Roebel staff
The Roebel bar is a device for improving the efficiency and increasing the performance of large electrical machines . It was founded in 1912 by Ludwig Roebel von Brown, Boveri & Cie. patented.
Classic Roebel staff
It represents a special splitting of a rod-shaped electrical conductor into several parallel sub-conductors. These sub-conductors are insulated from one another and specially layered. Such a specific and well-defined twist is applied in particular in electrical machines greater power and serves the effects of resistance serhöhung at AC power - due to the Nutstreufeld or Nutenquerfeld generated current displacement (as skin effect at Hochfrequenzlitzen hereinafter) - to reduce. This cross-groove scattering field is greatest in the vertical direction at the upper part of the groove, as a result of which there is a current displacement towards the groove head. Since each sub-conductor section of a grooved conductor has on average the same vertical position in the slot of an electrical machine over the length of the slot, the current density in all twisted parallel conductors is approximately constant and current displacement is avoided. An increase in resistance caused by current displacement, which would inevitably be associated with increased ohmic losses, can thus be greatly reduced. Because of the very complex production of the conductors and the associated high costs, the use of Roebel bars is limited to machines with a high output in the upper kilowatt and megawatt range.
When the electrical conductor is supplied with current within a slot , the non-uniform magnetic resistance above and below a vertical slot segment results in a non-uniform flux distribution. Because of the air or copper path for the magnetic flux, there is a significantly greater magnetic resistance (air path) above the groove segment than below the groove (iron path). The following figure shows a subdivision of the groove cross-section into three equally large, vertically displaced surface elements and the associated flux linkages or field lines of the individual surface segments through which the current flows. Furthermore, the superimposition of these three flow chains is shown.
With a direct current supply, there is no voltage induction, no eddy currents and no current displacement in the slot because of the unchangeable magnetic field. The result is the qualitative curve of the current density J, the magnetic induction B and the leakage flux linkage Φ shown below.
With direct current feed, the current density is constant over the entire groove flank x. If, however, the electrical conductor within a slot is supplied with alternating current on the basis of an applied alternating voltage, the variable flux leads to a voltage induction which is also greatest at the slot base due to the greatest flux linkage, cf. following figure.
As a result, the greatest current displacement occurs at the groove base. An increase in resistance can be seen over the entire conductor cross-section. AC voltage feeds from windings occur in three-phase asynchronous and synchronous machines. This current displacement can be counteracted by twisting the partial conductors (Roebel winding).
High temperature superconductor Roebel cable
Analogous to the classic Roebel rod, high-temperature superconductor cables are made from RE BCO tapes. In these cables, superconductor tapes, which have been punched into a meander structure, are interwoven. This is shown schematically in the adjacent figure. Such a high temperature superconductor (HTS) Roebel cable exhibits complete transposition of all ribbons and therefore low coupling currents and alternating current losses. It is often referred to as a “ Roebel assembled coated conductor ” (RACC) cable.
Web links
- A groundbreaking invention in the field of generator construction - The Roebel rod by Ludwig Roebel (1878–1934)
- 1910–1919 Roebel bars and turbines ( Memento from October 30, 2013 in the Internet Archive ) - Chronicle of the ABB company for the years 1910–1919 (English)