Undesirable or excessive heat can occur in an electrical machine, such as a generator, and in particular, in the rotor of a generator. Undesirable heat can cause the rotor and the generator to run less efficiently, and in cases of more severe heat buildup, the excessive heat can cause damage to the rotor components or other parts of the generator.
One method to cool a rotor comprises radial flow cooling. With radial flow cooling, a cooling fluid can flow through one or more rotor subslots, each of which can be a path extending longitudinally through an internal portion of the rotor. The cooling fluid can flow longitudinally through the rotor subslot and then branch, venting radially out of the rotor via radial vent paths defined, at least in part, by holes through the body of the rotor, as well as holes through other rotor components (e.g., components attached to the rotor).
Rotor components that define vent holes can comprise a rotor body, rotor windings, creepage blocks, amortisseurs (otherwise referred to as amortisseur windings and amortisseur bars, amongst other names known by those skilled in the art), slot springs, and slot wedges, each of which can be positioned in a longitudinally-extending rotor slot which is in the periphery of the rotor body.
A plurality of the rotor slots can be spaced around the circumference of the rotor, with rotor teeth between the rotor slots. Rotor windings, or coils, which can be made up of a number of copper turns separated by insulation, can be set in the coil slots. The rotor windings, as well as the other rotor components in the rotor slots, can be held from moving radially outward by the slot wedges. The slot wedges can be inserted serially in the rotor slots radially outside the rotor windings and, in turn, retained radially by a dovetail-shaped structure of the rotor slots. The creepage blocks can be interposed between the slot wedges and the rotor windings, and the creepage blocks can serve as insulating barriers between the slot wedges and the rotor windings.
One or more types of amortisseurs can serve to conduct electrical energy between major rotor components and, in doing so, decrease or prevent an axially flowing current loop. Like that of radial flow cooling, one effect of amortisseurs is to reduce heat, increasing generator efficiency and decreasing the risk of damage to the generator. In some generators, such as static start generators, which can be used as synchronous motors to start gas-turbines, magnetically induced subsynchronous torques occur on the generator and turbine rotors. Magnetic fields are produced in the generator rotor body as a result of subsynchronous current flow, which produces eddy currents. The torques can occur at frequencies near turbine and generator rotor resonant frequencies, which can detrimentally affect the physical integrity of the generator components. The eddy currents cause the components conducting them to undesirably heat. The amortisseurs have a relatively low resistance as compared to other components of the rotor, so the amortisseurs can carry the electrical energy away from the rotor and the other rotor components with minimal heat generation caused by electrical resistance. Carrying the electrical energy between major rotor components also reduces the effects of the magnetically induced subsynchronous torques.
An end winding amortisseur can be placed on each end of the rotor, under a retaining ring, which the end winding amortisseur serves to protect from overheating. The end winding amortissuer can be a ring, or can be segmented portions that form a ring around the end of the rotor. In some instances, the end winding amortissuer has finger elements that extend into the rotor slots and overlap and mate with slot amortisseurs to make electrical contact with the slot amortisseurs.
Slot amortisseurs can be arranged in the rotor slots under the slot wedges. The slot amortisseurs can conduct electrical energy that might otherwise flow axially through and heat up the slot wedges, rotor teeth, and/or other components. To maintain solid contact with the slot wedges, slot springs can be arranged under each slot amortisseur to exert and maintain a radial force on each slot amortissuer and press each slot amortissuer radially outward against the slot wedges. The slot springs can be leaf springs or other types of springs.
In order to facilitate the radial flow cooling, each slot wedge, slot amortisseur, and slot spring, when used, has one or more vent hole so that over a length of a rotor slot, a plurality of vent holes can be spaced. The vent holes in the slot wedges, the slot amortissuers, and the slot springs are aligned with vent holes through the rotor windings and the creepage blocks to form radial vent paths.
The slot wedges are packed into the coil slots tightly with the slot amortissuers and/or the slot springs. This tight fit is intended to maintain the alignment of the rotor components, including the alignment of the vent holes to maintain a clear vent path and facilitate efficient cooling and operation of the rotor. Unfortunately, the friction between the slot springs (when used), the slot amortisseurs, and the slot wedges, created by the tight radial fit of these rotor components, does not suffice to retain the components, and the alignment of the vent holes. High rotations per minute, high accelerations, and vibrations of the rotor, as well as other causes, can cause migration of the slot amortisseurs and/or the slot springs in either longitudinal direction, thereby causing blockage of the radial vent paths, which in turn, can cause overheating, physical imbalance, and problems associated with overheating and imbalance.
It would be advantageous to reduce or eliminate migration of the slot amortisseurs and/or the slot springs in order to reduce or prevent migration of the rotor components and misalignment of the vent holes, and thereby reduce or prevent the associated problems described herein above.