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Z0000

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Originally appearing in Volume V09, Page 232 of the 1911 Encyclopedia Britannica.
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Z0000 16000 e 16000 .0l .4000 4yf /2000 10000 2000 2000 4000 2000 H 0 t0 20 30 40 50 60 70 FIG. 3. form of rings by the ballistic method, the rings of sheet-metal being stamped or turned in the flat. The wire ring No. X. was coiled and annealed after coiling. Magnetic VII. VIII. IX. X Flux Density B Transformer- Transformer- Transformer- Transformer- Units). plate plate of plate of plate of wire. Swedish Iron. Scrap Iron. Steel. H µ H H A H µ 1,000 o•81 1230 1.08 920 0.60 1470 1.71 590 2,000 1.05 1900 1.46 1370 0.90 2230 2.10 950 3,000 1.26 2320 1.77 1690 1.04 2880 2.30 1300 4,000 1.54 2600 2.10 1900 1.19 3360 2.50 1600 5,000 1.82 2750 2.S3 1980 1.38 3620 2.70 1850 6,000 2.14 2800 3.04 1970 1'59 3770 2.92 2070 7,000 2.54 2760 3.62 1930 1.89 3700 3.16 2210 8,000 3'09 2590 4'37 1830 2.25 3600 3'43 2330 9,000 3'77 2390 5.3 1700 2.72 3310 3'77 2390 10,000 4.6 2170 6.5 1540 3'33 3000 4.17 2400 11,000 5'7 1930 7'9 1390 4.15 2650 4.70 2340 12,000 7.0 1710 9.8 1220 5.40 2220 5.45 2200 13,000 8.5 1530 11.9 1190 7.1 1830 6.5 2000 14,000 11.0 1270 15.0 930 I0.0 1400 8.4 1670 15,000 15'1 990 19.5 770 .. .. 11.9 1260 16,000 21.4 750 27.5 580 .. .. 21.0 760 Some typical flux-density curves of iron and steel as used in dynamo and transformer building are given in fig. 4. The numbers in Table III. well illustrate the fact that the permeability µ=B/H has a maximum value corresponding to a certain flux density. The tables are also explanatory of the factthat mild steel has gradually replaced iron in the manufacture of dynamo electromagnets and transformer-cores. Broadly speaking, the materials which are now employed in the manufacture of the cores of electromagnets for technical purposes of various kinds may be said to fall into three classes, namely, forgings, castings and stampings. In some eases the iron or steel core which is to be magnetized is simply a mass of iron hammered or pressed into shape by hydraulic pressure; in other cases it has to be fused and cast; and for certain other purposes it must be rolled first into thin sheets, which are subsequently stamped out into the required forms. For particular purposes it is necessary to obtain the highest possible magnetic permeability corresponding to a high, or the highest attainable flux density. This is generally the case in the electromagnets which are employed as the field magnets in dynamo machines. It may generally be said that whilst the best wrought iron, such as annealed Low Moor or Swedish iron, is more permeable for low flux densities than steel castings, the cast steel may surpass the wrought metal for high flux density. For most electro-technical purposes the best magnetic results are given by the employment of forged ingot-iron. This material is probably the most permeable throughout the whole scale of attainable flux densities. It is slightly superior to wrought iron, and it only becomes inferior to the highest class of cast steel when the flux density is pressed above i8,000 C.G.S. units (see fig. 5). For flux densities above 13,000 the forged ingot-iron Permea0//qty Carves of /to and Steel moo ;~ =_~~^~~1 moo 1700 AlIG ~-~l11 1600 isoo- I1 'MIMI I400 ^ 0 ~i•d~_~ z0 noo ~MIMII•1 loo. 900. eoo ~~ j____ 600 t\ 0 1111~~l1• 400 _ , ^ik \_U 300 -^ 111111\~~ zoo II ~UUI1W 1I ig m.l $% a!,gas Mayn4Pic flux 0ensily Or induction B C 0441nitl has now practically replaced for electric engineering purposes the Low Moor or Swedish iron. Owing to the method of its production, it might in truth be called a soft steel with a very small percentage of combined carbon. The best description of this material is conveyed • by the German term " Flusseisen," but its nearest British equivalent is " ingot-iron." Chemically speaking, the material is for all practical purposes very nearly pure iron. The same may be said of the