Showing papers in "Transactions of The American Institute of Electrical Engineers in 1927"

Graphical Determination of Magnetic Fields Practical Applications to Salient-Pole Synchronous Machine Design

Abstract: There are three methods of obtaining the flux distribution in a magnetic field. First: By test. Templets or models can be made of the field to be explored and the flux distribution can be obtained by test as described in a companion paper ``Graphical Determination of Magnetic Fields?Comparison of Calculations and Tests'' by Messrs. E. E. Johnson and C. H. Green. Second: By mathematical analysis. This method is accurate and consistent results can be obtained. If the problem is very complicated, however, the mathematical solution is very laborious and sometimes impossible. Third: By the graphical method. This method is quite accurate and it can be used for design calculations. It is comparatively easy and it is the quickest method of the three. The graphical method of plotting magnetic fields used in this paper is described in a companion paper, ``Graphical Determination of Magnetic Fields?Theoretical Considerations'' by Messrs. A. R. Stevenson, Jr. and R. H. Park. This paper shows how the graphical flux plots can be used very successfully in design calculations. In fact, the performance characteristic of a salient-pole synchronous machine can not be predetermined accurately without the use of flux distribution coefficients.

Synchronous Machines-III Torque-Angle Characteristics Under Transient Conditions

Abstract: This is the third part of a series of papers on the subject of synchronous machines. The first two were: I. An Extension of Blondel's Two-Reaction Theory, II. Steady State Power-Angle Characteristics. The present paper deals with the power-angle, or torque-angle, characteristics under transient conditions, namely, A. Cyclic variation of impressed torque, B. Sudden angular displacement, C. Synchronizing out of phase. It is shown, as in Fig. 6, that although the slope of the torqueangle characteristic (which is an important factor in the determination of the resonant frequency) under the oscillatory condition is greater over a large range of values of the average angle ?' than under steady operation, nevertheless in the range of normal operation, i. e., from ?' = 0 to ?' = 25 deg., the two slopes, in the case of salient-pole machines, are practically the same. Hence, it is only in rather rare, special cases that a correction in the slope for the oscillatory condition is necessary. For such cases, Equation 27 gives the correction. Referring to condition B, Fig. 13 shows the steady state torqueangle characteristic and also the characteristics for the condition of sudden angular displacement, the latter occurring from various given points on the steady state curve. The slopes indicated by dotted line segments in Fig. 6 merely correspond to parts of the complete characteristics shown in Fig. 13. The latter are calculated from Equation 46.

The Measurement of Surge Voltages on Transmission Lines Due to Lightning

Abstract: This paper, after referring to the work of previous investigators in the application of the photographic Lichtenberg figures to the measurement of surge voltages, describes results of additional work in this field by the authors. Laboratory calibrations of photographic Lichtenberg figures, using the cathode ray oscillograph and the lightning generator, are shown. Data are presented relative to the accuracy obtainable with these figures as a means of measuring surge voltages. An extension of instrument design is described in which two recording elements are used to give greater certainty of result. Means for connecting a surge voltage recorder instrument to a transmission line by an insulator-string potentiometer are described, and calibration of the instrument with potentiometer is given up to 1400 kv. Specimen field records of surge voltages up to 2000 kv. are shown.

Design of Reactances and Transformers Which Carry Direct Current

Abstract: It is usually necessary to place an air-gap in the core of a reactance or transformer which carries direct current in order to secure the greatest inductance The work here reported points out a direct method of designing such reactances or transformers including the determination of the best value of the air-gap

A New Electronic Rectifier

Abstract: A new rectifier utilizing a Tartially oxidized disk of copper as a rectifier unit is described. The disks may be arranged into groups suitable for all fields of rectifier applications. The rectification appears to take place at the junction between the copper and the oxide without observable physical or chemical changes, and is similar in character to rectification by the hot-cathode type of rectifiers. The physical characteristics of assembled rectifiers and a method of designing the same for special purposes are outlined and some of the design problems discussed. It is pointed out that the new rectification phenomenon is of a radically different nature from those observed in structurally some what similar contact rectifiers. The usual theories of contact rectification, which are based on electrolysis or thermoelectricity, are not applicable to the present case. The new phenomenon is discussed in the light of more recent theories based on electron affinities of copper and copper-oxide, which are in better accord with the observations. Some applications are given for which the rectifier seems to be especially suited.

