The machine is very nearly self-regulating in itself. It is therefore not considered necessary or desirable, except under special circumstances, to provide other than a simple hand-regulation at the dynamo for the purpose of controlling the potential difference. By the arrangement of the armature-coils it is easy to obtain various combi nations if desired. This circumstance is made use of for simplifying the measurement of the potential. Instead of taking the reading across the terminals, which would necessitate the use of an electrometer, or a very high resistance voltmeter, an ordinary voltmeter, indicating to 100 or 150 volts, is placed across one of the coils, the machine being fitted with a special pair of voltmeter terminals for this purpose. The indication thus obtained, multiplied by the total number of armature-coils, gives the potential difference between the arma ture terminals. The voltmeter may, if desired, have its dial marked to directly in dicate the total potential difference. It is said that this dynamo of 50 to 60 horse power—the first of its type—was built di rectly from the first design, without re course to any preliminary experiments. The machine illustrated has an output of 40,000 watts, at a potential of 2,000 volts, and a speed of 650 revolutions per min. In the latest type of this machine a separate exciter for the revolving field-magnets is employed, coupled directly to the same shaft.
The Westinghouse Alternating-Current Arc - Light (Constant - Current) Dynamo (Fig. 98) resembles very closely the West inghouse alternating incandescent machine in outward appearance, but its operation is quite distinct from the former, and is effected by the peculiar construction of the armature, which is designed so as to cause the machine to deliver a current of constant strength at all loads automatically. The armature is shown in perspective in Fig. 99, and in longitudinal and transverse sections in Figs. 100 and 101. It will be noted that the armature-coils are not, as in the incandescent machine, in the shape of flat coils placed on the periphery, hut consist of oblong coils, which are wound separately, as shown in Fig. 102, and then by means of the clamping-tool (Fig. 103) are clamped in position around the cores of the armature-projectinns, which are provided with overlapping teeth. After the coils have been placed in position the spaces between the teeth are filled out with wooden wedges, which are dovetailed and slid in from the side, so that no further fastening is required to keep the coils or the wedges themselves in position. The peculiar construction of the armature with the overlapping teeth has the effect of maintaining the current constant at all loads, so that there is no regu lating apparatus whatever required for that purpose. The armature is built up of thin
wrought-iron sheets stamped out to the required shape, and the teeth are so designed, and are of such length, that they slightly overlap the distance between two consecutive pole-pieces, so that one tooth is not out of the field of any one magnet before another enters that field.
At the side of the dynamo in Fig. 98 there will be noted an apparatus consisting of a solenoid with a single core. It is a short-circuiting apparatus, and its object is to protect the machine from the results of a break in the line. In the continuous-current machine a break in the line is generally followed by the cessation of current in the armature when a series machine is employed ; but in this system a break in the external circuit causes the generation of a heavy current in the armature, which, if not prevented, would eventually cause its destruction. To avoid this, the apparatus shown is employed, and its function is to short circuit the armature. By this means the counter-electromotive force generated in the arma ture is such as to cut down the heavy current to the normal strength, so that no dangerous heating of the armature-coils can take place: The apparatus is so constructed that an excess of electromotive force generated in the armature, such as would be caused by a break in the line, causes a spark to pass between two points ; this allows sufficient current tq pass to ener gize the solenoid, which pulls up its core and makes contact between two points that short circuit the armature. Normally, the distance apart of these points is so regulated that any excess of current beyond 12 amperes causes the spark to jump and effect the short circuiting.
There is still another safety device of the same nature, which consists of two metal points placed opposite each other on the armature-shaft, and connected respectively to the collector rings. Upon the current exceeding a certain value the spark formed between these two points causes a short-circuiting of the machine, and a consequent cutting down of the current clue to the increased self-induction.
The machines built vary in capacity from 25 lights to 240 lights, and the table shows their various sizes and capacity. It will be noted that the speeds of these machines are consider ably lower than those employed in the incandescent system, and that the number of alterna tions per second is also far below that of the former, the average number approximating 7,500 alternations per min., as compared with 16,000 for the incandescent machines. The machines Nos. 2 and 3 are provided with two sets of windings connected to two pairs of collectors, so that two independent circuits can be run from one machine.