cast steels now much employed for the production of dynamo magnet cores. The cast steel which is in demand for this purpose has a slightly lower permeability than the ingot-iron for low flux densities, but for flux densities above i6,000 the required result may be more cheaply obtained with a steel casting than with a forging. When high tensile strength is required in addition to considerable magnetic permeability, it has been found advantageous to employ a steel containing 5 % of nickel. The rolled sheet iron and sheet steel which is in request for the construction of magnet cores, especially those in which the exciting current is an alternating current, are, generally speaking, produced from Swedish iron. Owing to the mechanical treatment necessary to reduce the material to a thin sheet, the permeability at low flux densities is rather higher than, although at high flux densities it is inferior di 0 h so !o loo ^^^^^^^^^^s^~^^p!m:=a-4aa&G SN; ..^^^^^^^PEMEE EEi0iiir^J^^^^^^^^ ...^^^/S:/iCGi^^^^^^^^^^^^^^^^^^ ,, ^^M Ers0^^^^^^^^^^^^^^^^^^^^^^^ c, ^74^~^^^^^^^^^^^^^^^^^^^^^^^^^^ m,.00^IIFI^^^^ RTMEfl MMIT122TI 7^^^^^^ ,....^'!%^^^^^i UZZOSZIMf iLUIZZL7^^^^^^^ 11f1L^^^^^^^^^^^^^^^^^^^^^^^^^EMR ,I.. i,,N/^^^^^^^^^^^^^u.uu^^ M =ISM ...011 IEWIMI^^^^NIE^^EMMS^_^^^^^^^^^^ - $•... .IIllIMINE ^^ ^!%~C^^^^^^^^^^^^^^^^ oo^^^^O-a/MrNMEMEM NMESU^^^^^^^ 0 IINEEMCA^^^^rCISI~crrT!rl~!:nr.TIME RTa^^^^ EpN^^^^^EEMCIMSS"'.~i'1MMME'TZ I k . „fN^A^^^^^^^EMmPnl -EfTl4l1~'df'~'~fL1171r3.Ral7^TM f ...INI//^^^^^^^^[ EEEMIMEE ~^^^^^i ,... dlll^^^^^^^^^^^^^^^^^^^ ^^^^^^•^ ".0Cli^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ..u^^^^^^^~sl 1METIMM «eNI^M^^^^^^ q t 0. 90 ,0,Y ~Zc /70000.0.0. to, the same iron and steel when tested in bulk. For most I TABLE VII.-Observations on the Magnetic Hysteresis of Cast Iron. purposes, however, where a laminated iron magnet core is required, the flux density is not pressed up above 6000 units, and it is then more important to secure small hysteresis loss than high permeability. The magnetic permeability of cast iron is much inferior to that of wrought or ingot-iron, or the mild steels taken at the same flux densities. The following Table IV. gives the flux density and permeability of a typical cast iron taken by J. A. Fleming by the ballistic method:- For most practical purposes the constructor of electromagnetic machinery requires his iron or steel to have some one of the following characteristics. If for dynamo or magnet making, it should have the highest possible permeability at a flux density corresponding to practically maximum magnetization. If for trans-former or alternating-current magnet building, it should have the smallest possible hysteresis loss at a maximum flux density of 2500 C.G.S. units during the cycle. If required for permanent magnet making, it should have the highest possible coercivity combined with a high retentivity. Manufacturers of iron and steel are now able to meet these demands in a very remarkable manner by the commercial production of material of a quality which at one time would have been considered a scientific curiosity. It is usual to specify iron and steel for the first purpose, by naming the minimum permeability it should possess corresponding to a flux density of 18,000 C.G.S. units; for the second, by stating the hysteresis loss in watts per lb per too cycles per second, corresponding to a maximum flux density of 2500 C.G.S. units during the cycle; and for the third, by mentioning the coercive force required to reduce to zero magnetization a sample of the metal in the form of a long bar magnetized to a stated magnetization. In the cyclical reversal of magnetization of iron we have two modes to consider. In the first case, which is that of the core of the alternating transformer, the magnetic force passes through a cycle of values, the iron remaining stationary, and the direction of the magnetic force being always the same. In the other case, that of the dynamo armature core, the direction of the magnetic force in the iron is constantly