The Law of Corona and the Dielectric Strength of Air-IV The Mechanism of Corona Formation and Loss

Abstract: The mechanisms of corona and corona loss have been studied with the cathode-ray oscillograph. High voltage power of the order of 0.1 watt can be measured with an accuracy of 1 per cent with this instrument. The measurements show that the loss follows the quadratic law above the visual critical voltage. On polished wires there is no loss until the visual critical voltage is reached. The loss then starts quite suddenly and takes a finite value on the quadratic curve. On cables and imperfect conductors there is a loss below the visual critical voltage on brushes at local ``rough'' spots. The loss due to these irregularities can be represented by the probability law. This is quite in accord with former work. In practise it is important not to mutilate the conductors in stringing. The really important factor in design is the irregularity factor, m e , for weathered conductors. No line should be operated with a corona loss under fair weather conditions. It is not necessary from the economic stand point since large diameters can be obtained with special types of conductors. The visual critical corona voltage can be calculated with great accuracy. As the applied a-c. voltage is increased above the visual critical value, the instantaneous critical voltage becomes lower and lower until finally corona starts at the zero point of the wave. This occurs when the applied voltage is twice the visual critical voltage. At still higher voltage, corona starts below zero or on the falling wave.

Additional losses of synchronous machines

Abstract: I. H. Summers: Referring to Fig. 3, in which data are given for a 600-volt winding, one bar per slot, the load loss at 100 per cent kv-a. is slightly more than 20 per cent of the full-load loss. Referring to Fig. 4, in which data are given for a 2300-volt winding, 2-coil sides per slot, the load loss at 100 per cent kv-a. is approximately 3 per cent of the full-load loss.

Graphical Determination of Magnetic Fields Theoretical Considerations

Abstract: Three papers on the general subject, ``Graphical Determination of Magnetic Fields,'' are being presented simultaneously to cover three phases of the subject: 1. Theoretical Considerations, 2. Comparison of Calculations and Tests, by E. E. Johnson and C. H. Green, 3. Practical Applications to Salient-Pole Synchronous Machine Design, by R. W. Wieseman. In the following paper, which is the first of this series, the authors have reviewed the history of the subject, hare briefly stated the ordinary rules for plotting magnetic flux in air and in current-carrying copper, have developed additional rules for checking the accuracy of field plots, and have given theoretical methods for mathematically calculating the distribution of field in certain cases commonly encountered in practise. The authors have called attention to the great value of the mathmatical work by the German engineer, Rogowski, and the graphical methods by the French engineer, Lehmann. These German and French articles contain the only extensive practical applications of the plotting of magnetic fields in current-carrying regions with which the present authors are familiar. Since it is much more difficult to plot fields in current-carrying regions, and since the majority of readers are less familiar with this phase of the subject, the greater part of the first paper, ``Theoretical Considerations,'' is devoted to a study of such fields.

Methods Used in Investigating Corona Loss By Means of the Cathode Ray Oscillograph

Abstract: Methods of employing the cathode ray oscillograph for investigation of corona loss are described. By deflecting the ray of cathode particles (electrons) in one direction within the tube by a voltage proportional to the applied voltage, and in a transverse direction by a voltage proportional to the current, a closed figure representative of the loss, is thrown upon the screen. Methods of obtaining photographic records of these figures and of calculating, therefrom, accurate values of the power expended are given. The instrument used is well adapted for this work. Powver measurements of 0.1 watt can be measured with an accuracy of 1 percent. From the volt-ampere cyclogramns the characteristics of the positive and negative loss on the a-c. wave are readily observed. The instantaneous voltage at which the loss starts and the instantanteous values of the conmbined corona and capacity currents can be accurately determined. Measurements of the corona starting point and loss on various conductors check the laws of corona established by Mr. Peek in 1910. The formulas of ``critical disruptive gradients'' and ``visual disruptive gradients'' were closely checked. The loss was found to follow a quadratic above the visual critical corona voltage, e v . For cables and roughened conductors the excess loss below e v due to surface irregularities approximately follows the probability law. For smooth, polished conductors the loss suddenly jumps front zero to a definite value at e v , and then follows the quadratic.

