Temperature field inside and outside the motor housing

The temperature field of the load described as Operating Point V in Tab.2.1is presented in Fig. 4.1in the middle vertical cross-section of the computational domain crossing the shaft axis. In that figure, the motor is on the left-hand side and the generator is on the right-hand side. As it can be seen, the highest temperature, approximately 78^◦C, occurred in the winding region. The cross-section does not cut the model directly through the

wind-ings. However, the visible hot-spot temperatures are noticeable in the air being in the near-est vicinity of the windings. Additionally, the highnear-est temperature can also be observed in the cooper paths located on the PCB plate. The external shape of the machines is visible in the presented temperature field because the surrounding air temperature was notably lower.

Moreover, it is observed that the air temperature in this view was higher directly above the end caps of both machines. The highest air temperature outside the motor occurred above the back-end cap of the generator. The remaining temperature field located 0.1 m above the machines was practically uniform. Therefore, only a lower half of the analysed cross-section through the domain was presented. The temperature field can be treated as the representa-tive case for all the investigated operating points. The highest temperature was always ob-served in the windings vicinity and in the cooper paths region. The temperature was lower in the components located closer to the surrounding air under the acrylic glass cover and the lowest temperature of the solid components was observed in the region of the motor brack-ets and coupling. In the fluid zone, the lowest temperature in the numerical domain was noticed at the boundary condition location of the constant inlet pressure.

GENERATOR

Figure 4.1: Temperature field in^◦C displayed in the domain cross-section through the motor axis at the load of 3 resistors and the rotational speed of 3500 rpm (Operating Point V)

The temperature field obtained for the operating point being the closest to the rated power is presented in Fig.4.2. This operating point, namely Operating Point II, is also charac-terised by the highest temperatures that occurred in the numerical domain and was recorded

during the experiment. In the presented figure, the temperature field in the horizontal and vertical cross-sections is presented. Moreover, the temperature field on the external surfaces of the windings and on the PCB plate is illustrated. One can see that the highest temperature occurs on the external surface of windings being in contact with the air within the motor. The same temperature occurs in the copper path located on the PCB plate. The heat generated in these regions is caused by the current flow through the copper conductors. The described temperature field presented in Fig. 4.2can be treated as the representative for all the anal-ysed operating points. However, for the other operating points, the temperature values are lower.

Figure 4.2: Temperature field in^◦C displayed in the domain cross-section through the motor axes and windings surfaces at the load of 4 resistors and the rotational speed of 3500 rpm (Operating Point II)

A comparison of the temperature results in specific locations obtained using CFD and ex-perimental measurements is presented in the following part of this section. The results col-lected in the specific locations are presented in four groups, which are separated by the dot-ted green lines. Those thermocouples locations marked as red coloured points are schemat-ically shown in Fig.2.3of Section2.1:

• inside at the front part of the motor: points #1-#7,

• inside the motor and in the rear part of the motor: points #8-#11,

• on the external part of the motor housing: points #12-#19,

• air above the motor: points #20-#24.

Moreover, as it was mentioned in Section2.1, the errors of the temperature measurement were estimated at the level of ± 0.3 K after the calibration procedure. Therefore, the error bars of the measured temperature were not introduced in the following results because they will be lower than the points indicating the temperature values.

In Fig. 4.3, the results obtained at the highest load with the highest rotational speed of the motor are presented. Therefore, the presented Operating Point II was characterised by the highest investigated temperature occurring in these work conditions. In that figure, the thermocouples fixed on the windings recorded the highest temperature values between 90 and 96^◦C. Thermocouple #5 measured the temperature at the end of the rotor tooth.

This point in the model is characterised by the lower temperature at the level of 80^◦C. The last thermocouple in the first group shows a value of 80^◦C, while in the numerical model, it reaches approx. 72^◦C. The experimental data, in the first group of thermocouples, located on the motor windings, show the temperature that varies within the range of 6 K. The same points in the CFD results shows more uniformly values at 97^◦C because the computed values vary within the range of 2 K.

