DYNAMO yoke on either side of the armature to be pushed inwards until it nearly meets the armature on a horizontal diameter, and the direction of winding of one coil to be changed. Two salient poles and two consequent poles are thus obtained, and the machine is like that of Fig. 24, but with only two poles wound. The leakage across from pole to pole is great in a four-pole magnet of this type,
Fig. 25.
Fig. 26.
but in alternators where a large number of poles are required it may be convenient to wind only half their number, owing to their close proximity to one another. The multipolar equivalent of Fig. 21 is shown in Fig. 27, and again the weight of copper on the divided magnetic circuits must be greater than in the preceding multipolar magnets. Fig. 28, which is best suited to four
poles, may be obtained from Fig. 22 by dividing the pole-pieces and reversing the direction of winding on one side. The two single horse-shoe magnets thus formed have the peculiarity that half of the flux passing from one pole returns at once through the
a high value the ampere-turns are very largely increased owing to its approaching saturation, and this implies either a large amount of copper in the field coils or an undue expenditure of electrical energy in their excitation. Flence there is a limit imposed by practical considerations to the density at which the magnet should be worked, and this limit may be placed at about B = 16,000 for wrought-iron or steel, and at half this value for cast-iron. For a given flux, therefore, the cast-iron magnet must have twice the sectional area and be twice as heavy, although this disadvantage is partly compensated by its greater cheapness. If, however, castiron be used for the portion of the magnetic circuit which is covered with the exciting coils, the further disadvantage must be added that the weight of copper on the field-magnet is much increased, so that it is usual to employ forgings or cast-steel for the magnet cores on which the coils are wound. If weight is not a disadvantage, a cast-iron yoke may be combined with the wrought-iron or caststeel magnet cores. An absence of joints in the magnetic circuit is only desirable from the point of view of economy of expense in machining the component parts during manufacture ; when the surfaces which abut against each other are drawn firmly together
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by screws, the want of homogeneity at the joint, which virtually amounts to the presence of a very thin film of air, produces little or no effect on the total reluctance by comparison with the very much longer air-gaps surrounding the armature. In order to reduce the eddy-currents in the pole-pieces, due to the use of toothed armatures with relatively wide slots, the pole - pieces must be laminated in the same direction as the armature core is laminated ; the poles are then built up of thin sheets of charcoal iron or mild steel, and these are sometimes fixed by casting them into the outer yoke-ring. However it be built up, the mechanical strength of the magnet system must be carefully considered. Any two surfaces between which there exists a field of density B3 experience a force tending to draw them together proportional to the square of the density. B2 and having a value of ^ ^q(; R* Per square inch of surface, over which the density may be regarded as having the uniform value B,,. Hence, quite apart from the torque with which the stationaiy part of the dynamo tends to turn with the rotating part as soon as current is taken out of the armature, there exists a force tending to make the pole-pieces close on the armature as soon as the field is excited. Since both armature and magnet must be capable of resisting this force, they require to be rigidly held ; although the one or the other must be capable of rotation, there should otherwise be no possibility of one part of the magnetic circuit shifting relatively to any other part. An important conclusion may be drawn from this circumstance. If the armature be placed exactly concentric within the bore of the poles, and the two or more magnetic fields be symmetrical about a line joining their centres, there is no tendency for the armature core to be drawn in one direction more than in another ; but if there is any difference between the densities of the several fields, it will cause an unbalanced stress on the armature and its shaft, under which it will bend, and as this bending is continually reversed relatively to the fibres of the shaft, they will eventually become weakened and give way. Especially is this likely to take place in dynamos with short air-gaps, wherein any difference in the lengths of the air-gaps produces a much greater percentage difference in the flux-density than in dynamos with long air-gaps. In toothed armatures with short air-gaps the shaft must on this account be of great stiffness ; even when the shaft is sufficiently strong to withstand the stress without appreciable bending, any unbalanced pull on the armature as a whole should be avoided, since it may produce greater friction in the bearings, and cause them to develop an undue amount of heat. Reference has already been made to the importance in dynamo design of the predetermination of the flux due to a given number of ampere-turns wound on the field-magnet, or, conversely, of the number of ™agnetlc ampere-turns which must be furnished by the circuit. exciting coils in order that a certain flux corresponding to one field may flow through the armature core from each pole. An equally important problem is the correct proportioning of the field-magnet, so that the useful flux 7ja may be obtained with the greatest economy in materials and exciting energy. The key to the two problems is to be found in the concept of a magnetic circuit as originated by Rowland and Bosanquet;1 and the full solution of both may be especially connected with the name of the late Dr J. Hopkinson, from his practical application of the concept in his design of the EdisonHopkinson machine, and in his paper on “ DynamoElectric Machinery.”2 The publication of this paper in 1886 begins the second era in the history of the dynamo; it at once raised its design from the level of empirical rules - of - thumb to a science, and is thus worthy to be ranked as the necessary supplement of the original discoveries of Faraday. The process of predetermining the necessary ampere-turns is described in a simple case under Electromagnet. In its extension to the complete dynamo, it consists merely in the division of the magnetic 1
And extended by Kapp, “ On Modern Continuous - Current Dynamo-Electric Machines,” Proc. Inst. C.E. vol. Ixxxiii. p. 136. 2 Drs J. and E. Hopkinson, “Dynamo-Electric Machinery,” Phil. Trans., May 6, 1886 ; this was further expanded in a second paper on “ Dynamo-Electric Machinery,” Proc. Roy. Soc., Feb. 15, 1892, and both are reprinted in Original Papers on Dynamo-Machinery and Allied Subjects. S. III.— 74
