This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
In the instruction paper on "Power Stations and Transmission," a method is given for calculating alternating-current lines by means of formulae, and data are given regarding power factor and the calculation of both single-phase and polyphase circuits. For short lines, secondary wiring, etc., however, it is probably more convenient to use the chart method devised by Mr. Ralph D. Mershon, described in the American Electrician of June, 1897, and partially reproduced as follows:
When alternating currents first came into use, when transmission distances were short and the only loads carried were lamps, the question of drop or loss of voltage in the transmitting line was a simple one, and the same methods as for direct current could without serious error be employed in dealing with it. The conditions existing in alternating practice to-day - longer distances, polyphase circuits, and loads made up partly or wholly of induction motors - render this question less simple; and direct-current methods applied to it do not lead to satisfactory results. Any treatment of this or of any engineering subject, if it is to benefit the majority of engineers, must not involve groping through long equations or complex diagrams in search of practical results. The results, if any, must be in available and convenient form. In what follows, the endeavor has been made to so treat the subject of drop in alternating-current lines that if the reader be grounded in the theory the brief space devoted to it will suffice; but if he do not comprehend or care to follow the simple theory involved, he may nevertheless turn the results to his practical advantage.
Calculation of Drop. Most of the matter heretofore published on the subject of drop treats only of the inter-relation of the E. M. F.'s involved, and, so far as the writer knows, there have not appeared in convenient form the data necessary for accurately calculating this quantity. Table X (page 47) and the chart (page 46) include, in a form suitable for the engineer's pocketbook, everything necessary for calculating the drop of alternating-current lines.
The chart is simply an extension of the vector diagram (Fig. 40). giving the relations of the E. M. F.'s of line, load and generator. In electric wiring due to the impedance of the line. The component c is made up of two components at right angles to each other. One is a, the component overcoming the IR or back E. M. F. due to resistance of the line. The other is b, the component overcoming the reactance E. M. F. or back E. M. F. due to the alternating field set up around the wire by the current in the wire. The drop is the difference between E and e. It is d, the radial distance between two circular arcs, one of which is drawn with a radius e, and the other with a radius E. The chart is made by striking a succession of circular arcs with Fig. 40, E is the generator E. M. F.; e, the E. M. F. impressed upon the load; c, that component of E which overcomes the back E. M. F.
0 as a center. The radius of the smallest circle corresponds to e, the E. M. F. of the load, which is taken as 100 per cent. The radii of the succeeding circles increase by 1 per cent of that of the smallest circle, and as the radius of the last or largest circle is 140 percent of that of the smallest, the chart answers for drops up to 40 percent of the E. M. F. delivered.
The terms resistance volts, resistance E. M. F., reactance volts, and reactance E. M. F., refer, or course, to the voltages for overcoming the back E. M. F.'s due to resistance and reactance respectively. The figures given in the table under the heading "Resistance-Volts for One Ampere, etc." are simply the resistances of 2,000 feet of the various sizes of wire. The values given under the heading "Reactance-Volts, etc." are a part of them,ccalculated from ables published some time ago by Messrs. Houston and Kennelly. The remainder were obtained by using Maxwell's formula.
The explanation given in the table accompanying the chart (Table X) is thought to be a sufficient guide to its use, but a few examples may be of value.
Fig. 40. Vector Diagram.
Problem. Power to be delivered, 250 K.W.; E. M. F. to be delivered, 2,000 volts; distance of transmission, 10,000 feet; size of wire, No. 0; distance between wires, 18 inches; power factor of load, .8; frequency, 7,200 alternations per minute. Find the line loss and drop.
Remembering that the power factor is that fraction by which the apparent power of volt-amperes must be multiplied to give the true power, the apparent power to be delivered is
250 K.W. =312.5 apparent K.W.
The current, therefore, at 2,000 volts will be
312,500 = 156.25 amperes
From the table of reactances under the heading "18 inches," and corresponding to No. 0 wire, is obtained the constant .228. Bearing the instructions of the table in mind, the reactance-volts of this line are, 156.25 (amperes) X 10 (thousands of feet) X .228 = 356.3 volts, which is 17.8 per cent of the 2,000 volts to be delivered.
From the column headed "Resistance-Volts" and corresponding to No. 0 wire, is obtained the constant .197. The resistance-volts of the line are, therefore, 156.25 (amperes) X 10 (thousands of feet) X .197 = 307.8 volts, which is 15.4 per cent of the 2,000 volts to be delivered.
Starting, in accordance with the instructions of the table, from the point where the vertical line (which at the bottom of the chart is marked "Load Power Factor" .8) intersects the inner or smallest circle, lay off horizontally and to the right the resistance-E. M. F. in per cent (15.4); and from the point thus obtained, lay off vertically the reactance-E. M. F. in per cent (17.8). The last point falls at about 23 per cent, as given by the circular arcs. This, then, is the drop, in per cent, of the E. M. F. delivered. The drop, in per cent, of the generator E. M. F. is, of course, 23 - = 18.7 per cent. 100+23.