In the following section, the test results of Experimental Campaign I described in Sec-tion 2.2 are presented. The experimental results were used to validate Thermal Model II described in Section3.2. As it was mentioned, at this stage of the study, only thermal param-eters were presented and compared between the developed model and experiment data.
As the first results of the validated Thermal Model II, the temperature for the motor load denoted as Operating Point #1 (Tab. 2.2) is presented. The temperature results recorded during the experiments by the calibrated thermocouples and obtained from the numerical model are presented in Fig. 4.26 in six variants from Variant A to Variant E and Variant E+D. The mentioned variants were described in Section 1.4. The thermocouple positions were numbered on the x-axis in Fig. 4.26to be consistent with the tags presented in Fig.
2.4and their positions discussed in Section2.2. In Fig. 4.26, the CFD results were marked using the cross signs and the experimental results were presented using dots. Black colour corresponds to Variant A, the blue colour refers to Variant B, red colour denotes Variant C, green colour represents Variant D. In this figure, the sensor numbering for Variants from A to D is consistent with that presented in Fig. 2.4described in Section2.2where the tags
"T" are used. The last two configurations represent Variants E and Variants E+D and are expressed by orange and green colours, respectively. Their tags were introduced as "F" in Section2.2and their locations were presented in Fig. 2.4. As it was mentioned in Chapter2 describing the experimental part of the study, the thermocouples were calibrated to achieve the accuracy within the range of ± 0.3 K. Therefore, as in the previous description of the temperature field, the error bars range would be smaller than the presented points.
The temperatures presented in Fig. 4.26clearly show that the highest values were ob-served at all the positions on the motor windings. These hot-spot locations are consistent with all the investigated variants. The proposed dissipation intensifications comparing the original state from Variant A to Variant E are characterised by a lower value of the hot-spot temperature. For the reference variant (Variant A), the average temperature of the windings measured in four points reached approx. 95◦C, while the ambient temperature was mea-sured at the level of 26.2◦C. Therefore, for the reference Variant A, the hot-spot temperature rise of 68.8 K above the ambient temperature was noted.
For Variant B that represented the motor modification based on the higher emissivity layer covering all the external motor walls and its fixings, the average temperature of the hot-spot reached the 64.9 K rise above the ambient temperature. For this variant, the ambient
EXP, Variant A EXP, Variant B EXP, Variant C EXP, Variant D EXP, Variant E EXP, Variant E+D CFD, Variant A CFD, Variant B CFD, Variant C CFD, Variant D CFD, Variant E CFD, Variant E+D Temperature, °C
Figure 4.26: Temperature in◦C measured during the experiments by specific thermocouples (dots) and the corresponding values taken from the numerical models (crosses) for different variants at Operating Point 1
temperature was at the level of 27.8◦C. Comparing the results for Variant A and Variant B, it is worth noticing that the ambient temperature was different. Therefore, the motor temperature field between these two cases is greater when comparing only the temperature rise above the ambient temperature.
The next analysed (Variant C) represented the application of the smaller radiator on the motor housing. This configuration allowed for the temperature rise above the ambient tem-perature of the hot-spot at the level of 59.5 K, while the room temtem-perature was 27◦C.
The application of the bigger radiators in Variant D allowed for the temperature reduc-tion at the hot-spot to the level of 52.7 K above the ambient temperature. The results of the winding temperature obtained from the numerical model for the described variants from Variant A to Variant D were consistent with the measured values. However, the maximum difference between the model and experimental results reached 2.8 K in the hot-spot loca-tion. That was observed in Variant A for the thermocouple #T4.
The presented results for the thermal filler application represented by Variant E and Vari-ant E+D show higher inconsistencies between the performed experiment and the numerical model results. In these cases, the hot-spot temperature occurring on the windings can be expressed as the averaged value of the thermocouples from #F2 to #F5 and #F7. This aver-age was 50.3 K above the ambient temperature for Variant E and 39 K above the ambient temperature for Variant E+D. Therefore, the combined Variant E+D turned out to be the most effective one and allowed for the hotspot temperature reduction of 29.8 K when com-pared to the reference case (Variant A). For these two variants, the temperature from the numerical model was overestimated. Namely, the maximum difference of the hot-spot tem-perature comparing the numerical model results to the experiment data reached 8 K. The higher inconsistencies for these variants can be caused by, e.g., the influence of the thermal filler on the internal windings structure which was not investigated numerically. The inter-nal winding structure could be changed by penetration of the thermal filler. In this manner, effective thermal conductivity of the windings could be increased. Therefore, it could be the reason that the experimental results show lower temperature than those calculated numeri-cally. Moreover, the potting material affected the reduction of the thermal contact resistance between windings and bobbin and housing. However, the consistency of the numerical re-sults with the experimental records was very satisfactory.
Besides the hot-spot temperature presented in the previous paragraph that is crucial in the thermal analysis, the remaining thermocouple recordings were compared to the model results in the specific locations. Within the motor, the locations that presented the highest temperature after the windings were in the area between the windings of the neighbouring phases represented by Point #T5.
