Thermo-mechanical fatigue of electric machine windings

Transcription

1 Thermo-mechanical fatigue of electric machine windings Project manager: Oskar Wallmark Researchers: Oskar Wallmark, KTH Royal Institute of Technology, Project manager Giovanni Zanuso, KTH Royal Institute of Technology, Researcher/PhD student Jörgen Engström, Scania AB, Researcher Ahmed Elschich, Karlstad University, Researcher/MSc thesis student Partners: KTH Royal Institute of Technology and Scania AB Cash funding: 300 ksek Funding organization: Swedish Electromobility Centre Duration: Swedish Electromobility Centre

2 Sammanfattning Att prediktera hälsotillståndet för en elmaskinlindning i en transient applikation såsom en elbilsdrift är idag en utmaning. I detta projekt har den termomekaniska påverkan på en elmaskinlinding studerats genom framtagningen av relaterad termo-mekaniska modeller i ett programpaket baserat på finita elementmetoden. Från dessa resultat kan det konstanteras att den termiskt inducerade stressen som spårisolationen utsätts för är tillräckligt hög för att utmatta ( low cycle fatigue ) isolationssystemet vilket verifierar att termomekanisk utmattning är en faktor som påverkar åldrandet hos isolationssystemet. Vidare har experiment utförts vid Scania AB av personal från KTH med avsikt att identifiera frekvensinnehållet hos de transienter i fasströmmarna som är resulterade av den kraftelektroniska switchiningen. Ett viktigt resultat från dessa mätningar är att den resulterande ringningen vid en switchning är i storleksordeningen en, eller ett fåtal MHz. Detta är inom eller nära den banbredd som kan uppnås med moderna strömgivare baserade på den magnetoresistiva effekten. Den begränsade skillnad som strömsvaret som uppmättes vid två olika mätningar på samma elmaskin kan idag inte förklaras då elmaskinen i dagsläget inte har genomgått någon vidare analys (exempelvis en post-mortem analys). Sammantaget motiverar dock de framtagna resultaten fortsatt arbete för att undersöka möjligheten att implementera ett system för online karakterisering av maskinlindningens hälsotillstånd. Fortsatt arbete finansierat av Vinnova har inletts vid KTH för att göra just detta. Summary Failure of the winding insulation system in intermittent applications such as the traction of an electric or hybrid electric vehicle is difficult to predict. In this project, the aging of electric machine winding insulation systems for intermittent applications (including automotive applications) has been studied by developing thermo-mechanical models of the winding insulation system implemented in a finite-element based software package. From the results, it can be concluded that the thermally induced stresses exposing the slot insulation are high enough to low cycle fatigue the electrical insulation system, thus thermo-mechanical fatigue is an ageing factor of the electrical insulation system. Further, experiments carried out at Scania AB by personnel from KTH have been carried out with the purpose of identifying the frequency content of the current ringing during the switching of the power electronics. An important result from these experimental tests is the confirmation that the ringing during a switching transition is in the range of one or a few MHz. This is within or close to the bandwidths that modern, magnetoresistive-based current sensors are specified at. The minor observed change in the peak value of the stator current ringing during the two last experimental tests carried out cannot be explained at this stage since the machine windings have not undergone any additional characterization (e.g., a post-mortem analysis). Page 2 (9)

3 However, the obtained results motivate further studies aimed at implementing a system to investigate the feasibility of an on-line characterization of the state of a machine winding. Continued work funded by Vinnova has been initiated at KTH with the aim of investigating this potential. Background To be able to predict the expected life time of a component is important. In order to do that the following must be known: 1) The ageing factors in the actual operation of the component. 2) The ageing process of each factor. For Scania, 1) is found through either from physical tests or from simulations. The ageing factors depend of course on the type of component. For electric machines, the main factors are: a) Ageing caused by temperature (of the winding and insulation system). b) Ageing caused by thermal cycling. Factor a) is well known and is modelled by a material degradation by temperature and is considered to follow Arrhenius equation which is well established in the electrical engineering industry. For thermo-mechanical ageing/wear on electric machines there is the IEC TS publication which aims for form-wound high voltage machines. Application to low voltage machines (<1000 V) is seldom seen in literature. An electric machine in a heavy vehicle powertrain has a duty that is strongly intermittent. This means that the thermal cycling is more pronounced compared to normal industry grade machines. It also means that the ageing due to thermal cycling could be more important and therefore it is necessary to investigate this. General project description The overall goal of this work has been to determine whether thermomechanical wear of machine windings used in an automotive application is an ageing factor or not. The milestones/scope of the work and to the extent milestones have been reached are briefly discussed below. Literature study on previous work. o A literature study on common lifetime-evaluation models and failure mechanism was presented in [1]. Adapt existing methods in structural strength theory and apply them to the varying thermal load of the electric machine. The load cycles in structural mechanics can be assumed to be very similar to the thermal load cycles in electric machines and therefore the same models should apply. o The models developed in [1] needs to be accompanied with additional experimental results for this milestone to be fully reached. Page 3 (9)

4 Simulation with a thermo-mechanical model of a core-winding object. This will help understanding what the important parameters are and will thus give input to both evaluation of test set-up and results. o This was carried out in [1] (see Fig. 1 for a sample result from the model developed). Figure 1: Developed model and computed von Mises stresses Analyze measured data from vehicle. o Experimental data from an actual vehicle was not carried out within the project. However, experimental data from an electric machine in Scania s laboratory was gathered with the purpose of analyzing the high-frequency current transients during the switching of the power electronics. An important goal with these measurements was to determine the frequency range of the current transients in order to determine suitable specifications for a future system for on-line characterization of the state-of-health of the winding system. Results from these experiments are described briefly below. The collaboration between KTH and Scania allowed PhD student Giovanni Zanuso to perform measurements at the Scania Technical Centre in Södertälje during three different dates. During the first time, on July 13th 2017, the developed measurement setup (Fig. 2) brought from KTH was successfully tested on Scania s laboratory test bed. Figure 2: Measurement setup at Scania Technical Centre. Furthermore the presence of the ringing phenomenon in the stator current was attested, as it can be seen in Fig. 3. Page 4 (9)

