The Design of Turbo Field Magnets for Alternate-Current Generators with Special Reference to Large Units at High Speeds

1910.] WALKER: DESIGN OF TURBO FIELD MAGNETS. 319 THE DESIGN OF TURBO FIELD MAGNETS FOR ALTERNATE-CURRENT GENERATORS WITH SPECIAL REFERENCE TO LARGE UNITS AT HIGH SPEEDS. By MILES WALKER, Member. (Paper received November 10, 1909, and read in London on March 10, 1910.) SUMMARY. This paper re-opens the controversy, between the salient pole type of field magnet and the cylindrical type. Reasons are given why the latter type is better suited for obtaining the greatest possible output from a given diameter. The methods adopted by various makers of supporting the windings are described, and the possible limits of output considered. THE NECESSITY OF PROVIDING LARGE GENERATING UNITS OF HIGH SPEED. In the discussion which followed the paper by Dr. Kloss, " Practical Considerations in the Selection of Turbo Alternators," before the Manchester Section of the Institution of Electrical Engineers in November, 1908, an argument arose as to whether the field magnets of turbo-generators should be made with salient poles or should be of the cylindrical type with a distributed winding. Some difference of opinion exists among designers on this point, and each method has been adopted in quite a large number of successful turbo-generators. In the opinion of the author the matter is one of great importance, because, in the future, makers of turbo-generators will be called upon to build machines of larger capacity than ever before contemplated, and the makers of steam turbines to drive them will call for extremely high speeds, so that the turbo-generator of the future will have to be capable of giving an extremely large output from a diameter which is not excessive, and will have to be constructed in a manner which permits of a high factor of safety. If we look at the growth in the kilowatt capacity of machines during the last thirty years, we are driven to the conclusion that the kilowatt capacity of large units in the immediate future will be as great as 15,000 or 20,000 k.w. In the year 1880 a 10-k.w. machine was considered large; in 1885 a 100 k.w. ; in 1890 a 300 k.w.; in 1895 a 500 k.w.; in 1900 a 1,000 k.w.; in 1905 a 5,000 k.w.; in 1910 we have 320 WALKER : DESIGN OF TURBO FIELD MAGNETS [March 10th, io,ooo-k.w. steam turbine-driven generators and 17,000-k.v.a. water turbine-driven generators. The cheapening of the cost of generation of electricity and the reduction in the capital required per kilowatt installed are already being felt" in the widening of the field in which electricity is being used, so that in the near future we may expect the output of electricity stations in all our large towns and manufacturing districts to be enormously increased, and the great requisite will be large generating stations at low capital cost. For capital cost is one of the main items which go to make up the price at which electricity can be supplied. These large units will in many cases consist of impulse turbines of very high speed and very large capacity. THE SALIENT POLE TYPE versus THE CYLINDRICAL TYPE OF FIELD MAGNET. The author has therefore thought it would be well to open again the discussion upon the salient pole type and the cylindrical type of FIG. 1. field magnet, and to consider the advantages and disadvantages of each of these types, with special reference to machines of very large output and very high speed. Let us consider first the essential features of a field magnet and the duties which it has to perform. We will take for granted that the field magnet is the rotating element, because it is difficult to support satisfactorily a rotating armature winding wound for a high voltage, and it is not desirable to collect current generated at 6,000, or 11,000 volts from slip-rings. Fig. 1 shows a section through the iron parts of a stationary arma- 1910.] FOR ALTERNATE-CURRENT GENERATORS. 321 ture with the winding turbo-alternator. The duty of the field magnet, which we will take as having four poles, is to produce a magnetic flux which crosses the gap in a radial direction, and returns through the iron of the armature back to the core of the magnet. To produce this flux, we shall require a winding placed so that it produces a magneto- motive force in the direction indicated by the arrows in Fig. i. The Two Factors in the Output.—The output of a field magnet is proportional to the product of two factors:— 1. The cross-section of the iron of the path indicated by the arrows (Fig. i). 