ELECTRIC ALTERNATING CUR RENT MACHINERY. A loop of wire re volving in a magnetic field is the simplest form of an alternating current generator. The direc tion of induced electromotive force in the two halves of the loop, which cut the magnetic flux in opposite directions, is such that the combined electromotive force at the terminals is double that of either revolving conductor alone. This induced electromotive force is proportional to the rate of cutting the magnetic lines, and there fore to the sine of the angle by which the plane of the coil differs from the plane midway be tween the poles and normal to the magnetic flux. At its zero position, or when the planes coincide, the coil is cutting no lines of force and we have sine a= 0. The electromotive force, however, grows as we depart from this zero position, assuming uniform speed, until, when 90 degrees is reached the rate of cutting of the lines becomes a maximum, sine 90 degrees = 1. Passing on, the electromotive force dies away until 180 degrees is reached, when the value again is zero. From this to 270 degrees we have an increasing electromotive force, but of opposite polarity and at the end of the revolu tion, or 360 degrees, again reach zero. Thus we have in one revolution in a two-pole field two waves of pressure of the same form but of opposite sig i n. The one is called the posi tive wave and the other the negative. One such revolution, or one positive wave and one nega tive wave, constitute what is called a cycle, or period, which in technical literature is desig nated by the symbol ti =one sine wave. The great majority of systems have a frequency be tween 60 ‘•-•-, and 25 Both of these fre quencies are standard practice in this country, and the values between are chosen for special cases. Owing to the high frequency of commer cial systems, alternators are built with more than one pair of poles, in order to keep the re volving speed within reasonable limits.
Average and Effective Values.— If we plot the values of the instantaneous pressures as ordinates, with time as abscissa, we have a cor rect representation of the generation of alter nating currents, and the shape of the wave. When the total number of lines cut per revolu tion by a coil revolving at constant speed re mains the same, the average induced electro motive force remains constant, regardless of the distribution of the magnetic flux. The effective value, however — the value read by the metre and the value which corresponds in its heat ing effect to the direct current value — is not independent of this distribution.
The Place of Alternating Current Sys tems.— The direct current for the railway at 550 volts, and for the lighting and power systems of the densely populated centres of our large cities in the Edison three-wire system 110 to 220 volts, seems to have become standard practice.
Nevertheless the low radius of distribution with out excessive cost of copper, even in the 550 volt railway system with a grounded return, makes necessary a great multiplicity of moder ate-sized or small plants, operating at low effi ciency. It is here that the alternating current comes to the front. While commutators (q.v.) can be built for collecting direct current for 1,000 volts, alternators can be built for 12,000 volts and step-up transformers of h'gh economy are quite possible at 75,000 to 100,000 volts. Re membering that the copper cost is inversely as the square of the voltage, the great possibilities of the alternating current system are at once seen.
Energy from Waterfalls.—Electrical energy from waterfalls that a few years ago were merely points of scenic interest is now supplied to hundreds of cities in North America. There are numerous power plants of from 50,000 to 200,000 horse-power capacity, sending currents with voltage from 25,000 to 150,000 to distances up to 250 miles. See HYDRO-ELECTRIC DEVELOP MENT and ELECTRIC TRANSMISSION OF ENERGY.
The Alternator.— Small alternators and those of moderate potential usually collect their current from insulated rings mounted on the shaft and connected to the ends of the armature winding. Through brushes, the current is taken to the external circuit. In some machines a rec tifier is added for supplying sufficient undimen sional current to produce the necessary addi tional field to overcome the drop due to increase of load. All commercial alternators are sup plied with an exciter, or direct current dynamo, whose function is to supply current to the field windings. The field spools are usually con nected in series. The amount of current thus necessary on a full non-inductive load vanes from 11/2 to 3 per cent of the total output of the alternator. Owing to the difficulty of collecting large currents by means of brushes and of pre servmg good insulation between the rings and shaft, the revolving field type of machine is now used in almost all large installations, the field current from the exciter being supplied through cast-iron rings mounted on the shaft, or in the case of the inductor type, consisting of an annular ring surrounding the inductor or re volving element, which consists of laminated iron poles suitably spaced and keyed to the shaft. The windings being stationary, there are no moving connections, either for the field current or the main current of the machine. In either type the alternating current is taken from the terminals of the windings, usually at t,he bottom of the frame.