Hollow conductors are used in generator coils and stator bars to allow for the circulation of a cooling medium. By carrying away the heat generated as the conductors cut through magnetic field lines, they serve a vital cooling function. Their design is based on the "skin effect"-a phenomenon in alternating current (AC) where the current is concentrated primarily near the surface of a conductor, with lower current density toward the center. This characteristic justifies the use of a hollow structure, which not only conserves conductive material (such as copper) but also facilitates the internal cooling required during motor or generator operation.
In the rotors of "dual-water-cooled" (water-cooled stator and rotor) generators, hollow conductors must simultaneously carry high-intensity currents, provide efficient heat dissipation, and withstand immense centrifugal forces as well as the impact of the circulating cooling water. Consequently, they must meet stringent requirements regarding electrical conductivity, thermal conductivity, mechanical properties, and dimensional precision.
Hollow conductors are typically manufactured from copper (such as TU2 oxygen-free copper) or stainless steel. The production process is complex, involving a series of stages-including semi-continuous ingot casting, water-sealed extrusion, drawing, annealing, and the application of electrical insulation-with strict controls on wall-thickness precision (often requiring a tolerance of ±0.10 mm). With ongoing technological advancements, direct-cooling technology using hollow conductors has been extended to fields such as electric vehicles, helping to reduce motor weight, lower operating temperatures, and increase power density.
Internal corrosion of hollow copper conductors can lead to the degradation of cooling water quality, blockages that cause temperature spikes, and even the breakdown of electrical insulation. To prevent the accumulation of water inside the conductors and their exposure to the atmosphere, protective measures such as purging with dry air, dehumidification via vacuum pumps, or nitrogen-blanketing are employed.
