Showing papers in "Transactions of The American Institute of Electrical Engineers in 1939"

Self-Excitation of Induction Motors

Abstract: It has been known for some time that an induction machine whose rotor is driven mechanically may become selfexcited if capacitors are connected across its terminals. The present paper is concerned with the predetermination of the machine characteristics when operating under such conditions. The frequency of excitation is very close to the synchronous frequeney corresponding to the speed of the rotor. The voltage to which the machine will excite is dependent upon its no- load excitation characteristics at that frequency, the criterion to be satisfied being that the lagging volt-amperes of excitation equal the leading volt-amperes of the capacitors. Under load, similar criteria must be satisfied. Voltage conditions are determined by a cut-and-try solution such that the summation of reactive volt-amperes equals zero. The slip is then obtained from the relation that the summation of the real power equals zero. These relations have been applied to various types of loads, such as pure resistance and inductive resistance, single-phase and three-phase and also to induction-motor load. Excellent checks between test and calculated results have been obtained.

Breakdown Studies in Compressed Gases

Abstract: THE superior dielectric strength of compressed gases, their low dielectric loss, their low dielectric constant, their low cost, and the fact that they are self-healing make them very promising insulating media. Compressed-gas insulation has been used in precision condensers where low dielectric loss is essential; it has been tried in other types of equipment such as transformers; it has been suggested for electric power cables where the low dielectric constant is of considerable importance; and recently it has found interesting and important application in the production of very high voltages by electrostatic means for use in X ray and nuclear physics work.

Overvoltages During Power-System Faults

Abstract: OVERVOLTAGES may be produced by lightning, switching surges, faults, both solid and arcing, and the overspeeding of machines due to loss of load. The effect of circuit and machine characteristics on the duration and magnitude of these overvoltages has received considerable attention.1–10 Furthermore, field tests have been made in order to determine the effectiveness of different methods of grounding, and the accuracy of the methods used to calculate the magnitude of overvoltage during system faults.11–13

Effect of Restriking on Recovery Voltage

Abstract: IT IS well known that when any portion of an electrical circuit is either opened or closed a transient condition will usually exist for some period, depending on the damping, before the circuit assumes its new steady state. In power circuits, such switching operations are the application and removal of faults or loads and the connecting or separating of various parts of a system. The transient conditions resulting from these switching operations give rise in some cases to overvoltages, which can be calculated by straightforward and more or less well-known methods, if the transient circuit parameters are known. However, in some instances voltages much higher than those predicted by such calculations have been obtained, and various explanations1–9 have been offered for these occurrences.

Power-System Transients Caused by Switching and Faults

Abstract: This paper summarizes the results of an investigation of transient voltages on power systems caused by switching and faults. The transient voltages on power systems as measured by the “klydonograph” are reviewed and compared with the flash-over values of transmission-line insulation. It is shown that the higher values of transient voltages are produced by intermittent arcs. In part I, the various theories for the production of transient voltages of high magnitude as a result of intermittent arcs are reviewed and extended in order to obtain the highest voltages on typical polyphase systems with the range of natural frequencies and attenuation factors that are encountered in practice. Previous studies are of limited scope and apply principally to the case of an arcing ground on an ungrounded system. The present study shows broadly the range of transient voltages which may be produced with intermittent arcs and applies to switching operations as well as arcing grounds. A typical transmission system is studied with the aid of the a-c network calculator. One of the principal variable factors in this study is the method of system grounding and this includes a range of both resistance and reactance between the limits of a solidly grounded system and an ungrounded system. The study is carried out for four different conditions, namely: (1) arcing grounds, (2) de-energizing an unfaulted line section, (3) de-energizing a line section with a fault on one phase, and (4) de-energizing a line section with a fault on two phases. The results of this study are presented in graphical form in part II and show many interesting properties of systems with respect to the method of grounding, and the characteristics of transient voltages for the different switching and fault conditions. It is the authors' opinion that the transient voltages due to. faults and switching deserve more attention than they have received within the last few years.