2. The ampere-turns carried by the copper winding. FIG. 2. The cross-section of the path would be largest if the rotor consisted of a solid cylinder of iron filling the whole space, in which case there would, of course, be no room for the copper. The ampere-turns would be greatest if the whole space were filled with a copper winding, in which case there would be no iron for the flux. The best theoretical arrangement would be a disposition of copper and iron something like that shown in Fig. i, in which the iron and copper are put in those places where they are most useful, and least interfere with the presence of one another. Such an ideal disposition of iron and copper would, of course, be difficult to carry out in commercial manufacture, but the type of rotor which most nearly approximates to this, will be the one which will give the greatest output in the smallest space. One of the main considerations which determines the arrangement 322 WALKER: DESIGN OF TURBO FIELD MAGNETS [March 10th, of iron and copper on a turbo-rotor is the necessity of supporting the parts against the great centrifugal forces. The Salient Pole Construction.—The salient pole at first sight appears to offer very great facilities for supporting the copper winding, because it can be made with a projecting lip all round the pole. Fig. 2 shows the field magnet of one of the 4-pole turbo-generators lately described before this Institution.* This is mechanically a very good construction. In cases where it is decided to go to extremely high peripheral speeds, and additional support is required for the independent coils, the type shown in Fig. 3 may be employed. Here the coil on each FIG. 3. pole consists of four parts wound in slots cut out of a solid steel forging. Or if it is desired to construct the pole of laminations, the type shown in Fig. 4 may be employed, and great mechanical strength at the same time obtained. Very successful turbo-generators have been built according to the 'methods illustrated in Fig. 2, 3, and 4. It will be observed, however, from these figures that there is between all the poles a space which is not utilised at all, and that is the very space where the copper winding should be, according to the ideal arrangement in Fig. 1. This space can be utilised to a certain extent for the purpose of ventilation It is, however, frequently occupied by bronze coil supports which are found necessary to prevent side bulging of the coils. With the coils placed as shown in Figs. 2, 3, and 4, the cross-section * G. Stoney and A. A. Law : " High-speed Electrical Machinery," Journal of-the Institution of Electrical Engineers, vol 41, p. 300, 1908. 1910.] FOR ALTERNATE-CURRENT GENERATORS. 323 of the pole is necessarily reduced, and therefore one of the factors of output (No. i above) is reduced. It.will be seen that the reduction of the total flux of a generator reduces its output in a way that can never be quite compensated for by increasing the ampere-turns, because the capacity of the armature is in any case limited to a definite number of ampere-wires per inch of periphery on account of heating considerations, whereas the flux-carrying capacity of the armature iron can be extended by increasing the outside diameter of the punchings. A type of field magnet which very effectually gets away from the necessity of side supports for .the coils is that shown in Fig. 5, where each pole consists of a number of round, poles placed side by side on a central boss. This construction, however, has the drawback of still FIG. 4. further reducing the cross-section of the iron path, but it leaves a large amount of space available for ventilation. With the salient pole type the ends of the coils are easily supported by a projection from the ends of the poles. The coils, moreover, are comparatively simple in form and cheap to construct, and the length of mean turn is not great for a given cross-section of iron enclosed. The coils can be easily replaced. The Cylindrical Type Construction.—The advantage of the cylindrical type with distributed winding, as illustrated in Figs. 6 to 15, is that by placing the winding in slots between iron teeth we can utilise a great part of the periphery both for carrying flux and for carrying ampere-turns. In order to obtain a generator of good regulation, it is necessary to have the ampere-turns at no load upon the field magnet greater than the ampere-turns on the armature. Where the iron body of the pole and the iron core of the armature offer very little magnetic 324 WALKER : DESIGN OF TURBO FIELD MAGNETS [March 10th, reluctance, it is necessary to introduce into the magnetic circuit some part, such as a large air-gap, which will offer a large reluctance, and enable the ampere-turns on the field magnet at no load to be of the required amount.

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