Study On Air-water Multiple Cooling Technique Of Double-fed Induction Generator

The temperature rise of an electrical machine is directly related to itsperformances and economical indices. When the temperature rise of an electricalmachine reaches a certain value, the performances of the machine in mechanical,electrical, physical and other aspects will change. Firstly, there is an exponentialrelationship between the insulation life and the temperature in the electricalmachine during its continuously operating. The temperature rise in the localregion and in some components of the large electrical machine may be very high.If the ventilation and cooling system is not reasonable designed, the life of theelectrical machine will decrease. Secondly, due to the local overheating resultsfrom the unreasonable design of the ventilation and cooling system, the strengthand hardness of the metallic materials will drop with the temperature. As a result,the temperature distribution is more uneven, which will increase the thermalstresses. So the local overheat and the strenghened uneven temperaturedistribution will have the devastating effect on the mechanical properties of theelectrical machine. Again, the unreasonable design of the ventilation system willresults in over ventilation or lack of ventilation, any one will affect the efficientof the electrical machine. For all above reasons, the fluid field and thetemperature field are always accurately calculated during the design of themodern electrical machine. To improve the system operating efficiency, usually,the cooling medium in the electrical machine is rationally allocated, all heatingparts are cooled well, and the temperature distribution is made more reasonable.Based on the fluid dynamics theory, the equivalent model of the ventilationcircuit of the electrical machine was analyzed. According to the continuityequation and the energy equation, considering the air gap ventilation, the methodfor calculating the air ventilation system with multi-branch ventilation circuitswas derived. It can be seen that, the derived coefficient matrix of the wind resistances of the multi-branch distribution system is not a decoupled diagonalmatrix; the loss along one branch ventilation road that corresponds with oneresistance in the matrix is always affected by the flow quantity of that branchroad. Through the analysis of the factors that can affect on-way resistance andlocal resistance, it can be seen that, the wind resistance of each wind road isfunction of flow quantity and flow rate, so the coefficient matrix of theventilation network can not be seen as a constant coefficient matrix. So, a modelof topological ventilation network with variable parameters for ventilationcalculation was given, each wind resistance was described by a function of flowquantity, so the influence of the flow rate on the wind resistance was considered,and the calculation accuracy was improved.For an doubly-fed induction motor with both air cooling structure and watercooling structure, and with both axial ventilation and radial ventilation, aventilation and temperature directly coupled global three-dimensional model willneed high computation hardware requirements, and consume long computationtime. So a model of topological ventilation network with variable parameters isused to solve the motor ventilation system to get the air flow distribution, whichwas used to calculate the surface heat transfer coefficients of the stator and rotorin the electrical machine. The heat transfer coefficients were used as theboundary conditions during the temperature field calculation. The temperaturefields of the stator and rotor were calculated seperately to simplify thecalculation and shorten the calculation time. Because of the space limitation dueto the motor installation, and no ventilation protype model, it is unable to achieveall needed ventilation tests, but the temperature field of the rated condition wasgot from the experiment results list and compared with the calculated results, andthe calculation precision can meet the needs of the engineering.For a doubly fed induction generator with only axial ventilation system,because of the uneven distribution of the temperature along the axial direction, inthis paper, according to the distribution of the ventilation and the heat, a regionwith length in whole axial direction and being one of the sections with similartemperature distribution in the circumferential direction was taken as the physicalmodel, the ventilation calculation and the temperature calculation were directlycoupled. Given the boundary condition of the model, the equation was solved using the finite volume method, and the flow distribution and the flow velocity ofthe air at the rated operating conditions, as well as the temperature distributionsin each cross section of the stator, the rotor, and the water jacket were got. By thecomparative analyses between the calculated results and the test results, therationality of the calculation method was proved. The results also show, the waterjacket can effectively reduces the temperature of the stator yoke and the statorwindings, but the rotor winding temperature is a bit higher on the contrary, so theventilation capacity of the system and the heat transfer property of the framecooler shoud be optimizedFinally, the actually happened problem of the water jacket was analyzed andthe question for whether it is useful to place baffle infront the fan region wasanswered in this paper. From the experiment results, using the water jacket indifferent form will have very different effects on the temperature of the wallsurface. At the same time, due to the baffle, the ventilation losses were reducedand the system efficiency was improved.