Thermocouples #8 and #9 fixed at the back part of the motor show an internal air tem-perature of 79 and 76^◦C, respectively. In those locations, the CFD results are underestimated by approximately 5 K. Point #10 fixed inside the motor on the housing reached 65^◦C and discrepancy between the experimental and the numerical results does not exceed 2 K. The last measured Point #11 located on the PCB recorded a temperature of 95^◦C. This result corresponds to the difference of 1 K between numerical and experimental values. This tem-perature value was similar to the temtem-perature occurring on the windings. It was caused by the thermocouple directly fixed to the phase electric path. The current flowing through that path caused a local heat generation.

The next seven points were attached to the external surface of the housing. The tempera-ture at these points was approximately between 70^◦C and 75^◦C. Slightly higher temperatures were observed in the middle of the motor length. The value recorded in this case by couple #16 was burdened with a gross error. This could be caused by detaching the thermo-couple from the measuring surface. The maximum difference between points located on the external part of the motor housing was 4 K. Hence, temperature distribution in this group of the thermocouples was fairly uniform on the housing walls mainly due to the high thermal

conductivity of aluminium.

The last four temperatures were measured at different heights of the air above the consid-ered motor. The temperature of these points was at a level of approx. 37^◦C. The differences between the measurements and CFD results at these points are in the range of 2 K.

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Figure 4.3: The temperature comparison between the CFD and experimental results at the load of 4 resistors and the rotational speed of 3500 rpm in Operating Point II

In Fig. 4.4, the temperature results obtained at the middle value of the tested rotational motor speed are presented. In this case, the load configuration that consists of the three re-sistors. Therefore, the presented Operating Point IV was characterised by the lower investi-gated temperature occurring in these work conditions comparing to the described Operating Point II. In Fig.4.4, the thermocouples fixed on the windings recorded the highest tempera-ture values between 58^◦C and 62^◦C. The numerical results show there higher uniformity at the level of 63^◦C because these values vary within the range of 2 K. The last thermocouple in the first group shows a value of approx. 71^◦C and CFD results show lower value at the level of 64^◦C. The CFD results present more uniform temperature distribution, than it is observed in the experimental data, in the first group of thermocouples located on the windings and reach approximately 63^◦C.

Thermocouples #8 and #9 fixed at the back part of the motor show an internal air tem-perature of 70^◦C and 68^◦C, respectively. Point #10 fixed inside the motor on the housing

indicates 65^◦C. When analysing this sensor group, the CFD results show the highest incon-sistency with the experiment of 3 K at Point #8. The last measured point #11 located on the PCB recorded the temperature exceeding 80^◦C and this point was consistent to CFD model.

That temperature was at the level of the temperature that occured on the winding surface.

Similarly, as in the description of the previously analysed operating points, it was caused by the thermocouple directly fixed to the phase electric path. The current flowing through the mentioned paths caused the heat generation coming from the Joule heating phenomenon.

The next seven sensors were attached to the external surface of the housing. The tem-perature for these points was approximately between 63^◦C and 67^◦C. The value recorded in this case by Thermocouple #16 was also burdened by a gross error. This could be caused by detaching the thermocouple from the measuring surface. The last four temperatures were measured for different heights of the air above the considered motor. The temperature of these points was at a level of approx. 31^◦C. Thermocouple #19 reported temperature at the level of 30^◦C, while for this position numerical model produced the higher result at the level of 33^◦C. The differences between the measurements and CFD results at the rest positions in this sensor group was 2 K.

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Figure 4.4: The temperature comparison between the CFD and experimental results at the load of 3 resistors and the rotational speed of 2500 rpm in Operating Point IV

In Fig. 4.5, the comparison of the experimental and numerical results are presented for Operating Point V. These working conditions are characterised by similar rotational speed

as presented above for Operating Point II with the highest analysed temperature. Moreover, the applied load on the generator side was also characterised by Operating Point IV that was previously presented. Therefore, as it is proven in the following paragraphs, the temperature level obtained in this case was between those observed for two operating points presented previously. In these conditions, the lower load of the motor caused lower RMS values of the current and the resulting Joule heating was also lower. In Fig. 4.5, the thermocouples fixed on the windings recorded the highest temperature values between 80^◦C and 85^◦C. The last thermocouple in the first group shows a value of 71.4^◦C. The CFD results present more uni-form temperature distribution in the first group of thermocouples and reach approximately 84^◦C.