Load Ratings of Cable

Abstract: Operating and test data concerning the maximum safe loading of impregnated-paper-insulated lead-covered cable are presented. The results of the study may be summarized as follows: 1. The occasional operation of cable at higher temperatures than are permitted by present temperature rules effects considerable economy. 2. During emergencies, temperatures of 5 to 35 degrees centigrade (depending on kind of cable) above those permitted by the rules are safe for the insulation. 3. For extra-high-voltage solid-type cable, void formation in insulation and expansion of lead sheaths may limit allowable temperatures and temperature ranges. 4. Cracking of lead sheaths due to reciprocating cable movement into manholes may limit the temperature range for usual daily loading. Limitation is more severe for longer conduit lengths up to 500 feet, but changes little with increase from 500-to 1,000-foot lengths. 5. Cracking of sheaths in manholes due to cable movement may be reduced by improving manhole conditions. 6. For many cables a balanced design requires a lead-alloy sheath that gives increased resistance to effects of cable movement and of internal pressures. 7. Continuous field temperature surveys are essential to efficient use of large conduit and cable systems. 8. Only a small fraction of the cable ever operates at the higher temperatures. 9. Data on center empty-duct temperatures and on average heat losses over 24-hour periods give satisfactory results in heat calculations. 10. Other practices which increase load ratings are the use of different ratings for various periods of the year, the replacement of poor soil in special cases, and the use of extra-large conductors in warmer conduits.

Determination of temperature rise of induction motors

Abstract: The ``American Standards for Rotating Electrical Machinery'' (American Standards Association) prescribe that the temperature rise of motors shall be measured by the thermometer method for purposes of rating. Definite limiting values of temperature rise are established for each type of machine. The AIEE Standards No. 1 fix conventional allowances of 15 degrees centigrade, 10 degrees centigrade, and 5 degrees centigrade between the actual hottest-spot temperature and the highest observable value of temperature as determined by thermometer, resistance, and embedded-detector methods, respectively. This paper presents information on the relations between the measured values of temperature rise by different methods as found in tests on several hundred induction motors ranging from 10 to 1,000 horsepower in rating. It is shown that, with modern motor construction, variations of 20 degrees centigrade or more are sometimes obtained by the thermometer method on a given machine depending on location of thermometers or thermocouples, whereas the resistance measurements give relatively consistent values of temperature rise. It is therefore suggested that the standards for temperature-rise measurements be revised, and that the resistance method be adopted for all forms of enclosed or protected machines which are not readily accessible for application of thermometers on laminations, insulated windings, and other adjacent parts.

Rating of General-Purpose Induction Motors

Abstract: This paper reviews principles of rating of general-purpose a-c motors, particularly in relation to overload torque and temperature limits. The overload capacity of an electric motor is limited, first, by its stalling, or breakdown torque, and, second, by its operating temperature. The breakdown torque, analogous to the stalling torque of a gas engine, is roughly proportional to the motor size and the square of its magnetic flux density. The temperature limitation is exactly analogous to the temperature limit of a transformer, the useful life of the insulation being reduced exponentially as the temperature is raised. The increased variety of motor uses in recent years, especially for automatic operation of refrigerator compressors, air conditioning, pumps, and other mechanical devices, has led to more exact methods of application, utilizing motor overload capacity and matching torque characteristics to the driven equipment. Under this economic pressure, motor overload capacities have been increased over the requirements of present standards, and small motors are now commonly used on intermittent overloads far beyond their continuous ratings. It is proposed, therefore, that American standards be revised to provide for increased values of breakdown torque and service factor in the smaller motor ratings, and that permissible intermittent duty cycles be denned, enabling the full economic life of the motor to be utilized. It is also proposed that the starting current be recognized as a convenient and accurate measure of induction-motor breakdown torque, or momentary torque capacity, and that starting current values be established on a logical and consistent basis for both single-phase and polyphase motors. Specific recommendations are given in the paper for rated characteristics and operating limits under this starting current-temperature system of rating, and the economic advantages to the industry to be gained by their adoption are pointed out, including safer wiring and control systems and a reduced variety of special motors.

Two-Phase Co-ordinates of a Three-Phase Circuit

Abstract: IN RECENT years there has been an increasing interest in the use of substituted variables in the analysis of unbalanced three-phase circuits and machines. The best known and most widely used of the substituted variables are symmetrical components, introduced by Fortescue in 1918. In the method of symmetrical components the actual currents and voltages of the three phases of a three-phase circuit are replaced by the zero-sequence, positive-sequence, and negative-sequence currents and voltages, and, furthermore, the three-phase circuit itself is replaced for purposes of analysis by three fictitious single-phase circuits known as the sequence networks. This method has achieved extensive use in the analysis of polyphase machinery and in the making of fault studies on three-phase power systems. Another example of substituted variables is furnished by the direct-axis and quadrature-axis quantities as defined by Park2 in 1929, which have greatly facilitated the mathematical analysis of salient-pole synchronous machines.

Duty Cycles and Motor Rating

Abstract: The horsepower rating of a motor carries a dual implication — first, torque ability; second, temperature rise. The two are frequently confused. In selecting motors for duty-cycle jobs the two concepts should be considered separately. The motor rating and type should be chosen to fit the torque requirements of the job. The proper time rating or service factor to associate with this horsepower rating to insure satisfactory insulation life can be determined from the duty cycle. Use of oversize continuously rated motors instead of short-time-rated motors to secure high torque ability on variable load jobs imposes an economic loss. Frequent starting and reversing or starting high inertia loads imposes a temperature hazard frequently greater than heavy overloads. Horsepower rating as now understood is not a satisfactory criterion of a motor's reversing ability. These topics are developed by means of simple hydraulic analogies.

