[COPYRIGHTED] U. S. NAVAL INSTITUTE, ANNAPOLIS, MD. A FEW NOTES ON ALTERNATING CURRENTS INCLUDING REMARKS ON INDUCTION MOTORS AND RADIO TELEGRAPHY By LIEUT. COMMANDER LESLEY B. ANDERSON, U. S. Navy There is no doubt that alternating currents have come in the navy to stay. At present they are applied to ship propulsion, radio, gyro-compasses, submarine signalling, navy yard light and power; and, on a smaller scale, to gun firing, three-wire generators, testing sets, magnetos, engine speed indicators, and electric welding. There is such a large field for development that the use of alternating currents will probably be greatly extended within the next few years. The theory and application of alternating currents is a subject that is highly technical and most difficult. To master it requires several years of handling apparatus as well as book study. The NOTE.-I. This paper is submitted for the use of the Institute if so desired. 2. The matter covered is elementary in character. It deals with alternating currents, their generation and application to induction motors and to radio. 3. There are a great many officers in the service who never had the benefit of any instruction in alternating currents while at the Naval Academy. For them there is a great deal of time and energy to be expended if they would understand anything about the subject. They are required to know something if they are to pass examinations, for alternating currents are becoming more and more important each day. 4. These notes fill in the space between the starting of the subject and the place where most papers, lectures and pamphlets on the various applications of alternating currents begin. In order to understand even a simple description of electric drive, an elementary knowledge of the subject is necessary. 5. There are no books that I know of that give the elementary conception of the subject in concrete form. They all seem to go into such detail that it is almost impossible for the beginner to get a general idea from them. 6. I believe that these notes give the general idea in a simple manner. They are mostly explanations that were given to midshipmen and reserve officers in conjunction with the study of the text. They were used in lectures and talks in the recitation room. average naval officer is not interested to that extent. He is interested from a professional point of view more in what can be done, than in how it is done; and yet at the same time he is required to know something about how it is done. The information regarding how it is done is not readily available. It is generally camouflaged in a mass of integral signs. But there are certain peculiarities of alternating currents that can be understood without introducing higher mathematics, and I believe that a general idea of the subject can be obtained without taking up a very great amount of time. Alternating currents can be used for most of the things that direct currents can, and they are cheaper. They also can be used for some things that direct currents cannot. On the other hand, there are some things that alternating currents cannot do; for instance, only direct current can be used for charging storage batteries and, as will be shown later, for exciting the field of an alternator; so although A. C. is coming into greater use every day, it can never entirely replace direct current. To start with, every one knows that an alternating current flows half the time in one direction and half the time in the opposite direction; and that reversals of direction are frequent. The question is, How can it be made to do work in one direction, so to speak, such as making a lamp burn steadily or turning over the New Mexico's propellers? In the case of the lamp the explanation is easy. Electric current flowing in a wire will heat the wire. If the wire can be made hot enough it will give out light. Suppose the wire in question is the filament of a lamp. It does not make any difference to the filament whether the current is passed through it from starboard to port or from port to starboard. It is heated if current is passed through it. We can change the direction of the current as often as we please. It is the current that does the heating, and not the direction. It follows that alternating current can be used for the purpose of heating the filament just as well as direct current. Of course, the current must be supplied at proper voltage, and the frequency must be above 60 in order to prevent flickering. If these precautions are observed, the light is just as steady and effective as any that a direct current could produce. Alternating currents require a circuit to run around on just as direct currents do. Voltage is necessary to produce any kind of current, but it must be an alternating voltage to produce an alternating current. This brings us to the first peculiarity of alternating currents, which is, that alternating voltages and currents do not of necessity act together; that is, they are not necessarily in phase. In some circuits the current is ahead of the voltage, while in others the voltage is ahead of the current. Alternating currents may "lead," "lag," or be "in phase," depending on the characteristics of the circuit. There are three factors which taken together impede the flow of alternating current resistance, inductive reactance, and condensive reactance. While resistance tends to keep the current in phase with the voltage, both inductive and condensive reactance