changing, and at the same time undergoing a change in magnitude. It has been shown by F. G. Baily (Prot. Roy. Soc., 1896) that if a mass of laminated iron is rotating in a magnetic field which remains constant in direction and magnitude in any one experiment, the hysteresis loss rises to a maximum as the magnitude of the flux density in the iron is increased and then falls away again to nearly zero value. These observations have been confirmed by other observers. The question has been much debated whether the values of the hysteresis loss obtained by these two different methods are identical for magnetic cycles in which the flux density reaches the same maximum value. This question is also connected with another one, namely, whether the hysteresis loss per cycle is or is not a function of the speed with which the cycle is traversed. Early experiments by C. P. Steinmetz and others seemed to show that there was a difference between slow-speed and high-speed hysteresis cycles, but later experiments by J. Hopkinson and by A. Tanakadate, though not absolutely exhaustive, tend to prove that up to 400 cycles per second the hysteresis loss per cycle is practically unchanged. Experiments made in 1896 by R. Beattie and R. C. Clinker on magnetic hysteresis in rotating fields were partly directed to determine whether the hysteresis loss at moderate flux densities, such as are employed in transformer work, was the same as that found by measurements made with alternating-current fields on the same iron and steel specimens (see The Electrician, 1896, Loop. B (max.) Hysteresis Loss. Ergs per cc. Watts per lb per . per Cycle. Too Cycles per see. I. 1475 466 .300 II. 2545 1.288 .829 IV. 5972 7.397 4'765 V. 8930 13.423 8.658 Cast Iron. H B H B µ H B .19 27 139 8'84 4030 456 44'65 8.071 181 •41 62 150, To•6o 4491 424 56'57 8.548 151 1.1I 206 176 12.33 4884 396 71.98 9.097 126 2.53 768 303 13'95 5276 378 88'99 9.600 To8 3.41 1251 367 15.61 5504 353 106.35 Io,o66 95 4.45 1898 427 18.21 5829 320 120.60 10.375 86 5'67 2589 456 26.37 6814 258 140.37 10.725 76 7.16 3350 468 36'54 7580 207152.73 10.985 72 The metal of which the tests are given in Table IV. contained 2% of silicon, 2.85% of total carbon, and o.5 % of manganese. It will be seen that a magnetizing force of about 5 C.G.S. units is sufficient to impart to a wrought-iron ring a flux density of 18,000 C.G.S. units, but the same force hardly produces more than one-tenth of this flux density in cast iron. The testing of sheet iron and steel for magnetic hysteresis loss has developed into an important factory process, giving as it does a means of ascertaining the suitability of the metal for use in the manufacture of transformers and cores of alternating-current electromagnets. In Table V. are given the results of hysteresis tests by Ewing on samples of commercial sheet iron and steel. The numbers VII., VIII., IX. and X. refer to the same samples as those for which permeability results are given in Table III. Maxi- Ergs per Cubic Centimetre Watts per lb at a Frequency per Cycle. of Too. mum Flux Density VII. VIII. IX. X. VII. VIII. IX. X. Forged Soft B. Swedish Scrap- Ingot- Iron Iron iron steel. Wire. 2000 240 400 215 600 0.141 0.236 0.127 0.356 3000 520 790 430 1150 0.306 0.465 0.253 0.630 4000 830 1220 700 1780 0.490 0.720 0.410 1.050 5000 1190 1710 woo 2640 0700 1.010 0.590 1.550 6000 1600 2260 1350 3360 0.940 1.330 0.790 1.980 7000 2020 2940 1730 4300 1.200 1.730 1.020 2.530 8000 2510 3710 2150 5300 1.480 2.180 1.270 3.120 9000 3050 456o 2620 638o 1.800 2.68o 1.540 3'750 Swedish Iron. Maximum Flux Ergs per Cubic Centimetre Watts per lb at a Density B. per Cycle. Frequency of Iola. 2000 220 0.129 3000 410 0.242 4000 64o 0.376 5000 910 0.535 6000 1200 0.710 7000 1520 0.890 8000 1900 I•I20 9000 2310 1.360 In Table VII. are given some values obtained by Fleming for the hysteresis loss in the sample of cast iron, the permeability test of which is recorded in Table IV. In Table VI. are given the results of a magnetic test of some exceedingly good transformer-sheet