Consulting for CFD Fow & Thermal Calculations
Large alternative current machines are used as drives for large industrial plants or as generators for the net power generation. Large direct current motors are still used in few old rolling mills and factories.
The asynchronous and synchronous rotating electrical machines have the same stator composition: a wound stator core composed of core sheet layers with grooves in which copper layers are regularly placed throughout the circumferential extent. The winding coil is isolated according to its voltage and isolation classes. These copper layers are connected in three phase-windings. The parts of the winding emerging from the stator core, also called core ends, are connected through a terminal box to the electrical network. The core ends are mechanically supported and stiffened with one another so that the current forces that occur do not cause permanent damage, even in exceptional situations. It is highly challenging to perform CFD-calculations for these geometries.
Coil end with distributed winding
The asynchronous three-phase machines, also called induction machines have either squirrel-cage rotors or slip-ring rotors. Squirrel-cage rotors have grooves which are filled with non-isolated compact copper or aluminium stems which are short-circuited on each end by a ring. Slip-ring rotors have isolated windings that are connected to slip rings on the rotor shaft.
The rotors of synchronous machines are excited with direct DC current, unless they use permanent magnets.
Slowly running synchronous machines have salient-pole rotors, which have distinctly shaped poles.
This design is particularly suitable for low rotational speeds and a high number of pole pairs.
In contrast, fast-running synchronous machines have cylindrical rotors, as these are better able to withstand the high rotational speeds and the resulting centrifugal forces.
The poles of salient-pole rotors consist of individual, clearly defined poles with a magnetic core and a field winding.
In comparison, cylindrical rotors are mainly made of laminated steel sheets with grouped slots, in which the insulated rotor winding is arranged to form individual north and south poles.
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Turbo rotor with radial fan fixed on the shaft (older design)
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 most 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.
