A novel sensor concept for electrical machines based on flexible printed circuits is proposed. The idea of passive detection coils within the air gap of an electrical machine is picked up and extended by an active excitation and efficient signal processing. It allows for simultaneous measurement of the spatial and temporal distribution of the internal magnetic flux as well as the magnetic reluctance of a rotating machine. A multitude of internal quantities can be derived, such as temperatures in the rotor, eccentricities or centrifugal deformations. The system features high mechanical and electrical robustness and is suitable for installation in common electrical machines. Experimental results are provided to show the sensor concept's practicability.