Showing papers in "Transactions of The American Institute of Electrical Engineers in 1913"

Temperature and Electrical Insulation

Abstract: The problem of permissible temperature limits in electric apparatus is largely that of the durability of the insulation used. As this may consist of materials of widely varying heat-resisting qualities, the problem resolves itself into one of consideration of the properties of the materials themselves.

Law of Corona and Dielectric Strength of Air-III

Abstract: Air at the surface of small wires has an apparently greater strength than air at the surface of large ones. It has been found that the breakdown gradient may be expressed $g_v = g_0\left(1+{k \over \sqrt{r}}\right)$ This means that at breakdown the gradient is always constant and equal to g 0 , at k √r cm. from the conductor surface, independent of the size of the conductor. The explanation seems to be that energy is necessary to start rupture and that, therefore, rupture can not start at the surface, but only after the surface gradient has been increased to g v , in order to store the rupturing energy between the conductor surface and k √r cm. away in air, where the gradient is go. Theoretically g 0 should vary directly with the air density, or $g_0^{\prime} = \delta g_0$ If the energy theory is true the energy storage distance should also vary with δ or energy storage distance = Φ (δ) k √r. This has been found to be the case, that is, ${\rm Energy\ storage\ distance}\ = k \sqrt{r \over \delta}$ Therefore g v docs not vary directly with δ but $g_v = g_0 \delta \left(1 + {k \over \sqrt{\delta_r}}\right)$ The effect has been found to be the same whether δ is varied by change of pressure, or temperature (over range where there is still no chemical change due to temperature). According to the above theory if the wires were spaced very close together, so as to interfere with the free energy storage distance, that is, in the order of k √r cm., it would be expected that g v would increase in value in order to store sufficient energy to start rupture in the limited distance. It has been found that the gradient begins to increase at a spacing of 2 k √r cm. and, within the limits of the tests, g v values as high as 200 kv. per cm. or 500 kv. per inch have been reached. Spheres were used in these tests as it is impractical to adjust cylinders at small spacings. The electron theory may also be nicely applied to the above when the distance k √r may be thought of as the “accelerating distance.”

An Oscillograph Study of Corona

Abstract: The features or characteristics of the corona around wires brought out by oscillograms of the charging and leakage currents may be thus summarized: 1. The distortion in the current wave form, which results from copious ionization at the time of the voltage peak, is observed simultaneously with the hissing sound and bluish discharge from the wire. 2. At air pressures below 20 cm. of mercury, copious ionization,?as indicated by the hump on the current oscillograms,?sets in at a lower voltage during the half cycle for which the wire is the cathode than during the half cycle for which it is the anode. 3. From the oscillograms thus far obtained at atmospheric pressure it has been impossible to determine whether there is any difference in the voltage at which the distortion appears in the two half cycles, at pressures near atmospheric. 4. The boundary of the ionized region does not gradually expand outward from the wire with the increase in voltage during the half cycle. The loss of insulating properties in the air surrounding the wire must be conceived of as taking place with extreme suddenness. The evidence of this is the oscillation in the current shown on the oscillograms. 5.

Air as an Insulator When in the Presence of Insulating Bodies of Higher Specific Inductive Capacity

Abstract: The purpose of the paper is to show the conditions that determine the disruptive strength of an air path along the surface of a solid dielectric of higher specific inductive capacity, and what steps must be taken to insure the most efficient use of the two dielectrics in combination. Part II of the paper discusses the effect on the electric field between two given terminals of the presence of a solid dielectric. An experimental means of determining the shape of the field is given. An explanation of the conditions that exist at the surface of the dielectric when in an electric field is given. The conclusion is drawn that if the surface of the dielectric be made tangential to the lines of force at every point, the strength of the path between the two given terminals will not be weakened. Part III shows the results of applying this theory to practical purposes, such as insulating a terminal rod passing through a casing, and directing the external field on the standard form of condenser type terminals. In this portion of the paper, several curves are given, showing the distribution of potential over the surface of the dielectric. Part IV gives the summary and conclusions, which are to the effect that breakdowns of an air path over a surface have been obtained, which average as high as 9.4 kv. per cm. effective value (23,900 volts per inch) over a distance of 17.0 cm. (6.7 inches). Conditions of design are such that these same averages may be maintained for any voltage, by increasing all dimensions of the structure proportionately. The strength of the air path in this form of design is independent of the specific capacity of the dielectric, and an important thing, therefore, is to shape the terminals properly in order to obtain a high average intensity over the given path.