The lower values were temperature recorded in the space between the windings and the plastic core cover and temperature from the thermocouple fixed to the stator tooth end de-noted as #T6 and #T7, respectively.
After the passive cooling modification including the thermal filler, it is visible that wind-ing surface connectwind-ing the plastic core cover (bobbin) represented by Point #F6 is at a sim-ilar level as the surface connecting windings with thermal filler (Points from #T9 and #F1).
Therefore, one can notice that the temperature of the windings cross-section is more uni-form after the thermal filler application.
The lower temperature than those described earlier was recorded and modelled in the space between core and housing #T9 and #F1. This temperature was slightly higher than that of the housing temperature (#T8, #T11 – #T16 and #F12). In this case, the thermocouple
probably touched the internal wall of the housing. The analogous point was found in the numerical domain and confirmed this value.
The next lower temperature level was represented by the points located on the motor housing (#T8, #T11 – #T16 and #F12). The specific heat transfer intensifications based on the lowering motor housing temperature also influenced the winding temperature according to the presented results of the hot-spot.
The lowest measured temperature in the motor region was recorded by thermocouples located on the motor fixings. The numerical simulations presented the same tendency. How-ever, the consistency of the simulation results at these points when compared to the exper-imental recordings was worse than at the other points. In the passive cooling modifications excluding the thermal filler application, the maximum difference between the experimental and model results reported by thermocouple from the rear motor fixing achieved 3.5 K. It was noticed for Variant A. Higher inconsistencies reaching 8 K were reported for the cases with the thermal filler.
The lowest presented temperatures were recorded during the experimental study for the ambient (room) temperature. For each case, those temperatures were used as a boundary condition in the model described in Section 3.2. All the presented absolute temperatures should be referred to the ambient temperature to compare the specific variants with each other. The presented results can be treated as representative for other operating points at the lower temperature field.
The results of the numerical and experimental investigation at Operating Point 4 are pre-sented in Fig. 4.27. The tendency is similar as in the previously described operating point.
However, the temperature field is lower. As in the previously discussed operating point, the highest temperature occurred in the winding region. This hotspot was lowered by introduc-ing the next variants of the passive coolintroduc-ing improvements. However, at Operatintroduc-ing Point 4 characterised by a lower temperature than Operating Point 1, the winding temperature re-duction above the ambient level is lower than in the previous description.
It is also worth adding that in the results of Experimental Campaign II presented in the current section, the non-uniformity of the recorded measurements in the winding region is smaller than that reported in Experimental Campain I description. Therefore, the non-uniformity of the temperature, presented in Section4.1.1, resulting from the experimental part of the research could be caused by the thermocouple unideal sticking to the measur-ing surface. Moreover, the reasons for the presented difference between the results of the
EXP, Variant A EXP, Variant B EXP, Variant C EXP, Variant D EXP, Variant E EXP, Variant E+D CFD, Variant A CFD, Variant B CFD, Variant C CFD, Variant D CFD, Variant E CFD, Variant E+D Temperature, °C
Figure 4.27: Temperature in◦C measured during the experiments by specific thermocouples (dots) and the corresponding values taken from the numerical models (crosses) for different variants at Operating Point 4
measurements and those of the developed models can also be caused by a definition of ther-mal contact resistances, the homogeneous character of the losses implemented in selected regions and finally the errors in the losses estimation caused by the uncertainty of the mea-suring equipment and the meamea-suring procedure. Moreover, it is worth mentioning that heat sources in the numerical model were indirectly calculated from the set of measurements, while the iron losses were calculated on the basis of the motor characteristics created during the set of the idle state measurements implemented in the previously described Experimen-tal Campaign I.
The selected infrared thermal images recorded during the experiment and the results of the numerical model for the variants from Variant A to Variants D are presented in Fig.4.28.
The thermal images were recorded for the same motor load denoted as Operating Point #1 in Tab.2.2which was close to the rated load. In that case, the motor was powered by the source with the voltage of 24.44 V. The thermal images were taken with the surface emissivity of 0.95.
The same temperature range was applied to all the analysed cases in the presented figure for
the thermal images and the numerical results. All the proposed heat transfer intensifications allowed for the temperature reduction of the external motor walls and, in consequence, the temperature reduction of the motor windings. The most effective solution comparing in-tensifications focusing on the outside parts of the motor was the one with the maximum external surfaces development to reduce the thermal resistance between these walls and the ambient temperature. Analysing the cases from Variant A to D, the temperature of the exter-nal motor walls was lowered from 72◦C to approx. 50◦C. The presented temperature fields in Fig.4.28include the selected temperature points presented in Fig.4.26. The inconsistencies between the presented thermal images and model results can be caused by reflections (visi-ble especially in Variant A in the region of the bracket), the non-uniform emissivity of all the motor components and also inconsistencies presented in the previous discussion.
a
b
c
d
Temp, °C . .
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Figure 4.28: Temperature in◦C of the external motor walls in a form of the infrared photos (left) and the CFD model results (right) in four variants: (a) original test rig before modifi-cations (Variant A), (b) external surfaces covered by graphite (Variant B), (c) radiator with smaller fins (Variant C) and (d) radiator with bigger fins (Variant D)