In this work, some analytical formulas based on experimental tests are presented that can be used to estimate the heating of the crimped connections used in construction of electrical machines, saving the experimenter from additional, long and expensive tests. The formulas are based on the energy balance equation and combine practical results obtained from previous steady-state heating cycles for a set of 6 crimped connections of the same type, at currents close to the nominal value. There are standards that allow the estimation of the heating of the elements in the circuit at small variations of the test current. Thus, based on the heating obtained at one value of the current, the heating at another, slightly different value can be estimated, without the need for the actual experiment. For a more accurate estimation, the variation with temperature of the electrical resistances of the 6 connectors must be taken into account, which allows the extension of the estimation range. In this case, if there are two heating cycles performed at two different values of the current, estimations can also be made at other values, much different from the two. A global temperature coefficient of resistance can be deduced that can simplify the formulas while keeping the precision, serving to estimate the average heating of the connectors. The formulas have been validated for several types of crimped connections used in the construction of electrical machines.

The paper presents the mathematical support and the results of some simulations related to a new method of analyzing the stability of induction motors powered at variable frequency. After a brief introduction to the proposed issue, the mathematical model of the motor, written with relative values, is detailed. Taking this model as a starting point, a Matlab stability analysis program was created. By running this program, a series of graphs were obtained, detailed in the paper (two graphs are represented each, obtained for different conditions, which facilitate their comparison and obtaining conclusions). They refer to four specific cases regarding the modification of static inductance, rotor inductance, stator resistance and rotor resistance. Analyzing these graphs, the necessary conclusions were drawn. The conclusions are important, mainly in the design phase of the machine, to finalize the structure corresponding to the most stable operation of the drive system with asynchronous motor and static voltage and frequency converter. They highlight the influences of the most important parameters of the motor on the stability of the functioning of systems with such a structure. The paper ends with a representative bibliography for the proposed study.

This paper presents some experimental aspects related to the operation of asynchronous motors powered at variable voltage. A modern system for actuation of low-power asynchronous motors is presented, equipped with a voltage and frequency converter. Also, a data acquisition system is presented, with a sampling frequency of up to 100 kHz, supervised by an acquisition program that allows the visualization of the variations of the main characteristic sizes of the motor. The program contains both graphic interfaces developed by the manufacturer and by the authors of the paper. This program solves, among other things, an electromagnetic compatibility problem. The experimental tests carried out with the help of this system (in permanent non-sinusoidal regime at various power supply frequencies) are detailed. Technical details of the main system components are provided. The tests are accompanied by photos taken during the experiments and many original explanatory graphics. The authors’ contributions are mainly in the software area. Thus, in the original program, a series of windows for harmonic analysis and visualization of the characteristic phasors of the analyzed electrical quantities were implemented. The paper ends with the main conclusions resulting from the completion of the study and with a representative bibliography.