Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2003-06-26
2004-09-28
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S179000, C310S180000, C310S195000, C174SDIG002, C174SDIG002
Reexamination Certificate
active
06798107
ABSTRACT:
TECHNICAL FIELD
The invention relates to a rotating electric machine comprising a magnetic circuit with a magnetic core and a winding. Such electric machines comprise synchronous machines which are mainly used as generators for connection to distribution and transmission networks; commonly referred to below as power networks. The synchronous machines are also used as motors and for phase compensation and voltage control, in that case as mechanically idling machines. The technical field also comprises double-fed machines, asynchronous machines, asynchronous converter cascades, outer pole machines and synchronous flux machines.
The magnetic circuit referred to in this context comprises a magnetic core of laminated, normal or oriented, sheet or other, for example amorphous or powder-based, material, or any other action for the purpose of allowing an alternating flux, a winding, a cooling system, etc., and may be located in either the stator or the rotor of the machine, or in both.
The invention also comprises a method for manufacturing a magnetic circuit for a rotating electric machine.
BACKGROUND ART THE PROBLEM
In order to explain and describe the machine, a brief description of a rotating electric machine will first be given, exemplified on the basis of a synchronous machine. The first part of the description substantially relates to the magnetic circuit of such a machine and how it is constructed according to classical technique. Since the magnetic circuit referred to in most cases is located in the stator, the magnetic circuit below will normally be described as a stator with a laminated core, the winding of which will be referred to as a stator winding, and the slots in the laminated core for the winding will be referred to as stator slots or simply slots.
Most synchronous machines have a field winding in the rotor, where the main flux is generated by de, and an ac winding which is in the stator. The synchronous machines are normally of three-phase design. Sometimes, the synchronous machines are designed with salient poles. The latter have an ac winding in the rotor.
The stator body for large synchronous machines are often made of sheet steel with a welded construction. The laminated core is normally made from varnished 0.35- or 0.5 mm electric sheet. For larger machines, the sheet is punched into segments which are attached to the stator body by means of wedges/dovetails. The laminated core is retained by pressure fingers and pressure plates.
For cooling of the windings of the synchronous machine, three different cooling systems are available.
In case of air cooling, both the stator winding and the rotor winding are cooled by cooling air flowing through. The cooling air ducts are to be found both in the stator laminations and in the rotor. For radial ventilation and cooling by means of air, the sheet iron core at least for medium-sized and large machines is divided into stacks with radial and axial ventilation ducts disposed in the core. The cooling air may consist of ambient air but at powers exceeding 1 MW, a closed cooling is substantially used.
Hydrogen cooling is normally used in turbogenerators up to about 400 MW and in large synchronous condensers. The cooling method functions in the same way as in air cooling with heat exchangers, but instead of air as coolant hydrogen gas is used. The hydrogen gas has better cooling capacity than air, but difficulties arise at seals and in monitoring leakage. For turbogenerators in the power range of 500-1000 MW, it is known to apply water cooling of both the stator winding and the rotor winding. The cooling ducts are in the form of tubes which are placed inside conductors in the stator winding.
One problem with large machines is that the cooling tends to become non-uniform and that, therefore, temperature differences arise across the machine
The stator winding is located in slots in the sheet iron core, the slots normally having a rectangular or a trapezoidal cross section. Each winding phase comprises a number of coil groups connected in series and each coil group comprises a number of coils connected in series. The different parts of the coil arc designated coil side for the part which is placed in the stator and end winding for that part which is located outside the stator. A coil comprises one or more conductors brought together in height and/or width.
Between each conductor there is a thin insulation, for example epoxy/glass fibre.
The coil is insulated from the slot with a coil insulation, that is, an insulation intended to withstand the rated voltage of the machine to earth. As insulating material, various plastic, varnish and glass fibre materials may be used. Usually, so-called mica tape is used, which is a mixture of mica and hard plastic, especially produced to provide resistance to partial discharges, which can rapidly break down the insulation. The insulation is applied to the coil by winding the mica tape around the coil in several layers. The insulation is impregnated, and then the coil side is painted with a graphite based paint to improve the contact with the surrounding stator which is connected to earth potential.
The conductor area of the windings is determined by the current intensity in question and by the cooling method used. The conductor and the coil are usually formed with a rectangular shape to maximize the amount of conductor material in the slot. A typical coil is formed of so-called Roebel bars, in which certain of the bars may be made hollow for a coolant. A Roebel bar comprises a plurality of rectangular, copper conductors connected in parallel, which are transposed 360. degrees along the slot. Ringland bars with transpositions of 540 degrees and other transpositions also occur. The transposition is made to avoid the occurrence of circulating currents which arc generated in a cross section of the conductor material, as viewed from the magnetic field.
For mechanical and electrical reasons, a machine cannot be made in just any size. The machine power is determined substantially by three factors. The conductor area of the windings. At normal operating temperature, copper, for example, has a maximum value of 33.5 A/mm2. The maximum flux density (magnetic flux) in the stator and rotor material. The maximum electric field strength in the insulating material, the so-called dielectric strength.
Polyphase ac windings are designed either as single-layer or two-layer windings. In the case of single-layer windings, there is only one coil side per slot, and in the case of two-layer windings there are two coil sides per slot. Two layer windings are usually designed as diamond windings, whereas the single-layer windings which are relevant in this connection may be designed as a diamond winding or as a concentric winding. In the case of a diamond winding, only one coil span (or possibly two coil spans) occurs, whereas flat windings are designed as concentric windings, that is, with a greatly varying coil span. By coil span is meant the distance in circular measure between two coil sides belonging to the same coil, either in relation to the relevant pole pitch or in the number of intermediate slot pitches. Usually, different variants of chording are used, for example short-pithing, to give the winding the desired properties.
The type of winding substantially describes how the coils in the slots, that is, the coil sides, are connected together outside the stator, that is, at the end windings.
Outside the stacked sheets of the stator, the coil is not provided with a painted semiconducting ground-potential layer. The end winding is normally provided with an E-field control in the form of so-called corona protection varnish intended to convert a radial field into an axial field, which means that the insulation on the end windings occurs at a high potential relative to earth. This sometimes gives rise to corona in the coil-end region, which may be destructive. The so-called field-controlling points at the end windings entail problems for a rotating electric machine.
Normally, all large machines are designed with a two-
ABB AB
Dykema Gossett PLLC
Mullins Burton
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