Fig. 73.

Organs Of Dynamos As Constructed In Practice Part  30076

It is worth while to mention the peculiarity of form of the Burgin armature, consisting of 8, or, in the newest machines constructed by Crompton, of 10 rings, set side by side. Each ring is made of a hexagonal coil of iron wire, mounted upon light metallic spokes, which meet the corners of the hexagon. Over this hexagonal frame, 6 coils of covered copper wire are wound, being thickest at the 6 points intermediate between the spokes, thus making up the form of each ring to nearly a circle. Each of the 6 coils is separated from its neighbour, and each of the 10 rings is fixed to the axis 1/60 of the circumference in advance of its neighbour, so that the 60 separate coils are in fact arranged equidistantly (and symmetrically, as viewed from the end) around the axis. There is a 60 - part collector, each bar of which is connected to the end of one coil and to the beginning of the coil that is 1/60 in advance; that is, to the corresponding coil of the next ring. This armature has the great practical advantages of being easy in construction, light, and with plenty of ventilation.

In the Elphinstone - Vincent dynamo is a drum - armature of a somewhat distinct order, the separate coils being made of a rectangular form, and then laid upon the sides of a hollow papier - mache drum in an overlapping manner, and curved to fit it. The field is complex, with 6 external and 6 internal poles, and is very intense, owing to the proximity of these poles. The parallelogram - shaped coils are connected together so as to work as 3 machines, and feed 3 pairs of brushes; which may again be united, either in series or in parallel, or may be used to feed 3 separate circuits.

Collectors

On p. 123 the main points to be observed in the construction of collectors are enumerated. Collectors of such a type are common to all dynamos of the first class, except the Brush, in which there is a multiple commutator, instead of a collector. The collector of Pacinotti's early machine differed only in having the separate bars alternately a little displaced longitudinally along the cylinder, but still so that the same brush could slip from bar to bar. Niaudet's modification, in which the bars are radially attached to a disk, is a mere variety in detail, and is not justified by successful adoption. In the collector used in Weston's dynamo, and in some forms of Schuckert's, the bars are oblique or curved, without any other effect than that of prolonging the moment during which the brush, while slipping from contact with one bar to contact with the next, short - circuits one section of the coil.

In a well - arranged dynamo of the first class, the sections of the collector are traversed by currents, which run from the negative brush in 2 directions round the successive coils, and meet at that bar of the collector which touches the positive brush. Each section of the coil thus traversed adds its own electromotive force to the current passing through it. Consequently, on measuring the difference of potential between the negative brush and the successive bars of the collector, it is seen that the potential increases regularly all the way round the collecting cylinder, in both directions, becoming a maximum at the opposite side where the positive brush is. This can be verified by connecting one terminal of a voltmeter to the negative brush, and touching the rotating collector at different points of its circumference with a small metallic brush or spring attached by a wire to the other terminal of the voltmeter. If the indications thus obtained are plotted out round a circle corresponding to the circumference of the collector, the values give a curve like that shown in Fig. 74, from observations upon a Gramme dynamo.

It can be seen that, taking the negative brush as the lowest point of the circle, the potential rises regularly to a maximum at the positive brush. The same values are also plotted out as or - dinates upon a horizontal line in Fig. 75. This form of diagram shows very clearly that the rise of potential is not equal between each pair of bars, otherwise the curve would consist merely of 2 oblique straight lines, sloping right and left from the central point. On the contrary, there is very little difference of potential between the collector bars close to the + brush on its right and left respectively. The greatest difference of potential occurs where the curve is steepest, at a position nearly 93° from the brushes; in fact, at that part of the circumference of the collector which is in connection with the coils that are passing through the position of best action. Were the field perfectly uniform, the number of lines of force that pass through a coil ought to be proportional to the sine of the angle which the plane of that coil makes with the resultant direction of the lines of force in the field, and the rate of cutting the lines of force should be proportional to the cosine of this angle.

Now, the cosine is a maximum when this angle = 0°; hence, when the coil is parallel to the lines of force, or at 90° from the brushes, the rate of increase of potential should be at its greatest - as is very nearly realized in the diagram of Fig. 73, which, indeed, is very nearly a true " sinusoidal " curve. Such curves, plotted out from measurements of the distribution of potential at the collector, show not only where to place the brushes to get the best effect, but enable us to judge of the relative " idleness " or " activity " of coils in different parts of the field, and to gauge their actual intensity while the machine is running. If the brushes are badly set, or if the pole - pieces are not judiciously shaped, the rise of potential will be irregular, and there will be maxima and minima of potential at other points. An actual diagram, taken from a dynamo in which these arrangements were faulty, is shown in Fig. 76, and plotted horizontally in Fig. 77; from these it will be seen, not only that the rise of potential was irregular, but that one part of the col lector was more positive than the positive brush, and another part more ne gative than the negative.