Additional Losses of Synchronous Machines

Abstract: In the case of high-speed turbine generators, the cost reliable means of determining the losses under actual operating conditions is to measure the weight and temperature rise of the cooling medium and to estimate the small part of the losses which is dissipated from the frame to the surrounding medium. The temperature rise of the cooling medium can be obtained by means of temperature detectors located at the inlet and outlet sections of the generator. In order to obtain reliable values of the average temperature rise for the machine, it is necessary to have approximately uniform velocities at both inlet and outlet sections and to measure the temperature rise at a large number of incremental sections. The volume of cooling medium passing through the machine can be determined by (a) introducing a definite amount of heat energy into the cooling medium and measuring its temperature rise, or (b) measuring the mean velocity head at the outlet section of a properly designed stock. Loss tests were made on five 3600-rev. per. min. turbine generators when operated as synchronous condensers. In the case of these machines, the additional losses including the increase in core loss at full kv-a. and zero per cent power-factor load varied from 3 to 22 per cent of the total losses. This corresponds to approximately 0.14 to 1.0 per cent of the generator input. The additional losses as measured under sustained short-circuit conditions were from 5 to 10 per cent less than the corresponding values for full kv-a.

The Electric Arc

Abstract: DEFINITION OF ARC. An arc is a discharge of electricity, between electrodes in a gas or vapor, which has a voltage drop at the cathode of the order of the minimum ionizing or minimum exciting potential of the gas or vapor. ARC CHARACTERISTICS. The relation of arcs to glow discharges and coronas is illustrated by discussion of ``generalized'' curve of the gas discharge characteristic. Empirical equations for arc characteristics are interpreted, and a dependence on the boiling temperature of the anode is shown. Seeliger's experiments on the transition from glow to arc, accompanied by the development of a cathode spot, show that the mechanism of the current at the cathode is fundamentally different in the two types of discharge. CATHODE SPOT. An analysis based on heat conduction in the cathode shows that the cathode spot has no sharp thermal definition, but does have a sharp boundary if defined by visual brightness or by thermionic emission. The phenomenon of moving cathode spots presents the problem of accounting for the observed temperatures. THEORIES OF CATHODE FALL. Compton's theory is based on space charge considerations and the assumption that the thickness of the fall space is equal to the electronic mean free path. Langmuir's theory differs from Compton's in assuming this thickness to be considerably less than a free path. Considerations of energy balance at the cathode definitely support Langmuir's rather than Compton's theory. ENERGY BALANCE AT CATHODE.

Starting Performance of Synchronous Motors

Abstract: This paper deals with the theory underlying the starting performance of the salient-pole synchronous motor equipped with damper windings. The theory, while involving some approximations, is accurate enough for practical engineering calculations. Formulas are developed for the starting torque, pull-in torque and inrush. A method is also given for calculating the speed torque curve from stand still to synchronous speed. Due to the fact that the damper winding is not continuous around the periphery and due to the presence of the single-phase field winding, the rotor circuit is not a perfect polyphase secondary but is unbalanced to some extent. In order to take care of this unbalance, it is necessary, in addition to the usual system of positively rotating vectors, to employ a second system of negatively rotating vectors as is done in unbalanced three-phase problems. The stator resistance has been disregarded in working out the general case of the unbalanced or partial polyphase secondary in order to obtain a torque formula which will be simple and at the same time accurate enough for practical calculations. Mr. Q. Graham is working on this problem and expects to present in an Institute paper, in the near future, the general solution including the stator resistance. The use of the double squirrel-cage type of damper winding in salient-pole machines has been examined both theoretically and experimentally. Other methods of obtaining unusual starting performance are suggested and the results of some actual calculations presented.

