This article proposes a temperature monitoring method that is able to get the local variation of temperature in several positions of the rotor of an electrical machine during its rotation. The measurement principle uses a structured Polymer Optical Fiber Bragg Grating (POFBG) sensor. The existing methods estimates the temperature based on the external casing or hot spots. The aim of the proposed method is to monitor the temperature of the rotor during its operation in order to detect early thermal aging of electrical machines. The proposed contactless measurement concept and implementation into an academic rotating machine are described. Then, optical modeling of the POFBG is realized and the optical heating system is calibrated. Finally, experimental setup is carried out to make possible the temperature measurements.