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This paper proposes a novel online condition monitoring system of dc-link metallized polypropylene capacitors in fault-tolerant drives for aerospace applications. The estimation technique makes use of voltage and current sensors which are already in place for protection and control purposes. The novel aspect of the proposed technique relates to monitoring capacitors in real time while the motor is operational. No external interferences, such as injected signals or special operation of the drive, are required. The condition monitoring system is independent of torque and speed and hence independent of a variation in the load. The condition monitoring system is validated using manual calculations, simulation, low-voltage experimentation, and high-voltage implementation on an aerospace drive.