The overall performance of an electric machine is closely linked to the thermal design of the machine and there is a trend to include more thermal analysis into the machine design procedure in academic literature. Thus, there is also a motivation to prototype experimental machines for accurate validations of the thermal analysis. One of the challenges is to measure rotor temperatures online while the machine is in operation. This paper presents some of the experience from designing an online temperature monitoring system for an experimental IPMSM. In total, 32 sensors was successfully placed inside the 3.4 liter volume of the small IPMSM and temperature data was transmitted from the rotor in the end of the shaft, using infrared light. It was found that the online temperature monitoring system was very reliable. Both the used analog and digital sensors exhibit equal results and EMI immunity, when placed inside the electromagnetically noisy environment inside the machine.