Generator rotors are provided with radial slots about the periphery thereof, for receiving field or rotor windings of coils made up of a number of turns in a radially stacked arrangement, each turn or winding separated by insulation. The windings are retained in each slot by one or more full-length wedges, with one or more creepage blocks interposed between the wedges and the windings. The function of the wedge block is to contain the contents of the rotor slot against outward radial forces arising from rotation of the rotor. The creepage block serves as an insulating barrier between the wedge and the copper windings. The creepage block is also formed with apertures which permit cooling gas to flow from radial passages in the copper windings to radial passages in the wedge.
Generators currently available from the assignee are placed in three major design classifications based on the cooling medium used: air, hydrogen or liquid (water). All hydrogen and water cooled generators use direct conductor cooling of the rotor windings for heat removal. Smaller two-pole and all four-pole generators use the radial flow design.
In such radial flow systems, the field windings imbedded in the rotor slots are ventilated by a radial cooling circuit consisting of an axial sub-slot and a series of radial cooling passages. To maximize cooling gas flow in the radial direction and electrical conductance in the axial direction of the winding, the axially spaced radial flow cooling ducts are elongated in the axial direction. To minimize stresses in the rotor wedges, the last leg of each radial cooling path is a round hole. The problem to be solved is a cost-effective optimization of the transition via the creepage block from elongated ducts in the copper windings to the circular holes in the wedge without incurring undesirable pressure losses that would undermine the thermal performance of the ventilation circuit.
Historically, the creepage block radially between the wedge and the coil windings has been made by introducing a multitude of geometric features for each radial passage. In one example, cylindrical holes are drilled concentric with each wedge hole. Circular arches are then machined on the radial inboard surface of each creepage block to match the elongated ducts in the windings. Finally, a shallow axial groove is machined through the radially outboard surface of each wedge to allow for small variations in alignment between the creepage block and the wedge block. This conventional arrangement is illustrated in FIG. 1 (described in detail further below).