November 26, 1915. THE COLLIERY GUARDIAN. 1079 The design of such a machine represents to a greater degree than usual a compromise between many con- flicting mechanical and electrical requirements. There is first the problem of constructing a rotor weighing about 60,000 lb.,and running with a peripheral speed of some 24,000 ft. per minute. Such a rotor, unfor- tunately, must be of the nature of a cage, being irregularly cut into from the periphery and carrying much metal which is not self-supporting. The centri- fugal force acting upon a 1 lb. mass at the periphery will be about 1 ton. The form and kind of material entering into the construction of the rotor demand the most careful consideration, and certain features assume importance which could be largely ignored if the working speed were reduced by, let us say, only 20 per cent. Rotor Material. In the first place, it is impracticable to use a through shaft with a hollow body or with dises threaded upon it. The rotor body could be made as a solid steel casting or forging in one piece with the shaft ends. For lower speeds or smaller diameters such a construction is very often used; but such properties of the material as are desirable in the case, of the larger machine in question are difficult to obtain commercially, since con- siderable radial tensile stresses have to be provided for, and the ductility in this direction is chiefly required far below the surface. Again, the radial tensile stress right into the centre is not inconsiderable, and were the central material removed by boring, the tangential tensile stresses, at normal speed, of the material near the bore would be about 20,000 lb. per square inch. To this must be added the local excesses due to such lack of symmetry as occurs. Several large rotors, weighing up to some 45,0001b., had been constructed successfully by the Westinghouse Company of America, using two steel castings united •A ■i-”''. Fin. 11.—“Diamond” Enclosed Electrically-driven, Longwall, Chain-type Coal-cutter. 149: r •''■■ . -r 1 I y Fig. 12.—“Diamond” Two-motor, Centre Jib, Longwall Chain Machine. on the rotor centre line; but, for the large rotors under consideration a decision was formed in favour of the material and construction described below. A difficulty, in some respects similar to that indicated above, had previously arisen in connection with large forgings for high-speed water-wheel-driven generators, and it had been overcome by the use of open-hearth rolled plates consolidated and mounted on a shaft, the thickest rolled plate that was commercially available being chosen. One batch of about 100 tons of such steel, about 2| in. thick, gave tests averaging approximately as follow :—Ultimate tensile strength, 80,0001b. per ,sq. in.; yield point, 53,0001b. per sq. in.; elongation, 26 per cent, in 2 in.; reduction of area, 60 per cent.; bending test, strips the full thickness of the plate bent through 180 degs. and closed down without fracture. Plate-built Rotors. For the steam driven turbo-generators under con- sideration, this plate material appeared very attractive, combining very low cost with entirely reliable properties from the periphery to the centre, and from end to end of the rotor. It would furnish a rotor in which practically the only internal stresses would be inten- tional ones; there would be nothing corresponding to the huge initial stresses to which solid forgings and castings are liable; and every cubic inch of material would be within an inch or two of a forged surface (or the equivalent). Eventually such a rotor was made, with the plates slightly rabbeted into one another, and into a flanged shaft at each end; and by means of a group of four or six chrome-nickel steel bolts, each about 4 in. in diameter, a 50 to 56-in. diameter rotor can be constructed with practically the solidity of a single piece. A tension in the bolts roughly corresponding to the desired amount is obtained by tightening the nuts until a specified elongation of the bolts has been attained, viz., over in. in many eases. This plate construction only partially overcomes the difficulties indicated above, as it is still necessary to have heavy forged shafts with large flanges forged on them of the full diameter of the rotor. However, the difficulty of securing a satisfactory forging has not only been reduced, but the axial length of the large diameter part can be kept down to such dimensions that the material can be worked in an end-wise, i.e., axial, direction, thus ensuring good properties in that direc- tion. To produce the flanged shafts, a large bloom is forged down to a diameter intermediate between that of the flange and that of the large end of the shaft, and stepped off; the forging is then placed with its axis vertical, in a die, under a forging press, and the increased flange diameter obtained by end-wise forging. Returning to the plates, when these can be obtained locally, it is frequently convenient to use ingots of such size that one disc is obtained out of each plate, so that the complete plate can be delivered .to the factory, put upon a boring mill, and the required disc parted out of it. It is hardly necessary to point out that this plate material has a clearly defined field of usefulness. While excellent in an application such as that described above, the material may fail completely if used in an unsuit- able manner. For instance, in directions at right angles to its plane the plate necessarily has poor properties; there is liable to be some lamination, and remnants of piping are likely occasionally to appear. The plates, therefore, should not be used in a manner which will involve much tensile stress at right angles to their plane, or even considerable shearing stress in their plane. Critical Speed. While many large rotors are running satisfactorily at operating speeds above their critical speeds, there is a distinct advantage in keeping the critical speed above the running speed, when feasible. Thus, experience has shown that in some four-pole machines, running perfectly under normal conditions, a short circuit of even a few turns of rotor winding on one pole may result in serious vibration. On the other hand, one of the machines now under discussion, and designed for a high critical speed, was purposely run with two-thirds of the winding of one pole short-circuited, and no con- siderable difference in the running was noticeable. If it is decided to run below the critical speed in the case of these large machines, the rotor body must be as short and light as possible, involving a high air gap density and a severe working of the stator material in the neighbourhood of the air gap. The shaft diameter must be large, and high peripheral speeds must be tolerated for journals and slip rings. In the present machines the journal speed approaches 6,000 ft. per minute; and for the slip rings a speed of about 11,000 ft. per