Energy storage systems based on pumped hydro storage, compressed air (CAES) and flywheels require electric machines working both as motors and generators. Each energy storage system has specific requirements leading to a variety of electric machine topologies. Hydro power and CAES stations have several configurations; they may have a turbinegenerator and an independent pump-motor group or a common turbine-motor/generator assembly, but in both cases the electric machines are coupled to turbines that are operated at constant speed and low electric frequency at steady state. Synchronous machines are the predominant technology for these applications.