Crossbar Dial Telephone Switching System

Abstract: IT IS the purpose of this paper to describe briefly the crossbar dial-telephone central-office switching system which has recently been developed by the Bell System for use in large cities. Sixteen years ago, in February 1923, a paper was read before the Institute, by E. B. Craft, L. F. Morehouse, and H. P. Charlesworth of the Bell System, which outlined the history and the problems involved in telephone central-office switching and described the panel dial central-office system which had just been developed and was being introduced in the large cities. The first central office of this type was placed in service in December 1921, and since that time 456 panel dial offices serving nearly 4 1/2 million subscriber stations have been installed in 26 different cities throughout the country. During these years many improvements have been made in the panel system to make it more serviceable to the telephone public and to meet the new problems which have arisen, but in addition, the engineers of the Bell System have continued their search to find new and better means for meeting telephone switching demands. This work has resulted in the adoption of the crossbar-type central-office switching equipment. Two offices of this type were placed in successful operation during 1938 and others are in process of manufacture and installation.

The electric strength of air at high pressure - II

Abstract: Results are given of an investigation of the electric strength of air at pressures up to 21 atmospheres. Sparking voltages are presented for air under pressure as measured with 60-cycle alternating voltage applied to sphere gaps. Deviation from the linear pressure-voltage relation that exists at low pressures is discussed, and certain characteristics of sparking in compressed air are noted. A study of the electric strength of air under pressure was reported three years ago.1 Because of increased interest in the subject this study has been extended, and parts of the work have been repeated with a higher degree of accuracy. Data thus obtained are of interest in the design of air-blast circuit-breakers and other types of compressed-air switches, gas-insulated transformers, power cables operating under pressure, gas-filled radio transmitting condensers, etc. The present paper presents data relating to sparking between spherical electrodes in air, the air pressure ranging from 1 to 21 atmospheres (0 to 300 pounds per square inch gauge pressure). Results obtained with spherical electrodes may be applied with reasonable accuracy to any spark gap in which the electric field is substantially uniform. The results of this investigation are therefore applicable to sparking between the parallel plates of condensers, electrostatic shields of high-voltage apparatus, and other smooth surfaces. Breakdown between points, and at sharp edges, and between small wires, is discussed in the earlier paper.1

Reverse-Rotation Test for the Determination of Stray Load Loss in Induction Machines

Abstract: VARIOUS methods for the accurate determination of stray load loss in induction machines have been devised in recent years. Also older methods have been improved and developed as the nature of the loss has become better understood. The employment of different methods of testing on a given machine seldom shows complete agreement in results, due to minor discrepancies and inaccuracies inherent in the methods themselves. Another difficulty is that all of the plans proposed to date require a high degree of skill and experience on the part of the testing technician. Thus industry today finds itself in need of a test which will give highly accurate results and at the same time be simple and easy to perform. The purpose of this paper is to describe a new method of testing — termed the “reverse-rotation test” — which will more nearly meet these requirements. The theoretical basis and the assumptions involved are discussed. Tests to determine the accuracy of the method have been made on two squirrel-cage motors of somewhat different characteristics by various methods of testing. The results of these investigations are described and curves are given showing the degree of exactness obtained. While the method includes assumptions regarding compensating effects involving minor components of the loss, its over-all accuracy for the motors tested is shown by comparison with the values obtained by other methods having high precision.

The Current-Limiting Power Fuse

Abstract: FUSES may be used on large central station and industrial systems in locations where a high interrupting capacity is required. The conventional fuse may clear a short circuit in a half cycle or several cycles. Associated apparatus is subjected to electromagnetic forces produced by the high momentary currents. Good engineering practice dictates that all disconnecting switches, contactors, motor starters, etc., shall have short-time current ratings at least equal to the initial root-mean-square fault current which the system is capable of producing at the point of installation. Where these currents are extremely high, a saving in size and expense of such apparatus can be effected if the required short-time current rating can be reduced. Many attempts have been made and accomplishment has been achieved by the use of a fuse which consistently limits the initial peak current to a fraction of that available in the circuit. The purpose of this paper is to describe the new fuse.

