Niagara Falls 100.000-Hp. Development

 22 ] Niagara Falls 100,000 Hp. Development perature rise of the armature teeth above the cooling air and the balance would be the temperature drop through the coil insulation. If by some means, such as doubling the quantity of cooling air or doubling the surface exposed to the air, the heat dissipation by con- vection could be doubled, the copper temperature rise would be reduced by 10 or 12 deg. If, on the other hand, the drop through the insulation could be cut in half by increasing the coil surface or by decreasing the thickness of insulation, then the copper temperature rise would be reduced by approximately 20 deg. This discussion is based on the assumption of long cores (as in the present case), in which the ventilation of the coil ends has very little influence on the temperature of the copper in the center of the core. Armature Coil Insulation.—One of the distinctive features of this generator is the heat-resisting qualities of the insulation. Mica is used as the insulating mate- rial beginning with the bare copper conductor. This type of insulation has been developed particularly for the larger 60-cycle steam-turbine driven generators, in which it is impossible to design the windings so that the copper temperature is within the safe limit for ordinary vegetable fiber insulation. This mica insula- tion is regularly guaranteed for operation up to a tem- perature of 150 deg., and it has been successfully tested up to 300 and 400 deg. for short periods. Its use in the present instance was not dictated by operating temperatures, as these are well below 100 deg. (even allowing for inevitable differences between measured temperatures and the actual copper temperature), but it was used in accordance with the builders’ policy to use the best materials and constructions available and thereby provide the maximum operating factors of safety. The detail construction of the armature coil will be understood by reference to accompanying illustrations. Each coil consists of three active conductors and each conductor is built up of four strands or wires. The single conductor is split up in this way in order to reduce eddy currents. The winding is also connected in several parallel circuits for this same reason, as well as for the additional purpose of obtaining the desired number of conductors. The coils are formed of bare copper ribbon. Two sizes of strap are used, so that complete strand insula- tion is provided by taping the two larger ribbons with mica tape. Each group of four strands, forming an active conductor, is insulated with mica tape and the insulation between conductors is reinforced by strips of hard-baked mica. The three conductors of one coil are then assembled and the complete coil bound with thin cotton tape, after which the straight parts to be embedded in the slots are brushed with bakelite, and these parts are pressed to the required dimensions in a steam press. This solidifies the bakelite and makes the straight parts of the coil sufficiently strong and rigid to withstand the twisting forces of the wrapping ma- chine. The ends of the coils, which must be relatively flexible, are impregnated in a vacuum tank with hydro- lene. After this impregnation, the mica wrapper is applied to the straight parts of the coil. The wrapper is first applied to the coil in sheet form by hand, being drawn around the coil rather loosely. It is then placed in a wrapping machine equipped with electrically heated ironing plates, which revolve around the coil, heating and softening the bond. This permits the different turns of the wrapper to slide, one on the other, and with the heavy pressure exerted by the ironing plates pro- duces an extremely solid and compact insulating wall. This machine-wrapping process has two important ad- vantages due to the compactness with which the insu- lation is applied. The relatively poor heat conductivity of all coil-insu- lating materials is due to the minute layers of air between the layers of insulating material. Conse- quently, the more tightly the insulation can be wrapped FIG. 29—SECTION THROUGH PORTION OF STATOR SHOWING SUPPORT OF END COILS. FIG. 30—SIMPLIFIED ARMATURE WINDING As seen in Fig. 30, parallel grouping of coils is accomplished are taken off (both ends of three phases) and the star and through buses shown by the six outside circular arcs. Six leads neutral connection made outside of the machine.