rolled from Swedish iron. 37, p. 723). These experiments showed that over moderate ranges of induction, such as may be expected in electro-technical work, the hysteresis loss per cycle per cubic centimetre was practically the same when the iron was tested in an alternating field with a periodicity of Too, the field remaining constant in direction, and when the iron was tested in a rotating field giving the same maximum flux density. With respect to the variation of hysteresis loss in magnetic cycles having different maximum values for the flux density, Steinmetz found that the hysteresis loss (W), as measured by the area of the complete (B, H) cycle and expressed in ergs per centimetre-cube per cycle, varies proportionately to a constant called the hysteretic constant, and to the 1.6th power of the maximum flux density (B), or W=71 B1•6. The hysteretic constants (rl) for various kinds of iron and steel are given in the table below: Metal. Hysteretic Constant. Swedish wrought iron, well annealed . •0010 to •0017 Annealed cast steel of good quality ; small percentage of carbon . . •0017 to •0029 Cast Siemens-Martin steel . . •0019 to •0028 Cast ingot-iron •0021 tO •0026 Cast steel, with higher percentages of •0031 to •0o54 carbon, or inferior qualities of wrought iron Steinmetz's law, though not strictly true for very low or very high maximum flux densities, is yet a convenient empirical rule for obtaining approximately the hysteresis loss at any one maximum flux density and knowing it at another, provided these values fall within a range varying say from 1 to 9000 C.G.S. units. (See MAGNETISM.) The standard maximum flux density which is adopted in electro-technical work is 2500, hence in the construction of the cores of alternating-current electromagnets and transformers iron has to be employed having a known hysteretic constant at the standard flux density. It is generally expressed by stating the number of watts per lb of metal which would be dissipated for a frequency of loo cycles, and a maximum flux density (B max.) during the cycle of 2500. In the case of good iron or steel for transformer-core making, it should not exceed 1.25 watt per lb per too cycles per 2500 B (maximum value). It has been found that if the sheet iron employed for cores of alternating electromagnets or transformers is heated to a temperature somewhere in the neighbourhood of 2000 C. the hysteresis loss is very greatly increased. It was noticed in 1894 by G. W. Partridge that alternating-current transformers which had been in use some time had a very considerably augmented core loss when compared with their initial condition. O. T. Blathy and W. M. Mordey in 1895 showed that this augmentation in hysteresis loss in iron was due to heating. H. F. Parshall investigated the effect up to moderate temperatures, such as 14o° C., and an extensive series of experiments was made in 1898 by S. R. Roget (Prot. Roy. Soc., 1898, 63, p. 258, and 64, p. 150). Roget found that below 4o° C. a rise in temperature did not produce any augmentation in the hysteresis loss in iron, but if it is heated to between 4o° C. and 135° C. the hysteresis loss increases continuously with time, and this increase is now called " ageing " of the iron. It proceeds more slowly as the temperature is higher. If heated to above 135° C., the hysteresis loss soon attains a maximum, but then begins to decrease. Certain specimens heated to 16o° C. were found to have their hysteresis loss doubled in a few days. The effect seems to come to a maximum at about 18o° C. or zoo° C. Mere lapse of time does not remove the increase, but if the iron is reannealed the :augmentation in hysteresis disappears. If the iron is heated to a higher temperature, say between 300° C. and 700° C., Roget found the initial rise of hysteresis happens more quickly, but that the metal soon settles down into a state in which the hysteresis loss has a small but still augmented constant value. The augmentation in value, however, becomes more nearly zero as the temperature approaches 700° C. Brands of