Considerations in Applying Ratio Differential Relays for Bus Protection

Abstract: THE PROBLEM of bus differential protection is receiving increased attention by relay engineers because of the serious consequences of a bus fault. Several cases of bus faults have recently occurred indicating that regardless of care used in the design of structures, some unexpected failure or accident may cause a disastrous outage. In spite of the many improvements that have been made in the protection of transmission lines in recent years, little attention has been given to the protection of station busses. This has been due partly to the impression that they were not particularly subject to faults and partly to the feeling that the protective relays would do more harm than good through incorrect tripping. Recent experiences, however, indicate that a bus fault, though of infrequent occurrence, can cause much greater damage and a longer outage period than many transmission line faults. Also, high-grade performance has been obtained when a bus protective scheme is properly installed with due consideration given to the many factors which might lead to faulty operation. The purpose of this paper is to discuss some of the important factors to be considered in bus differential protection and the operating characteristics and results of a field test on a new bus differential relay.

Temperature limits set by oil and cellulose insulation

Abstract: The life of cellulose insulation in oil has been investigated as a function of temperature under conditions of free access to oxygen and also in an inert atmosphere. Temperatures up to 140 degrees centigrade have been used. An attempt has been made also to study the life of oils as a function of oxygen concentration in an actual transformer at various temperatures. The amount of oxygen to produce a given acidity was also determined. On the basis of these data on oils, an attempt has been made to calculate rates of oxygen absorption by oil which may be used to estimate the relative deterioration in transformers of other dimensions. The results show cellulose deteriorates only mechanically, retaining its electrical properties. It is subject to both temperature and oxidation effects, the temperature effect, of course, taking place above 105 degrees centigrade.

Electromagnetic Horn Design

Abstract: The principles of designing electromagnetic horn ``antennas'' to obtain beams of specified angular spread, smoothness of contour and power gain are disclosed. Quantitative curves are given from which the design of sectoral and pyramidal horns may be readily made.

Copper-Oxide Modulators in Carrier Telephone Systems

Abstract: Copper-oxide modulators are widely used in telephone systems for translating either single speech channels or groups of speech channels to carrier-frequency locations on the lines. A number of simple circuit arrangements have been developed that enable suppression of certain undesired frequencies to a degree that is impractical in tube modulators. These modulators transmit equally well in either direction and the modulating elements are more nonlinear than in tube modulators. As a result numerous effects are found that ordinarily are not important in the tube arrangements. Analytical studies have been considerably simplified by the use of a small signal, and a large carrier controlling the impedance variation of the copper oxide. It is found in this case that the superposition and reciprocity theorems hold for all the circuits that it has been possible to analyze even though the modulator is made up of nonlinear elements. Open and short-circuit impedance measurements can be made use of as in four-terminal linear networks, and a generalized reflection theory developed. Performance data are given for an idealized modulator under a variety of operating conditions.

A 12-Channel Carrier Telephone System for Open-Wire Lines

Abstract: A new carrier telephone system is described, together with its application in the long-distance telephone plant. By its use, an open-wire pair which already furnishes one voice circuit and three carrier circuits may have 12 more telephone circuits added. Thus in all 16 telephone circuits are obtained on a single pair. Several such systems may be operated on a pole line. Various problems incident to the extension of the frequency range, from about 30 kilocycles, the highest frequency previously used, to above 140 kilocycles, are discussed. Among the more important of these are the control of crosstalk between several systems on a pole line, arrangements for taking care of intermediate and terminal cables, and automatic means for compensating for the effects of weather variations on the transmission over this wide frequency range.

Recent Developments in Generator Voltage Regulation

Abstract: This paper discusses the fundamental requirements for stability in a generator voltage regulating system having several time delays. A new rheostatic type of voltage regulator element is described, together with a method of introducing antihunting means into the regulating system. In the appendices, a mathematical treatment gives the requirements for stability and adequate damping for several typical voltage regulating systems. It is shown that with certain systems, theoretically perfect regulation and also high damping are possible.

Similitude of Critical Conditions in Ferroresonant Circuits

Abstract: RECENTLY a number of articles dealing with the critical conditions in ferroresonant circuits has appeared. However, no simple generalization is possible from these previous methods. The generalization presented in this paper offers certain information pertaining to similitudes of circuits in a surprisingly simple manner. Knowing the critical stable conditions for one reactor, the principle of similitude enables the predetermination of the critical stable conditions for any other reactor using the same grade of iron.

Measurement of temperature in general-purpose squirrel-cage induction motors

Abstract: THERE are three fundamental methods of temperature determination1 which might be applied to squirrel-cage motors, namely: 1. The thermometer method 2. The resistance method 3. The embedded-detector method

Loading Transformers by Copper Temperature

Abstract: THE PURPOSE of this paper is to describe a new practical method of utilizing latent short-time overload capacity to the fullest extent, dependent upon actual copper temperature and automatically taking into account the factors that affect the life of a transformer.