The Dielectric Strength of Thin Insulating Materials

Abstract: The object of this paper is to record the results of an investigation which has been made at the Electrical Testing Laboratories to determine the effect of electrode area on the apparent dielectric strength of insulating materials in the form of thin sheets. The following tests were made with two, similar, flat, circular, electrodes ranging from 1/64 to 6, 10, and 15 in. (0.39 mm. to 15.2, 25.4 and 38 cm.) in diameter and placed directly opposite each other: 1. On insulating cloth and thin sheet hard rubber in air. 2. On insulating cloth in air and in transformer oil. 3. On transformer oil with various spacings. 4. On air with various spacings. The results of the various tests show very conclusively that the apparent dielectric strength of insulating materials in thin sheet form is materially higher with small electrodes than with large ones. This probably applies generally to all dielectrics, gaseous, liquid and solid, although the amount of variation differs widely with different materials. The tests emphasize the need for standard specifications for testing insulating materials, especially when in the form of thin sheets. It does not seem probable that the dielectric strength of an insulating material, under all working conditions, can ever be predicted with exactness, but at least we can have standard methods of rating such materials so that a value for the dielectric strength will have the same significance to the manufacturer, the purchaser and the designing engineer.

Tungsten Lamps of High Efficiency-I

Abstract: 1. The efficiency at which the larger sizes of tungsten lamps may be profitably run, is limited principally by the blackening of the bulb. 2. It has usually been considered that the blackening of ordinary lamps was due very largely, if not entirely, to the presence of residual gases. The evidence which has led to this belief is discussed. 3. The sources of gases within the lamp are studied, and the principal gases are found to be water vapor, carbon dioxide, carbon monoxide, hydrogen, nitrogen, and vapors of hydrocarbons. 4. The specific effects produced by these and other gases are determined. It is found that water vapor, which has long been known to be harmful, is the only one that produces perceptible blackening of the bulbs. 5. The blackening by water vapor is due to a cyclic process in which the water oxidizes the tungsten and is itself reduced to atomic hydrogen. The tungsten oxide volatilizes and deposits on the bulb, where it is reduced by the atomic hydrogen to metallic tungsten and water vapor is again formed. 6. Attempts to materially improve the life of lamps by the more complete removal of water vapor result in failure. It is therefore concluded that, although water vapor is usually the cause of the short life of poorly exhausted lamps, yet it is not the cause of blackening in well exhausted lamps. 7.

The Heating of Cables Carrying Current

Abstract: The current-carrying capacity of an insulated cable may be calculated from data on the thermal resistivity of the insulation and of the surface. It has been shown that these data may be obtained for any type of insulation from experiments on the cable itself by applying a well-known method for determing heat conductivities.

Notes on Induction Motor Losses

Abstract: The data obtained from numerous tests on induction motors, check the losses as given by the Standardization Rules with the exception of the fixed and stray losses. The tests show that both losses are less than would be obtained by the Institute method. The true fixed loss should be taken as the power input, when the motor is running at rated voltage and without load, minus the stator copper loss produced by the magnetizing current. This copper loss is appreciable in slow speed motors. With stator cores built up of well varnished punchings and without drifting or filing of the slots, the stray loss in the magnetic materials is negligible. The stray or eddy current loss in the stator copper is proportional to the stator copper loss, the percentage loss depending upon the degree of lamination of the conductor. The average stray loss of the test for both 25-and 60-cycle machines is approximately 6 per cent of the stator copper loss. The writer is not aware of any satisfactory commercial method of determining the stray loss in induction motor stators. It therefore seems desirable that an average value of the stray loss in per cent of the stator copper loss be adopted as standard. In squirrel cage motors and small wound-rotor motors, where the slip is usually greater than 2 per cent, the total rotor loss should be determined from the slip reading when operating at full load. The rotor loss should be taken as the copper I2r loss from resistance measurements in large wound-rotor motors where the slip of less than 2 per cent and a full load slip reading cannot be readily obtained.

The Behavior of Synchronous Motors During Starting

Abstract: The paper presents experimental data mainly in the form of oscillograms bearing on the subject of the paper. The oscillograms show very completely the effect on field current and armature current of a large number of combinations of starting conditions. From the oscillograms and curves, the following practical operating results are deduced: 1. That the effect of the closed field circuit on initial starting torque is negligible if low resistance damper windings of good design are used. 2. That with open field circuit during starting dangerous voltages are induced in the field winding. 3. That for all ordinary applications, at least, motors should be started with closed field circuit. 4. That successively larger loads can be pulled into synchronism if synchronism is attained: (a) On the starting voltage without excitation; (b) On the starting voltage with excitation; and (c) On line voltage with excitation. 5. That for moderate loads the minimum starting current is obtained by considerably over-exciting the motor before changing to line voltage. 6. That for large loads during starting and synchronizing, minimum starting current is obtained by under-exciting the motor before changing to line voltage. 7. Still further reduction in starting current is obtained by the use of reactance between the change from starting to line voltage, in which case excitation is delayed until the motor is in, or very near, synchronism.

