Power semiconductors, or transistors, constitute the core part of adjustable speed drives, which are commonly used in motor drive applications. Overheat of the semiconductors can compromise their behavior and, eventually, shorten the lifespan of the whole system. Therefore, the thermal management of transistors is crucial both at the design stage and during operation. Commonly performed thermal simulations normally rely on transistor thermal resistance. However, this approach does not account for the thermal behavior in transient regime. Furthermore, it is inappropriate for intermittent applications, such as for drilling machines, where the motor is on and off in repetitive working cycles, and the transistors never reach the equilibrium temperature. The transient thermal behavior is described by the concept of transfer thermal impedance. The transfer junction-to-case thermal impedance is given in the datasheet and assumes a constant ambient temperature; an assumption that is, however, not true in real applications. In this paper, we overcome this main limitation by using a resistive sensor. We consider MOSFETs in a 3-phase inverter, and model their thermal behavior with a well-known algorithm that uses the junction-to-case and junction-to-ambient thermal impedances, along with application dependent parameters. The actual rise of ambient temperature of the circuit board is included in the algorithm by virtue of the resistive sensor. The method has been validated with lab measurements, for two different MOSFETs. The proposed method can be used both at the design stage and during operation of the motor drive. Future works will include refinements of the power loss formulas, of the junction-to-ambient impedance modeling, as well as aging effects of the transistors.