Figure 4.5: The temperature comparison between the CFD and experimental results at the load of 3 resistors and the rotational speed of 3500 rpm in Operating Point V

Thermocouples #8 and #9 fixed at the back part of the motor show an internal air temper-ature of 68^◦C and 70^◦C, respectively. One of the largest differences between measurements and CFD results occurs in those points. In particular, the CFD results are underestimated by approximately 5 K. Point #10 fixed inside the motor on the housing reached 65^◦C. The last measured point #11 located on the PCB recorded a temperature of 84^◦C. This temperature was similar to the temperature occurring on the windings. It was caused by the thermocou-ple directly fixed to the phase electric. The current flow through that path caused a local heat generation.

The next seven points, where the temperature was measured, were attached to the exter-nal surface of the housing. The temperature at these points was approximately 65^◦C. Slightly higher temperatures were observed in the middle of the motor length. The maximum differ-ence between these temperatures was 1 K. Hdiffer-ence, temperature distribution was uniform on the housing walls mainly due to the high thermal conductivity.

The last four temperatures were measured at different heights of the air above the consid-ered motor. The temperature of these points was at a level of 35^◦C. The differences between the measurements and CFD results at these points are of 1 K. Therefore, the air zone above the investigated motor was modelled with the high consistency with respect to the experi-mental records.

In Fig. 4.6, the comparison of the experimental and numerical results are presented for Operating Point point VIII. This work state was characterised by similar rotational speed as presented for the points with higher analysed temperature, i.e. Operating Points II and V.

However, the level of the temperature for Operating Point VIII was higher than for Operating Point IV which was conducted for lower load but having higher rotational speed. Therefore, it confirms that the rotational speed does not play the most important role in the power loss generation. It is again confirmed that the lower load of the motor caused lower RMS values of the current and the resulting Joule heating was also lower. In Fig. 4.6, the thermocouples fixed on the windings recorded the highest temperature values between 65^◦C and 69^◦C. Sim-ilarly, as in the previous operating points, the temperature recorded by Thermocouples #5 shows higher values at this location than in the CFD model. Again, the CFD results present more uniform temperature distribution in the first group of thermocouples and reach ap-proximately 68^◦C.

Thermocouples #8 and #9 fixed at the back part of the motor show an internal air tem-perature of 66^◦C and 67^◦C, respectively. One of the largest differences between measurement and CFD results occurs at Points #8 and #9 in this group showing slightly underestimation of the CFD values. Points #10 and #11 fixed within the motor reached the temperatures of 55^◦C 66^◦C, respectively and these values were consistent with the CFD results.

The next seven points were attached to the external surface of the housing. The tem-perature at these points was approximately 55^◦C. The maximum difference, between these temperatures from the experimental and CFD results, was 2 K.

The last four temperatures were measured at different heights of the air above the consid-ered motor. The temperature of these points was at a level of 32^◦C. The differences between

the measurement and CFD results at these points are less than 2 K and it was noted at the last thermocouple position in this group denoted as Point #22.

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Figure 4.6: The temperature comparison between the CFD and experimental results at the load of 2 resistors and the rotational speed of 3500 rpm in Operating Point VIII

In this section, the discussed results focused on the representative operating points. For all presented operating points, the numerical results show reasonable accuracy with the recorded values. One can notice that higher motor temperature resulting from higher losses caused the higher temperature non-uniformity in the measured and modelled domain. It was also visible that model results were characterised by higher uniformity of the specific machine component temperature than that obtained in the experimental results. The tem-perature non-uniformity of the specific components, presented in the recorded measure-ments, can also be caused by detaching of particular thermocouples from the measured sur-face or by changing its original position during the experiment which was not possible to control after the motor closing. The highest temperature in the numerical domain was no-ticed on the windings for all the investigated operating points. The similar temperature level being close to the hot-spot values could be observed on the copper paths located on the PCB plate.