Thermal analysis of asynchronous machines under intermittent loading

Z. Vondrášek, V. Ryženko*, M. Linda

Czech University of Life Sciences Prague, Faculty of Engineering, Kamýcká 129, CZ165 21 Prague, Czech Republic
*Correspondence: ryzhenko@tf.czu.cz

Abstract:

The operation of electric machines is accompanied by losses which are mostly converted to heat. The heat needs to be dissipated from the machine. With a properly dimensioned motor, the arising heat is balanced with dissipated one. After the motor is started at ambient temperature, all functional parts of the machine are gradually warmed until stabilized. Any overloading of the machine leads to stabilization at temperatures higher than expected by the designers. High temperatures in the machine could cause a crash by damaging an insulation. In case of machines with permanent magnets, the temperature affects their magnetic properties and can leads to demagnetization at the Curie temperature. Therefore, the measuring of temperature is so important for verifying the allowed warming of the motor. Contact and noncontact methods could be used for temperature measuring. Thermal warming and temperature distribution in an electric machine can be also determined by theoretical calculations based, for example, on the finite element method. This method is used by a number of computer software such as Ansys. The article deals with generation and propagation of heat in electric motors and with measuring of warming characteristics with a variable value of a load factor for intermittent periodic loading of asynchronous machine. The loading is carried out by the dynamometer. The temperature measurement is implemented by temperature sensors which are located on the stator winding of the asynchronous motor and are in operation for the whole time the motor is loaded.

Key words:

electric machine, load factor, loading, losses, warming

The estimation methods of oilseed rape harvesting losses

R. Domeika, A. Jasinskas, D. Steponavičius, E. Vaiciukevičius and V. Butkus

Department of Agricultural Machinery, Lithuanian University of Agriculture, Studentų 11, LT-53361 Kaunas, Lithuania; e-mail: Rolandas.Domeika@lzuu.lt

Abstract:

Oilseed rape harvesting losses, which occur during cutting, separation and cleaning and shaking, reach 5–10%; cutting and separation processes account for 80–90% of the total harvesting losses. A special test stand was prepared for the research of oilseed rape cutting and separation losses. It was established that the active twin-blade knife separator and the passive triangular separator on the header of a harvester have influence on the separation losses in 0.5 m on both sides of the separator motion line. Separation losses using the active twin-blade knife separator were twice less than using the passive triangular separator. The optimal active twin- blade knife separator moving speed is 5 km h-1. The analysis of the research results has revealed that traditional estimation methods of oilseed rape harvesting losses are not correct and it is necessary to use a 0.1×0.1 m wire frame for the estimation of cutting losses and a 0.1×0.5 m wire frame for the estimation of separation losses.

Key words: