Thermally Induced Mechanical Stress in the Stator Windings of Electrical Machines

Abstract

The lifetime of an electrical machine mainly depends on the thermal overloading. Modern day applications of electrical machines on one hand require compact machines with high power density, while on the other hand force electrical machines to undergo frequent temperature cycling. Until recently, in the case of electrical machines, the main factor related to the degradation of the winding insulation was thought to be the thermal oxidization of the insulation materials. It has now been revealed that thermal overloading can also induce mechanical stress in the windings of electrical machines, which over time could lead to fatigue and degradation. In this paper, a comprehensive study of the thermally induced mechanical stress in the windings of an electrical machine is presented. The study is performed using combined thermo-mechanical models. The numerical results are validated by experiments on a segmented stator winding set-up.Energies 2018, 11, 2113 2 of 18 from the stress caused by the thermal expansion mismatch is presented in Reference [8]. According to Reference [9], the mismatch of the thermal expansion coefficient can induce mechanical stress in electronic assemblies not only by the temperature cycling during normal operation, but also due to the high temperatures experienced during fabrication, shipping and storage. The failure of copper through silicon vias due to the thermally induced mechanical stress has been presented in References [10,11]. Mechanical deformations in electric interconnects due to thermal expansion mismatch has been reported in References [12,13]. The effects of the thermally induced mechanical stresses on light emitting diodes, plastic ball grid arrays and field-programmable gate arrays are studied and discussed in References [14][15][16].Similar studies on the effect of thermally induced stress on different composite and cement-based materials have also been reported in literature. In Reference [17], the authors analyzed thermal stresses in two phase composites both numerically and analytically. The modeling of the degradation and failures of IGBT modules considering the electrical, thermal and mechanical stresses has been studied in Reference [18][19][20]. It has been found that the bond wire/substrate interface is the most vulnerable as it is affected the most by the thermally induced mechanical stress.Although the thermally induced mechanical stress has been widely studied for electronics packages, power electronics and other fields, very little work was found for an electric motor. The development of shear stresses between winding components due to thermal cycling has been listed as one of the ageing mechanisms of the stationary armature winding in electrical machines in IEEE Standard-56 (2016) [21]. Nevertheless, very few publications dealing with research on the topic were found. One reason could be that such stress would be insignificant if the windings of the machine are not epoxy impregnated, because the coefficients of thermal expansion of copper and coating are similar. How...

Assessment of Different Cooling Techniques for Reduced Mechanical Stress in the Windings of Electrical Machines

“…Although a brief study about the possible failure due to the difference in the thermal expansion ratios in electric machines was first presented in [6], detailed studies were not published until recently [7,8]. In our recent study on a similar topic [9], very high stresses were seen in the epoxy and also in the copper wire. The effect of the copper fill factor, thermal expansion coefficient of the epoxy and local temperature on the magnitude of the stress was also studied.…”

“…One reason for this is that the measured temperature is about 6 • C higher than the simulated temperature in the steady state. The dependence of the stress with the temperature has already been validated in our previous study with both simulations and measurement [9]. The strain gauge sensor installed in the set-up can measure stress in three directions in a-at 0°, 45° and 90° with respect to the axis where the sensor is installed.…”

Simulations in a Digital Twin of an Electrical Machine

“…[CG17]. In [SS18], where also the coupling to the thermal field is considered, the authors use the software COMSOL Multiphysics to investigate the stresses caused by thermal effects. In some works, open source software packages are employed, as for instance Elmer which is used in [SBK + 16].…”

Developing New Thermal Protection Method for AC Electric Motors

“…In addition, heat exposure dries out the insulation, which leads to cracks and chipping of the insulating layer and, accordingly, to electrical breakdown. At the same time, thermal overload can also cause mechanical stress in AM windings, which may lead to fatigue and deterioration of their characteristics over time [1].…”