Safeguarding the Use of Electricity in Mines

Abstract: The sources of danger attending the use of electricity in mines are shocks, fires and explosions. Shocks are the electrical accidents that happen most frequently underground, the conditions there being especially favorable to the occurrence of such mishaps. Fires may be caused by leaks to ground, short circuits and blowing fuses. Injuries to electrical equipment, due to falls of roof often add to the fire hazard. Explosions may be result of the ignition of explosives, mine gas or coal dust. The paper discusses the underground conditions that are unfavorable to the proper installation and maintenance of electrical equipment and suggests, in general terms, some methods of making safer the use of electrical equipment in mines. These suggesions are finally summarized briefly under five heads. The closing paragraphs of the paper discuss electrical equipment as a means of promoting safety and mentions the telephone, the portable electric lamp, electric shot firing devices and storage battery locomotives as equipment that may be regarded as lessening the risks surrounding the underground worker.

Thermocouples and Resistance Coils for the Determination of Local Temperatures in Electrical Machines

Abstract: The paper discusses different devices for measuring temperatures in electrical machines and points out their various sources of error, and also the limitations of the uses of mercury and alcohol thermometers, thermocouples and resistance coils. Fluid thermometers can generally measure only surface temperatures, and temperatures of rotating parts must be measured after they come to rest, thereby introducing large chances of error due to equalization or rapid changes of temperature, slow rate of indication of thermometers, small contact between the bulb and the surface to be measured, emergent stem, etc. In the measurement of temperature by the electrical resistance method either the windings of the machine itself or fine wire coils placed in or about the windings are employed. In using the coils of the machine, the result depends upon the accuracy with which the coefficient of change of resistance of the copper with temperature is known. This method, of course, gives only the average temperature of the whole length of the winding and does not indicate the temperature of any part of the winding. Where coils of fine copper are used highly accurate measurements are possible, as the coefficient of temperature of the wire may be accurately known. The extent of the coil determines the extent to which the measurement is local. For accurate determinations, some sort of bridge for determining the resistance is required. The thermocouple is the most accurate device for measuring local temperatures, as the e.m.f. generated is a function of the difference in temperature between the junction of the wires of the couple and their free ends, and the temperature of the free ends can be accurately controlled. Thermocouples require the use of precision meters for reading their indications, and potentiometer arrangements may be applied to obtain any desired degree of accuracy. Ordinary thermometry is the simplest, quickest and least accurate method of temperature measurement, and usually applies only to surface conditions. The choice between the electrical methods of temperature measurement is largely determined by the ease of application in any particular case. The thermocouple will give the most rapid indications, although the resistance method is almost as rapid if the coil is of right proportions and is intimately applied to the parts whose temperature is to be measured.

A Proposed Wave Shape Standard

Abstract: The present wave-shape standard has several objections, principal among which are the use of the oscillograph, the lengthy calculations necessary and the discrimination in favor of higher harmonics. The proposed standard makes use of the change of effective reactance of a condenser with change of wave shape. The important advantages are: the wave shape distortion can be determined very quickly; the distortion depends in a very definite way upon the harmonics present, both as to order and magnitude; the apparatus is very simple and the test easily manipulated. The disadvantage is that it does not show which harmonics are present. The ratio of the condenser reactance on a sine wave to the condenser reactance on the distorted wave is called the distortion ratio. It is conveniently determined by impressing the wave shape to be measured, first on a condenser with a large inductive reactance in series, and then on the condenser alone. The current and voltage at the condenser terminals in each case determine the reactance. The measurement of the distortion ratio on generating apparatus should be made at full voltage no-load, since the wave shape at load is indefinite. Several oscillograms show wave shapes recently observed and accompanying them are the distortion ratios, the deviations and the analyses. The paper concludes with proposed substitutes for the present A. I. E. E. Standardization Rules. The derivation of the theoretical formula for distortion ratio is given in the appendix. Tables are also given to serve as a guide in determining permissible distortion.

A Suggestion for the Engineering Profession

Abstract: The engineer of today traces his ancestry along two distinct lines; one practical, the other theoretical. The development of the modern type of engineer has been along a great variety of special lines; even college courses and degrees have reflected this tendency towards specialties. As a result there is no united engineering profession, but instead a variety of engineering professions. This lack of unity has prevented the engineering profession from exerting its proper influence in the solution of economic problems, and has also prevented the profession having the standing that it ought to have among the learned professions. Colleges and technical schools will undoubtedly be effective in introducing this desirable unity, but the process will of necessity be a very slow one. Suggestons have been offered in the past for the formation of an American engineering society which would include all kinds of engineers. For very many reasons this has always been found impracticable. A suggestion is here offered for the formation of an American Engineering Association in which there would be no personal membership, but in which present national engineering associations would participate by electing representatives annually. No business, commercial or trade organizations would be included.