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.

Many rotating machineries are supported by bearings with clearance, which are further clamped in, compared to the bearing stiffness, weak pedestals. As have been reported in several studies, it is essential to choose a proper clearance in order to avoid unwanted vibrations. When rotating machineries supported by bearings with clearance are subjected to stationary loads (gravity, magnetic pull etc.), it may not loose contact between the shaft and the bearing (i.e. full contact). However, even in full contact unwanted dynamics can occur which is of interest in this paper. It is found that the clearance give raise to anisotropic pedestal stiffness. It is further shown that some of the resonance frequencies decrease with the clearance.

Operational Modal Analysis (OMA) has lately been a more and more common tool in structural dynamics. The benefits compared to Experimental Modal Analysis (EMA) are that one does not need any artificial excitation and that the structure does not have to be at rest, which is a requirement of EMA. So far, OMA has won terrain for modal analysis of large structures such as buildings and bridges, and for vehicles such as helicopters and ships. For rotating equipment, OMA has not yet been widely used or evaluated. Rotordynamic modes depend on the operating condition of the rotating equipment (e.g. speed). Hence the fundamental requirement of EMA that the testing object should be at rest is not fulfilled. Mapping of critical speeds during start-up and coursing down gives some important operating information, but it will not give all modal information needed to validate a theoretical model or to verify the requirements of damping. The aim of this paper is to evaluate OMA for rotordynamical modal analysis. A numerical model of rotor is used as test object. The response is analyzed with various SSI methods. The results are presented in a Campbell diagram, showing the frequency dependent rotor modes as well as the structural modes. Values for the damping are presented in a root locus plot. These results agree well when comparing the results with a direct naturalvalue solver for the numerical model. It is concluded that OMA can be applied to rotordynamical applications. However, identifying corresponding modes at different speeds can be difficult at field measurements; hence the authors suggest further research and development of Operational Modal Analysis in this specific area.