The Oscilloscope: a Stabilized Cathode-Ray Oscillograph with Linear Time-Axis

Abstract: A method is described for using a cathode-ray oscillograph for the simultaneous observation of a number of variable quantities by means of a distributor. A linear time-axis, obtained by means of a gas-discharge lamp connected to a source of direct-current through a resistance or thermionic tube, is stabilized by introducing into this circuit a small e. m. f. derived from the same source that supplies the unknown quantities under observation. By making stabilization definitive rather than casual, distortion is avoided. The unknown quantities are thus shown in a form convenient for observation, appearing as stationary curves plotted with time as abscissa. The curves may be superposed about a common zero line, or displaced with reference to each other with separate zero lines. The name oscilloscope is applied to the apparatus here described, assembled as an instrument.

Klydonograph Surge Investigations

Abstract: Since its recent development, the klydonograph has been used to investigate surge conditions on a number of transmission systems. Quantitative data, which show the characteristics of the actual surges present on transmission systems, are presented. In addition, these data are discussed in relation to various existing theories and practises regarding the production and the elimination of transient high voltages. The correctness of some of these ideas and the fallacy of others are indicated. The paper is subdivided, according to the nature of individual investigations, as follows: I. Open-wire Systems. II Cable Systems. III. Lightning Arresters.

The M. M. F. Wave of Polyphase Windings With Special Reference to Sub-Synchronous Harmonics

Abstract: The m. m. f. waves of fractional slot windings or other irregular windings are found to contain harmonic components having wavelengths greater than two pole pitches. These are designated as sub-synchronous harmonics since their harmonic order is lower than that of the synchronously rotating wave. They induce currents in the damper windings of synchronous machines which may produce noticeable loss. Some test data concerning losses are included. These harmonics have an effect upon reactance and, under certain conditions, they may cause vibration. An appendix covers the calculation of the m. m. f. of three-phase fractional slot machines.

Reduction of Armature Copper Losses The Inverted Turn Transposition for the Reduction of Losses Due to Non-uniform Current Distribution in the Armature Conductors of Large A-C. Machines

Abstract: A new method of reduction of armature copper losses is described, which consists in the inversion of the conductors of a multi-turn barrel type coil at one or more places in the end portions of the coil. Previous writers have described methods of transposing the conductors in the slot and of carrying insulated strands through successive positions in successive coils, but the new method now described presents distinct points of difference in theory and in construction from any of these earlier methods. The theory of the new form of transposition is briefly described, complete formulas are presented for the most useful cases, and illustrations of its use are given.

Reduction of armature copper losses

Abstract: G. H. Rockwood, Jr.: In order to compute the losses in these transpositions which Mr. Summers has described, it is necessary to determine the percentage of slots in the winding having a given value of the angle θ between current in the top and bottom coil sides. Necessity for visualizing the winding may be done away with and the computation simplified by the following procedure:

Measurement of Telegraph Transmission

Abstract: Various factors contribute to impair the quality of telegraph signals. For instance, there may be interfering currents either induced in the circuit or brought in by conduction, the proportioning of the circuit elements may be imperfect or batteries and relays may be out of adjustment. The result in any case is to distort the telegraph signals so that the received signals are not a true copy of those transmitted. The paper describes methods for measuring this distortion and for analyzing the results so as to indicate the nature and extent of the impairment and its probable cause.