minute is necessary. The span between the journal centres must be kept small, which limits the design of the stator end windings, and necessitates the omission of blowers from the rotor. Rotor Ventilation. A matter requiring early attention is that of rotor coils and ventilation. In order to obtain a small, short rotor, advantage must be taken of every means to that end. It is incumbent to use a ventilated rotor. The best system of ventilation in this ease is one com- bining an axial passage underneath the winding slots, with a radial discharge distributed over a fairly wide central portion of the rotor. With the plate construc- tion we can readily obtain the radial ventilating spaces by means of a simple “ slab-miller ” operation upon the individual plates. This provides a vent j in. wide in the region of the slots and teeth, leaving the plate solid in the region of the pole, where the extra metal is needed for magnetic reasons. Rotor Windings. A separately-formed edge-wound coil, dropped turn by turn into the slots, is used, with solid mica insula- tion, and a light steel cell to protect the insulation against the effect of the ventilating air. A compara- tively wide strip (about 1| in.) is used, with only three coils per pole, i.e., six slots per group, in the four-pole rotor. The field windings will be of substantial thick- ness, perhaps 0-1 in. for 220-volt excitation, and it is not an easy matter to get such windings down solidly in the slot so that no shifting will occur during running. A three-part wedge (bronze centre and steel liners) is very effective in overcoming this difficulty, inasmuch as the central bronze part of the wedge can be inserted in the slot and pressed down upon the winding under great pressure, while the two steel liners, one on each side, are driven into place without any friction on the top surface of the insulated coil. At the same time, the opening of the rotor slot is, from a magnetic point of view, greatly reduced, and a reasonable compromise is thereby effected. Coil Retaining Rings. As regards the part of the rotor windings external to the core, we have a very serious problem to meet in supporting this free copper. The most satisfactory method for machines of this size is the use of a weldless chrome-nickel steel ring, proportioned to carry its own load and that of the copper underneath it without any radial support from the rotor. The rings must be posi- tively driven by means of a key located in the rotor body. Steel is used for the retaining rings, because mechani- cal considerations must take precedence over everything else. The rings tend to form a magnetic short circuit from pole to pole, and, in fact, do account for a large leakage. The design of the tip of the ring, and its con- nection with the rotor body, are arranged to reduce this evil as far as possible, and a comparatively slight varia- tion in this respect can make a large difference in the leakage in the case of a four-pole rotor. The two-pole machines of large size saturate their ring cross section almost necessarily, and in this case the end disc, which centres the ring at its outer end, is made of manganese bronze to obviate further leakage. In the four-pole machine this disc is made of steel, without appreciably affecting the leakage. So long as the load on the generator is a nearly balanced three-phase (or two-phase) load, the objection to a magnetic end ring is chiefly on account of the extra flux with which it burdens the rotor. Occasionally, however, a balanced condition of phases cannot be obtained; in this case the leakage flux from the stator coils is no longer a simple rotating field, and very large losses would occur in the magnetic steel end ring in consequence. The unbalanced load affects the flux con- ditions in the main body of the rotor also, and must be provided for by heavy longitudinal dampers. In the slotted region of the circumference, the slot wedge is used for this purpose, being made in continuous lengths and of hard-drawn copper; elsewhere special dampers are inserted. These dampers complete their electrical circuit by means of all the circumferential copper and bronze of the end rings; and the design of the Ijronze section of the ring is such as to ensure the electrical connection being improved by the stresses arising upon rotation. In single-phase and three-phase short-circuit runs made upon these machines the power absorbed (less that due to friction and windage) was found to be only 10 per cent, greater when the machine was delivering rated current on a single- phase short circuit than for the same current on a three- phase short circuit, thus indicating only a very small additional loss due to the unbalanced single-phase effect. Rotor Coil Bracing. In the case of four-pole machines having only three heavy coils per pole of less than 90 degs. pitch, no very special means are needed; but with two-pole machines, where the coil span is twice as great, and there are more coils per pole, involving a greater length of straight extension from the slot, it is important to brace the windings effectively. The system of blocking adopted for a 21,000-k.v.a. two-pole machine was to locate a light built-up steel driving horn on the interpole centre line of the rotor, and attach it firmly to the shaft by heavy chrome-nickel steel bolts. The shaft diameter at this place is but little smaller than the inside diameter of the coils, so that a very effective stay is obtained. It is then merely necessary to block the succeeding coils from these and from one another by the use of a metal block carried by a radial chrome-nickel steel stud having some degree of flexibility, and acting merely as a tie and anchor. Substantial insulating shoes separate the coil from the block. Stator Construction. The electrical conditions to be met in the stator are by no means easily provided for. Some system of axial ventilation is practically imperative; and in these machines this is very complete, the provision for the air supply being particularly liberal immediately behind the teeth. Behind each slot are arranged three oval venti- lating openings, with only a narrow strip of punching separating them from one another and from the slot; in this way the amount of core cooling surface is pro- perly proportioned to the cross-sectional area of the air vent. On account of the length of these machines, the cool- ing air is admitted at each end of the stator, and dis- charged at the centre. This arrangement also simplifies the question of rotor ventilation. The total volume of cooling air required by such a machine is about 60,000 cu. ft. per minute : and thus a large central opening is necessary. Provision against concentration of flux must be made by properly proportioning the flux per inch of core immediately next to the vent; whilst the large stray flux leaving the rotor from this vent zone is taken care