Electrical machines are highly efficient; the electrical losses reach however such high levels that the produced losses must be removed. The winding resistance is temperature-dependent, and accordingly, an improvement of the cooling would bring about a decrease in the copper losses. Due to the high material costs, it is desirable to use this to its limit: through a more efficient cooling process, the current density can be increased the same machine size can reach a higher power.
The cooling system has to ensure homogeneous temperature distribution during the start-up, as excessive thermal stress provokes cracks. The life expectancy of the winding isolation strongly depends on temperature; an increase of the temperature level by 5 to 8 Kelvin results in a halving of the life expectancy. The insulation materials are therefore categorized in insulation classes and each class corresponds to a maximal allowable temperature by continuous operation of the machine. All these interrelations explain the main application area of CFD analysis. With the help of the results of a global or a partial analysis, the electrical machines companies can offer cheaper machines without exposing themselves to technical risks.
Cooling circuit of an ALSTOM air-cooled generator
The IECC European norms on efficiency will include more and more industry segments in the coming years. In order to stay below the temperature limits, the electrical machine companies will need to invest in more efficient cooling concepts. A better cooling design is mostly reserved for newer machines; however, a retrofit usually offers the possibility to make the cooling concept more efficient and thereby increase the power output.
The cooling medium takes the losses produced in the machine and removes them. The most widely available cooling medium is air, and accordingly, air is used for the majority of electrical motors and generators. Hydrogen under pressure reaches a significantly better heat transfer coefficient. The hydrogen must, however, be enclosed in a pressure safe casing due to the risk of the production of explosive gas. Large turbo generators are cooled with hydrogen. Demineralized treated water is very effective for stator cooling; however a water-cooled stator is difficult to construct and to operate, so water is used only for very large generators. Instead, oil is more commonly used for the cooling of transformers.
Turbo rotor with radial fan fixed on the shaft (older design)
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