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Rotor temperature is of concern in both short-term machine protection and in longer term condition monitoring systems for large induction machines. Unduly large temperatures can arise under severe operating conditions such as stall and overload. A cage rotor condition monitoring system using direct temperature measurement is described. Under running conditions, it continuously transmits temperature information about strategic points on the rotor via an optical link. Possible rotor circuit imperfections are broken bars and end rings, single or multiple, and the deterioration of bar/end ring joints. The possibility of using temperature information for the identification of these faults is also investigated.