Moving Picture News (Jan-Dec 1911)

THE MOVING PICTURE NEWS II ADVIC E OPERATORS ORAGLE PRACTICAL ARTICLED FOR ALL DEPARTMENT Bj WM. E. SMITH, E. E. Alternating Current Synchronous Generators In the last article there was a slight error made in the second column, third paragraph from the bottom, due to the proofreader not being familiar with electrical terms; the paragraph should read: "By revolving in a uniform field, the electromotive forces set up are proportional to the sine of the angle through which the coil has turned from the position in which it lay across the field." Before we proceed to go further into the study of alternators or alternating current generators it will be of advantage to have a little elementary talk as to the difference between alternating and direct currents. Direct current flows steadily in one direction through a circuit, just as water in a city water system flows regularly in one direction, and does not flow backward or reverse in the pipes. This is due to the pressure caused by pumps or by the reservoir situated on the hill. Just so with direct current; the pressure is steadily in the same direction, and forces the current through the circuit in that direction. It is true that the direct current may be made to flow in either direction through any conductor desired by merely reversing the connections of the conductor; but nevertheless the current flows steadily in one direction or the other until some change is made by an attendant. But alternating current is very dififerent from this, and does not flow steadily in one direction, but is actually flowing first in one direction through the entire circuit and then in the opposite direction. That is, the current flows through the whole circuit in one direction, then its value decreases until the current becomes zero, when it instantly begins to flow in the opposite direction, increasing in strength until some definite highest value is reached (the amount depending upon the circuit and the voltage), and then falling to zero only to begin flowing again in the original direction. It continues to go through these changes or reverses with great regularity, varying ordinarily in a fairly smooth curve and not by jumps. As to why it is necessary to have these two dififerent kinds of current and which is the better, it may be stated that direct current was studied and developed some years before alternating current was understood. All the early generators were for direct current, largely because arc lamps were the main use for current, and direct current was all that could be used on the early lamps. Then direct current motors were developed, and brought to a high state of efficiency before the alternating current was used at all commercially. FIGURE 36 FIGURE 35 When incandescent lamps became more extensively used it was soon found that direct current could not be transmitted very far without using large wires or wasting a large proportion of the power. It was found by study and experiment that alternating current could be generated and used at higher voltages than was possible with direct current, and that this higher voltage so reduced the current required to do a given piece of work that the wires could be kept within reasonable limits without increasing the losses to an impossibly high point. Alternators may be also classified into single phase and polyphase, according to whether their armature coils are so arranged that the currents generated in them all rise and fall at the same time or whether they have two, three or more circuits so arranged that the currents generated in one part are out of phase with those in another circuit. Power Factor Conditions A fact not always appreciated is that the demands upon a generator increase as the power factor is decreased. It must also be borne in mind that the power factor is determined solely by the load and not by the generator. The existence of loads of low power factor involves necessity for particular care on the part of a purchaser in the selection of both generators and prime movers. It should be remembered that the size of a generator is determined by the