tend to throw it out of phase; their action is at right angles to the action of resistance. At the same time, they act opposite to each other. Inductive reactance tends to make the current lag; while condensive reactance tends to make it lead. There are several ways of proving these statements, but as the principal difficulty in studying alternating currents is the mastering of this and other proofs, let us for the present be satisfied with saying that it can be proved. It is the very foundation of the subject, consequently of the utmost importance. The amount that the current lags or leads is really a time interval, but it is not measured as such. For the purpose of measurement we consider that a cycle is made up of 360 electrical degrees, then we state the amount of phase difference as an angle, called the phase angle, and measured in degrees. At this point it is necessary to introduce a few definitions. As an alternating current is continuously reversing its direction, it is convenient to consider one direction as positive, and the opposite as negative. Cycle-One complete set of positive and negative values of an alternating current-360 electrical degrees. Alternation-Half a cycle. Period-The time required for the current to pass through one cycle. Frequency-The number of cycles or periods per second. Radio Frequency-From 20,000 to 1,000,000 or 2,000,000 cycles per second. Audio Frequency-Below 20,000 cycles per second. NOTE. The frequency used for power transmission is about 25 cycles, for lighting about 60. Inductance-Every conductor carrying current is surrounded by a magnetic field, which is brought into existence by the current. Any change in the magnitude of the current causes a corresponding change in the magnetic field. The field then, in changing, reacts on the conductor and induces in it an E. M. F. The induced E. M. F. opposes the change of current. When the current is increasing, the induced E. M. F. tends to keep it from doing so, therefore acts in a direction opposite to the current ; when the current is decreasing in magnitude, the induced E. M. F. tends to keep it from doing so, therefore acts in the same direction as the current. Inductance, then, is the ability of a circuit to set up, of its own accord, a counter E. M. F. following changes in the circuit current. Capacity-The charge of electricity that a conductor will hold. Capacity depends upon the shape and size and location with respect to other conductors. The most common form of capacity is the condenser, of which there are many types. The quantity of electricity that a condenser will hold depends upon the voltage, as well as the capacity. A tank of so many cubic feet capacity will hold that many cubic feet of air, but more air can be put into the tank by increasing the pressure; also, air can be taken. out, thus reducing the pressure. It is the same thing with condensers; in that case voltage corresponds to pressure. There is thus a distinction between quantity and capacity. In this paper certain symbols, which are generally accepted as standard, will be used: A. C.-Alternating current. D. C.-Direct current. E. M. F. or simply E.-Voltage, or electromotive force. I-Current. R-Resistance. L-Inductance. C-Capacity. X-Inductance reactance, which is equal numerically to 2-fl. X-Condensive reactance, which is equal numerically to I 2πfc X-Reactance, the algebraic sum of the condensive and inductive reactances. Z-Impedance. -Phase angle, angle between the current and the voltage. f-Frequency. This is enough of the currents for the present. How they are generated is of more interest. After a short investigation of alternators we will be better prepared to understand the action of the currents a little better. There are two conceptions of the generation of E. M. F., either of which can be used when it is the one best suited to the conditions. The first is, that an E. M. F. is generated in a conductor when it cuts lines of magnetic force; the second is, that an E. M. F. is generated in a coil or loop when there is a change in the number of lines of magnetic force threading through it. The first is the conception that is easiest to apply to generators, the second to transformers. A generator consists essentially of an armature and a magnetic field. The field consists of magnetic lines of force. It is created by electromagnets called poles, which are designated North and South. The armature consists of the cutting conductors. The armature bars cut the lines of force and E. M. F.'s are induced in the bars. If the circuit is completed a current will flow. In connection with this conception relative motion of the cutting conductors and the lines of force is all that is required to generate an E. M. F., so it is immaterial whether the lines move or whether the conductors move. Generator armatures generate alternating E. M. F.'s, which can be rectified by means of a commutator if direct current is required. If alternating current is required, the commutator is left off and collector rings used instead. The leads from the armature are then connected to the rings, and brushes take off the current as with direct current machines. If rings are installed in addition to the commutator, and proper connections made, both alternating and direct current can be taken from the same armature at the same time. The direct current is taken off the commutator and the alternating current off the rings. Such machines are actually made and are called |