Niagara Falls 100.000-Hp. Development

Niagara Falls 100,000 Hp. Development [ 23 and the more completely these air pockets are elimi- nated the better will be the heat conductivity of the insulating wall. The total thickness of the insulating wall is also reduced, and thus relatively more space is available for the active materials. The curved ends of the armature coils are insulated with a number of overlapping layers of varnish-treated cloth, each layer being brushed with insulating varnish. Cloth insulation is best suited for this part of the coil on account of the curved form of the coil and the re- quired flexibility while placing the coils in the slots. The temperature of the coil ends is low compared to that at the center of the core, so that cloth insulation can be used with a liberal factor of safety notwithstand- ing its low safe temperature. Mica insulation requires no protection against the disintegrating effects of static discharge or corona, such as, for example, the grounded metallic sheath that is sometimes used when varnished cloth or paper is used in high- vol tage generators for the slot insulation. Many years’ experi- ence with mica insul- ation has proved be- yond question that this material is unaf- fected by corona at the voltage gradients ordinarily employed. At the same time sta- tic discharge is more commonly present— evidenced by ozone— in mica-insulated gen- erators than in those employing vegetable fiber insulation. This FIG. 31—SECTION OF COMPLETED ARMATURE IN PLACE These windings are connected in parallel with three similar sections by heavy buses shown at the top. Fig. 3 0 shows the armature winding scheme. is due, of course, to the higher voltage gradients that are possible with mica and to the higher specific inductive capacity of this material. Armature Coil Supports.—The coil ends are so shaped that the conical surface formed by the complete winding forms an angle of 60 deg. with the armature air gap surface. This is done to provide room for the coil bracing and to decrease the axial length of the coil ends. Liberal ventilating spaces are provided between coil ends, which are also necessary for the bronze bolts of the coil supports. This type of coil support has been developed for the largest steam-turbine generators and provides an unusually large factor of safety when applied to a 20-pole water-wheel generator. This is another instance of using the best available construction and providing a very liberal factor of safety. The straight parts of the coils immediately outside the core are sup- ported in a circumferential direction, by carefully fitted wood blocks. Rotor Construction.—The rotating part is guaranteed to operate safely at 100 per cent overspeed and at this speed the surface velocity is 15,400 feet per minute. This is a relatively low angular velocity—less than one- sixth that of large 60-cycle turbo-generators—so that the stress conditions are relatively easy and can be taken care of by any one of a number of well-estab- lished constructions. The spider hub and arms form a single casting. Each arm carries two small dovetail slots for spacing the punchings in building up the rim, but these small dovetails are not depended on to carry any of the radial load imposed by the rim, poles and field coils. Field Coils.—The field coils are formed of. bare strap, wound on edge and insulated between turns with asbestos. This is applied with shellac, and during the construction of the coil the coil is heated in an open gas flame to completely burn out the shellac. The insulation between the copper and pole is formed of molded mica and asbestos, and micarta washers are used at the top and bottom of the coil for mechanical protection. The field coil insulation, as in the case of the arma- ture coil insulation, is guaranteed to with- stand safely a total temperature of 150 deg. even though the operating tempera- ture is well be 1,ow this figure. Thrust Bearing.— The thrust bearing is of the well-known Kingsbury type. There are six babbit- ted shoes supported on hardened jack-screws, which are raised or lowered as necessary to adjust the shoes to take equal shares of the load. The use of these adjusting screws also permits any shoe to be re- moved for inspection without lifting the entire rotor. Occa- sional inspection is desirable because it may disclose, by the scoured appearance of the babbitt, the presence of dirt in the oil or other objectionable conditions and permit the correction of the difficulty before serious damage to the service has occurred. When filled with oil to the running level the bearing housing holds 270 gal. Lubrication.—There are but two points of lubrica- tion in the generator, the thrust bearing and the upper guide bearing. Both of these bearings are fed inde- pendently from a central station oiling system and drain independently to the station reservoir. The oil drainage from the guide bearing passes through a hole in the spider hub to a stationary oil pan below. There is an instrument board, located on the thrust bearing housing and facing the passageway to the station operating gal- lery, containing indicating dials for oil meters in the thrust-bearing line and in the guide-bearing supply line and indicating dials for thermometers in both thrust and guide bearings. Ventilation.—The generator rotor has inclined fan blades mounted on each side of the spider rim. Each end of the armature is enclosed so that the fans deliver