These are devices which are based on electronic semiconductor units which operate in a switching operation, i.e. only in an on-state or off-state. In variable three-phase drive systems, they have the specific task of generating (usually) three-phase voltage with a variable frequency and voltage amplitude for feeding the AC motor from single-phase or three-phase mains AC voltage with a constant frequency and amplitude. Previously, networks of different frequencies were coupled via rotating frequency convertors. However, in convertor stations for traction power, mechanical systems are still used.