5 Figure 3: Ringing phenomenon presence in the phase a stator current after the commutation of the upper IGBT leg. Different colours represent different measurements with various values of the stator current. A first important conclusion was drawn from such measurements: the ringing phenomenon depends on whether the stator current is positive or negative during the commutation, as shown in Fig. 3. This can be clearly observed in the FFT of the measurements, shown in Fig. 4. Figure 4: FFT of the stator current measurements displayed in Figure 1. Different colours represent different measurements with various values of the stator current. Fig. 4 confirms also the presence of two peaks in the current spectrum, which previously has been reported in the literature. On October 10th 2017, a second session of measurements was performed at the Scania Technical Centre. The aim was to measure all the phases of the electric machine and determine a reference measurement data set. After this date, longrun tests were performed in the same electric machine with the goal of ageing it. On January 18th 2018, a third session of measurements was performed aiming to observe a possible change of the ringing phenomenon due to electric machine ageing. The amplitude of the first peak of the ringing phenomenon in Fig. 3 was chosen as a comparison index between the second and the third measurement sessions. Fig. 5 shows that the peak values obtained during the second and the third measurement sessions do not significantly differ except Page 5 (9)

6 for a small variation for positive currents. A possible consequence is that the windings were not enough aged at that time. Interestingly, after the third session, the machine failed during its long-run tests. The cause of the failure has not been yet investigated Figure 5: Peak value of the stator current ringing with different values of stator current after the commutation of the upper IGBT leg. Black: measured during the second session. Red: measured during the third session. Results The results from the finite element analysis presented in [1] analysis indicate that the mechanical stresses induced will affect the lifetime of the electrical insulation system. Further, the results from the parameter sensitivity study performed in [1] indicate that the stator core length is too short to have a significant effect on the thermo-mechanical stresses induced. However, the variation of the temperature cycle rate and the temperature cycle amplitude increase the induced thermo-mechanical stresses. The thermally induced stresses exposing the slot insulation exceeds the yield strength of the material, therefore plastic deformation may occur only after one thermal cycle. The other components in the stator are exposed to stresses below the yield strength. The thermally induced stresses exposing the slot insulation are high enough to low cycle fatigue the electrical insulation system, thus thermo-mechanical fatigue is an ageing factor of the electrical insulation system. An important result from the experimental tests carried out at Scania Technical Centre is the confirmation that the ringing during a switching transition is in the range of one or a few MHz. This is within or close to the bandwidths that modern, magnetoresistive-based current sensors are specified at. The minor observed change in the peak value of the stator current ringing during the two last experimental tests carried out cannot be explained at this stage since the machine windings have not undergone any additional characterization (e.g., a post-mortem analysis). However, the obtained results Page 6 (9)

7 motivate further studies aimed at implementing a system to investigate the feasibility of an on-line characterization of the state of a machine winding. Utilization of results Apart from the published MSc thesis [1], the project results have been presented at the two following workshops organized by SEC: E-mobility centre day ( ). Thematic workshop on automotive electric drives ( ). Further, the sample results from the experimental tests carried out at Scania will be included in an accepted paper to be presented at the 2018 International Conference on Electrical Machines and Systems (ICEMS 18). The results gathered in this project, together with the results from the Vinnova funded project Smarta strömgivare för nätverk av elektriska drivsystem (which the project leader of this SEC project also was project leader for) have been used as a basis for the project Självpredicerande och kommunicerande elektriska drivsystem för optimering av industriella processer which recently was granted funding by Vinnova, with Viktoria Fodor, Professor at KTH in communication networks as project leader and ABB AB, Imagimob AB, and LumenRadio AB as project partners (Oskar Wallmark and Giovanni Zanuso are also taking part in this project). Hence, continued activities aimed at realizing goals including on-line characterization of the state-of-health of the winding insulation system are ongoing. Targets Predicting the aging of winding insulation systems used in electric machinery operated in intermittent load cycles is an important goal in order to simplify the introduction of electric drivetrains in the transportation sector. Hence, this project is of relevance to the actors within the SEC. The project s context is within the field of automotive electric machinery which has been identified as part of the roadmap for SEC-Theme 2 (electrical machines and drives). The project has not been evaluated by any external actor but the targets, and the extent that they have been reached are summarized in the General project description section above. It is the opinion of the project leader that the targets set up have been reached at least to a sufficient level. Industry contribution Jörgen Engström at Scania AB has provided the necessary information for Ahmed Elschich to carry out and complete his MSc thesis situated at Scania AB. Further, Jörgen has enabled access for Giovanni Zanuso at KTH to carry out the experimental tests in Scania AB s laboratory. Page 7 (9)

8 Collaboration Parts of this project have been carried out in collaboration with the closely related sister project Smarta strömgivare för nätverk av elektriska drivsystem which was funded by Vinnova and also with Oskar Wallmark as project leader. Dissemination of knowledge See the Utilization of results section. Miscellaneous The time period of this project was extended during autumn Papers and publications [1] Ahmed Elschich, Thermo-mechanical fatigue of electrical insulation system in electrical machines, MSc thesis, Karlstad University, Page 8 (9)

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