grid and a renewable energy source. In this paper a custom machine based on an ironless surface permanent-magnet structure is compared with iron core structures, with special reference to fault tolerance. In fact, the electrical machine shall work either as a motor or as a generator and it is the enabler for some critical specifications: high-discharge duration and compatible size/dimension. For this application, high rejection of faults is an important option, that requires an efficient fault detection system and a fault tolerant configuration. The proposed solution is based on the ironless dual-rotor surface permanent-magnet machine that is perfectly suited for these specifications. The ironless dual-rotor configuration features two main advantages: (1) the phases are magnetically and electrically isolated; (2) reactive components are much lower than resistive components. In addition, the ironless machine topology is less sensitive to unbalanced air gap radial forces that arise during fault operations due to a non symmetrical air gap magnetic field. These forces would strongly affect magnetic or air bearings, that are a mandatory option to reduce mechanical losses and to increase discharge duration. Hence, the proposed ironless structure is the best suited for the energy storage application.In this paper three synchronous electrical machines are modeled and compared via finite element analysis and a prototype of the dual-rotor ironless machines was realized and tested.