In the presented study, five measurement techniques were implemented. The most im-portant method used during experimental research was the temperature acquisition. The calibrated thermocouples allowed for the temperature recording with a small error in the range of ± 0.3 K. The thermography technique was also evaluated as a useful tool in this kind of thermal research, but the temperature field acquisition of the surfaces characterised by different emissivity was a challenging task. The additional difficulty was imposed by the high surface reflectiveness. However, when the high emissivity layer was implemented, this effect was not noticeable anymore. The CTA technique, applied during the experiment, allowed for velocity estimation above the investigated machine. However, the natural convection, having a transient character, implicated high measurement error estimated on the basis of a standard deviation. The conducted experiment also proved that LDA technique is a suitable technique for velocity acquisition within electric machine only when the investigated space allows for this technique application. The air gap between rotor and stator tooth, reaching 0.7 mm, did not allow for the velocity measurement in that motor space. Moreover, the high velocity gradient in that region caused an additional difficulty. However, the LDA technique was an effective tool during the velocity measurements in the internal back part of the motor housing where the space around the motor shaft allowed for the data acquisition. The ex-perimental part of this study pointed out that PIV technique with the smoke seeding was not suitable for this kind of research. The seeding medium containing solid particles was not taken into consideration due to possible damage of the bearings or other rotating compo-nents. Therefore, the PIV technique can be recognised as very challenging in similar cases.
The losses estimation on the basis of measurements is also a demanding task because of:
(i) the current measurement uncertainty, (ii) the loss division between electric commutator, cables and motor and (iii) low accuracy of torque measurement.
In the numerical modelling of the electric motor thermal behaviour, the CFD technique
should be assessed as an effective tool. The thermal motor behaviour including the temper-ature and velocity fields can be predicted with a satisfactory accuracy. Using this modelling formulation, the radiation phenomenon can be investigated using S2S or DO models result-ing in similar values of the temperature field. The DO model is more expensive computa-tionally than the S2S approach, but the view factors do not have to be estimated prior to the computational process as it is necessary in the S2S model. Another important aspect in the thermal modelling of the electric motors is the implementation of the anisotropic thermal properties of windings and core components. Such material property definitions allow for more accurate estimation of the temperature field. The other challenging task in the electric motor thermal modelling was to implement the contact resistances that appear on the in-terface between the two different components. It is especially important in the region where the windings are in contact with bobbin or core. The estimation of the power loss field distri-bution implemented as the volumetric source terms in the thermal model is possible using the coupling procedure within the EMAG solver. In the core, the losses are the effect of the magnetic field distribution and by this nonhomogeneous heat sources can be transferred within the coupling procedure. The temperature response from the thermal model to EMAG solver was an important aspect of the electric conductivity value estimation. However, in the investigated low power electric motor, the small temperature gradient that occurred within the windings did not influence significantly the electric conductivity of this component in comparison to the average and nonhomogeneous temperature field.
The passive methods applied for the heat dissipation intensification from small power electric motors, turned out to be an effective way for the motor elements temperature re-duction. The simplest, but not the most effective method, was the emissivity increase of the external motor surfaces. As the conducted study proved, the more effective method of heat dissipation is the external surface enlargement. Equally effective method can be the imple-mentation of the potting material within the motor to limit the stagnation zones of the air enclosed in the motor housing. This operation caused the thermal resistance reduction oc-curring between the windings and motor housing. As it was pointed out in the summary, the most effective tested variant of the heat dissipation intensification was based on com-bined method including an application of: high emissivity layer, bigger thermal radiator and filler material. For the motor working conditions that were close to the rated parameters, the winding temperature reduction of approx. 30 K was observed when compared to the case without any heat enhancement and referred to the ambient conditions. The average wind-ing temperatures for the case without heat dissipation enhancement achieved 68 K above the room temperature. Therefore, the mentioned value of the temperature reduction is al-most half temperature rise above the ambient temperature with respect to the primary motor
construction.
The temperature limitation, especially in the winding region, allowed for overloading the machine to higher level. In the presented research, the overload reaching 140% of the rated parameters was tested, while the winding temperature was kept at the same level as before introduced modifications. These overload tests were conducted for the heat dissipation im-provement covering the application of the bigger radiator. For the combined method, the constructed test rig made it possible to overload the motor to the level of 145% of the rated parameters. It was a maximum overload condition possible to be tested and limited by the test rig component constraints. The limitations were caused by: (i) the generator, which was not additionally cooled, and (ii) by the maximum working parameters of the power supplier.
However, for this overload operating point, the obtained temperature was 18 K lower than the temperature during the reference conditions operation. Therefore, the verified experimen-tally and numerically heat dissipation enhancement configuration allow for the significant motor overload with respect to the rated parameters. Moreover, the temperature limitation of the hot spot allows for reducing the risk of the motor overheat under the unfavourable ambient conditions. Additionally, the lower winding temperature influences the lower value of the copper losses.
The most effective combined method was based on the passive techniques. Therefore, the presented in Introduction hypothesis of the dissertation, formulated as: "The combi-nation of passive techniques for the heat dissipation improvement is an effective way to decrease the temperature of the windings in the PM BLDC small power motor, including application of high emissivity layer, extension of the external surface of the housing and ap-plication of the thermal filler (potting material)", was positively verified.
The winding temperature reduction, presented in the work, was the main achievement of the introduced combined solution. On the other hand, the application of the radiators and thermal filler caused the increase of the motor mass and material costs. However, the motor net price offered by producer was at the level of approx. 400 Euro when the overestimated cost of additional materials used in the research reached less than 30 Euro. In the applica-tion, in which the motor size does not have to be reduced, this solution can successfully be implemented.
All the investigated methods of the winding temperature reduction fulfilled their pur-pose. To increase the heat dissipation from the motor test rig, the application of high emis-sivity layer covering this stand allowed for achieving the goal. As it was shown in the research, more effective solution can be the development of the external motor housing surface.
Fur-thermore, an application of the potting material characterised by high thermal conductiv-ity in the dead motor zones can also improve the effect of winding temperature reduction.
Separate or combined methods can be taken into consideration depending on the motor application.
The CFD technique is the effective tool to estimate the fluid and temperature behaviour in the small power PM BLDC motors during its operation at different loading conditions.
The implementation of the coupled models covering the electromagnetic, thermal and flow phenomena enables to predict the motor operation before its construction. However, the validation procedure of the motor prototype should always be included in the full-scale de-signing process to verify the assumed input parameters used in the models.