Main requirements on an electrical traction drive in passenger cars are a high efficiency, a high power density with respect to size as well as to weight and a high robustness. Synchronous machines with interior permanent magnets made of NdFeB fulfill all these criteria and consequently they are applied oftentimes especially in hybrid electrical vehicles. Due to their brushless technology their robustness is mainly limited by the temperature resistance capacity of the windings and the magnets. Especially the temperature of the permanent magnets has to be considered as critical, as over temperature of the magnets may lead to permanent failure of the machine. In addition the characteristics of the machine with respect to the generated torque varies with the permanent magnet temperature, requiring the operating point control to consider the temperature in order to ensure a precise torque generation over the whole operating range of the drive. This work deals with designing and dimensioning of an observer for the temperature of the permanent magnets located on the rotor. This observer is based mainly on electrical measures of the machine. On the one hand it is characterized by a time discretization scheme, which is numerically stable in the whole speed range of the machine. On the other hand it is able to handle saturation effects which often can be found in highly utilized machines. For this purpose characteristic curves are applied inside the observer, which are derived previously by a machine characterization measurement. Furthermore, during the development of the observer a method for measuring the voltage on the terminals of the machine based on modulation has been developed and used for operating the observer. In comparison to model based approaches this method turns out to yield good results, yet being faster and more flexible to be put into use.