steel are now obtainable which do not age in this manner, but these non-ageing varieties of steel have not generally such low initial hysteresisvalues as the " Swedish Iron," commonly considered best for the cores of transformers and alternating-current magnets. The following conclusions have been reached in the matter:—(T) Iron and mild steel in the annealed state are more liable to change their hysteresis value by heating than when in the harder condition; (2) all changes are removed by re-annealing; (3) the changes thus produced by heating affect not only the amount of the hysteresis loss, but also the form of the lower part of the (B,H) curve. Forms of Electromagnet.—The form which an electromagnet must take will greatly depend upon the purposes for which it is to be used. A design or form of electromagnet which will be very suitable for some purposes will be useless for others. Supposing it is desired to make an electromagnet which shall be capable of undergoing very rapid changes of strength, it must have such a form that the coercivity of the material is overcome by a self-demagnetizing force. This can be achieved by making the magnet in the form of a short and stout bar rather than a long thin one. It has already been explained that the ends or poles of a polar magnet exert a demagnetizing power upon the mass of the metal in the interior of the bar. If then the electromagnet has the form of a long thin bar, the length of which is several hundred times its diameter, the poles are very far removed from the centre of the bar, and the demagnetizing action will be very feeble; such a long thin electromagnet, although made of very soft iron, retains a considerable amount of magnetism after the magnetizing force is withdrawn. On the other hand, a very thick bar very quickly demagnetizes itself, because no part of the metal is far removed from the action of the free poles. Hence when, as in many telegraphic instruments, a piece of soft iron, called an armature, has to be attracted to the poles of a horseshoe-shaped electromagnet, this armature should be prevented from quite touching the polar surfaces of the magnet. If a soft iron mass does quite touch the poles, then it completes the magnetic circuit and abolishes the free poles, and the magnet is to a very large extent deprived of its self-demagnetizing power. This is the explanation of the well-known fact that after exciting the electromagnet and then stopping the current, it still requires a good pull to detach the " keeper "; but when once the keeper has been detached, the magnetism is found to have nearly disappeared. An excellent form of electromagnet for the production of very powerful fields has been designed by H. du Bois (fig. 6). Various forms of electromagnets used in connexion with It; dynamo machines are considered in the article DYNAMO, and there is, therefore, no necessity to refer particularly to the numerous different shapes and types employed in electrotechnics. H. du Bois, The Magnetic Circuit in Theory and Practice; S. P. Thompson, The Electromagnet; J. A. Fleming, Magnets and Electric Currents; J. A. Ewing, Magnetic Induction in Iron and other Metals; J. A. Fleming, " The Ferromagnetic Properties of Iron and Steel," Proceedings of Sheffield Society of Engineers and Metallurgists (Oct. 1897) ; J. A. Ewing, " The Magnetic Testing of Iron and Steel," Proc. Inst. Civ. Eng., 1896, 126, p. 185; H. F. Parshall, " The Magnetic Data of Iron and Steel," Proc. Inst. Civ. Eng., 1896, 126, p. 220; J. A. Ewing, " The Molecular Theory of Induced Magnetism," Phil. Mag., Sept. 189o; W. M. Mordey, " Slow Changes in the Permeability of Iron," Proc. Roy. Soc. 57, p. 224; J. A. Ewing, " Magnetism," James Forrest Lecture, Proc. Inst. Civ. Eng. 138; S. P. Thompson, Electromagnetic Mechanism," Electrician, 26, pp. 238, 269, 293; J. A. Ewing, " Experimental Researches in Magnetism," Phil. Trans., 1885, part ii.; Ewing and Klassen, " Magnetic Qualities of Iron," Proc. Roy. Soc., 1893. (J. A. F.)
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