Tests on High-and Low-Voltage Oil Circuit Breakers

Abstract: Data from a large number of tests on several types of oil circuit breakers are given in this paper The tests were made on breakers with the following ratings: (a) 150 kv, 1,500,000 kv-a; (b) 35 kv, 250,000 kv-a; (c) 7000 volts, 7500 kv-a; (d) 132 kv, 1,250,000 kv-a, and (e) 132 kv, 750,000 kv-a These tests were made on power systems having sufficient connected capacity to make the tests conclusive Complete data are tabulated and oscillograms are shown Some valuable conclusions resulted from the tests

Space Charge and Current in Alternating Corona

Abstract: 1. The physical nature of ionization in a corona discharge in air is studied; by means of the corona spectrum, the saturation current in air around the corona voltage, and the influence of the material of the wire on the saturation current. The results indicate that nitrogen only is ionized in a corona discharge in air; that the ionization of the nitrogen results in the separation of an electron from the nitrogen molecule; and that the electron quickly attaches to a molecule or group of molecules, probably water or oxygen, to form an ion. 2. The free charge in the neighborhood of the wire called the space charge, is found to be alternating in character and to have a definite boundary. The space charge formed on any half-wave returns to the wire on the next succeeding half-wave. 3. The mobility of the ions is calculated from the boundary of the space charge. A limiting value of about 10 cm./sec. per volt/cm. is indicated for the positive ions. The negative ions show no sign of a limiting value and the mobility varies from about 1.6 to 10 cm./sec. per volt/cm. as the maximum impressed voltage rises from the corona voltage to twice the corona voltage. 4. Ionization is found to occur at lower voltages on the positive half-waves than on the negative half-waves and the ionization on the positive half-waves becomes much more copious with the beginning of ionization on the negative half-waves. 5.

Oil Breakdown at Large Spacings

Abstract: Much work has been done on the breakdown of insulating oil at small spacings between electrodes. Information for electrode separations of several inches is not as complete. It has been found that sources of ionization external to the gap influence the gap breakdown, so that the design of electrode supports and parts is of great importance. Data on several sizes of spherically terminated rods or cylinders are presented. Short-time breakdown tests are shown to be quite erratic and a form of long-time test schedule was developed which gives more consistent results. The final test used is called a tenminute-hold and yields values for a given condition representing the maximum voltage that can be held consistently. This is of special interest in design. The empirical curves of oil breakdown are analyzed by mathematical methods. A general equation for breakdown voltage in terms of electrode diameter and separation is developed which agrees quite well with the experimental data. Evidence is presented to show that water in globular form suspended in oil may increase the breakdown potential considerably with spherical electrodes if the separation is several times the diameter.

A Precision Measurement of Puncture Voltage

Abstract: A description is given of an apparatus for the automatic determination of the dielectric strength of thin sheet insulation. The machine makes about 1000 breakdown tests in a day. Almost no attention is required, thus reducing the human element to a minimum. The results of over 100,000 punctures are given. An investigation of the effect of temperature indicates that none of these punctures occurred according to the thermal theories of breakdown. Tests to determine the effect of electrode area are also described. It is shown that statistical theory can be applied to the results with some success. The standard deviation of the breakdown values is a measure of the inherent variability of the material, and thus gives promise of value in the specification of insulation.

Voltage Standardization of A-C. Systems From the Viewpoint of the Electrical Manufacturer

Abstract: In outlining the history of voltage standardization, it is observed that there has been separate standardization of voltages of various types of apparatus, rather than standardization of a complete operating system. The results of a questionnaire answered by 22 operating companies are analyzed and the conclusion is drawn that the use of the old standard transformer voltages involved, in many cases, over-exciting the transformers or generators in order to maintain satisfactory voltage at the consumers' terminals. The reason is that the existing transformer voltage standards do not compensate for the line drop in transmission lines and feeders. The voltage standards of the International Electrotechnical Commission are set forth, indicating a partial agreement as to maximum system voltages with the proposed standards, although arrived at by a different method. The I. E. C. standardization, however, is not so complete as the proposed system. In describing the proposed standards, certain basic principles are laid down as the conditions which must be fulfilled. The proposed system of voltage standards starts with already standardized Utilization voltages at the low end of the scale and suggests tranformner voltage ratings and ratios which will allow proper voltage to be supplied to the consumers without over-exciting any of the transformers or generators in the system, and ties in the transformer and apparatus voltages with system voltages, based on the A. I. E. E. definition of rated circuit voltages. The proposed standards thus cover the whole field of voltages of a-c.