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 DYNAMO

589 which heat is generated in the coils, and k is some constant engine; or, if approaching the higher limit above mentioned, depending upon the exact method of reckoning their cooling it is often directly coupled to the crank-shaft of the steam surface. As a general rule the cooling surface of a field-coil is engine. If used in conjunction with an accumulator of reckoned as equal to the exposed outer surface of its wire, the influence of the end flanges being neglected, or only taken into secondary cells, it is shunt-wound, and must give the account in the case of very short bobbins wound with a consider- higher voltage necessary to charge the battery; otherwise able depth of wire. In the case of the rotating armature a it is compound-wound, in order to maintain the pressure on similar formula must be constructed, but with the addition of a factor to allow for the increase in the effectiveness of any given the lamps constant under all loads within its capacity. cooling surface due to the rotation causing convection currents in The compound-wound dynamo is likewise the most usual the surrounding air. Only experiment can determine the exact for the lighting of steamships, and is then directly coupled effect of this, and even with a given type of armature it is de- to its steam-engine; its output seldom exceeds 60 kilopendent on the number of poles, each of which helps to break up watts, at a voltage ranging from 60 to ICO. For centralthe air-currents, and so to dissipate the heat. For example, in two-pole machines with drum bar-armatures, if the cooling surface station work, economy in the distributing mains dictates a be reckoned as equal to the cylindrical exterior plus the area of the higher voltage, especially in connexion with a three-wire two ends, the heating coefficient for a. peripheral speed of 1500 system; the larger dynamos may then give 500 volts, and feet per minute is less than half of that for the same armature when at rest. A further difficulty still meets the designer in the be connected directly across the two outer wires. A pair correct predetermination of the total loss of watts in an armature of smaller machines coupled together, and each capable of before the machine has been tested. It is made up of three giving 250 volts, are often placed in series across the separate items, namely, the copper loss in the armature winding, the system, with their common junction connected to the loss by hysteresis in the iron, and the loss by eddy-currents, which again may be divided into those in the armature bars and end- middle wire; the one which at any time is on the side connexions, and those in the core and its end-plates. The two carrying the smaller current will act as a motor and drive latter items are both dependent upon the speed of the machine ; the other as a dynamo, so as to balance the system. The hut whereas the hysteresis loss is proportional to the speed for a steam dynamo may be said to have pracgiven density of flux in the armature, the eddy-current loss is directly-coupled proportional to the square of the speed, and owing to this differ- tically displaced the belt- or rope-driven sets which were ence, the one loss can be separated from the other by testing an formerly common in central stations. Though in Great armature at varying speeds. Thus for a given rise of tempera- Britain the bipolar machine has been generally retained ture, the question of the amount of current which can be taken for outputs up to 150 kilowatts, or even more, the tendency out of an armature at different speeds depends upon the proportion is towards the multipolar machine, which is largely used on which the hysteresis and eddy watts bear to the copper loss, and the ratio in which the effectiveness of the cooling surface is altered the Continent and in the United States, even for comparaby the alteration in speed. Experimental data, again, can alone tively small outputs. The generating units of the central decide upon the amount of eddy-currents that may be expected in station are arranged in progressive sizes, rising from, it given armatures, and caution is required in applying the results may be, 250 or 500 horse-power up to 750 or 1000, or of one machine to another in which any of the conditions, such as the width of the inductors, the number of poles, or the extent of in large towns to as much as 5000 horse-power. They lamination of the core, are altered. are usually shunt-wound, the regulation of the voltage, It remains to add, that the rise of temperature which may be to keep the pressure constant on the distributing system permitted in any part of a dynamo after a prolonged run is very generally placed at about 70° Fahr. above the surrounding air. under the gradual changes of load, being effected by Such a limit in ordinary conditions of working leads to a final variable resistances in the shunt circuit of the fieldtemperature of about 170° Fahr., beyond which the durability of magnets. the insulation of the wires is liable to be injuriously affected. Generators used for supplying current to electric tramUpon some such basis the output of a dynamo in continuous work- ways are commonly wound for 500 volts at no load and ing is rated, although for short periods of, say, two hours the are over-compounded, so that the voltage rises to 550 volts normal full-load current of a large machine may be exceeded by at the maximum load, and thus compensates for the loss of some 25 per cent, without unduly heating'the armature. volts over the transmitting lines. Since the changes of For the electro-deposition of metals or the electrolytic load are very rapid, it would not be possible to shift treatment of ores a continuous current is a necessity; but, copper gauze brushes so that they should always be at the Uses of apart from such use, the purposes for which exact position to avoid sparking; hence carbon brushes coatinu- the continuous-current dynamo is well adapted are employed. For arc lighting it was formerly usual to ous-curren tare so numerous that they cover nearly the employ a class of dynamo which, from the nature of its yaamos. w}10}e field 0f electrical engineering, with one construction, was called an “open-coil” machine, and important exception. To meet these various uses, the which gave a unidirected but pulsating current. Of such pressures for which the machine is designed are of machines the Brush and Thomson-Houston types were equally wide range; for the transmission of power over very widely used; their E.M.F. ranged from 2000 to long distances they may be as high as 3000 volts, and for 3000 volts for working a large number of arcs in series, electrolytic work as low as five. Each electrolytic bath, and by means of special regulators their current was mainwith its leads, requires on an average only some four or five tained constant over a wide range of voltage. But as volts, so that even when several are worked in series the their efficiency was low and they could not be applied to voltage of the dynamo seldom exceeds 60. On the other any other purpose, they have been largely superseded in hand, the current is large and may amount to as much central stations by closed-coil dynamos or alternators, which as from 1000 to 14,000 amperes, necessitating the use of can also be used for incandescent lighting. In cases where two commutators, one at either end of the armature, in the central station is situated at some distance from the order to collect the enormous current without excessive district to which the electric energy is to be supplied, heating of the sectors and brushes. The field-magnets are voltages from 1000 to 2000 are employed, and these are invariably shunt-wound, in order to avoid reversal of the transformed down at certain distributing centres by concurrent through polarization at the electrodes of the bath, tinuous-current transformers (see Transfobmers and lor incandescent lighting by glow lamps, the requirements Electricity Supply). These latter machines are in of small isolated installations and of central stations for reality motor-driven dynamos, and hence are also called the distribution of electrical energy over large areas must “ motor-generators ”; the armatures of the motor and be distinguished. For the lighting of a private house or dynamo are often wound on the same core, with a comlarge factory, the dynamo giving from 5 to 100 kilo- mutator at either end, the one to receive the high-pressure watts of output is commonly wound for a voltage of 100, motor current, and the other to collect the low-pressure and is driven by pulley and belt from a gas, oil, or steam current furnished by the dynamo.