Showing papers in "Transactions of The American Institute of Electrical Engineers in 1940"

Frequency Modulation

Abstract: RADIO communication makes use of a medium common to the whole world for the transmission of many signals simultaneously. In order to accomplish this a high-frequency electromagnetic wave has one of its characteristics varied in accordance with the instantaneous variations of the signal to be transmitted. The control of these variations is called modulation. The various simultaneous messages can then be separated by: (a). Differences in the frequency band used. (b). Differences in signal strength. (c). Differences in direction of the source.

Load-Rating Theory for Multichannel Amplifiers

Abstract: The amplifiers of multichannel telephone systems must be so designed with regard to output capacity that interchannel interference caused by amplifier overloading will not be serious Probability theory is applied to this problem to determine the maximum single-frequency output power which a multichannel amplifier should be designed to transmit as a function of N, the number of channels in the system The theory is developed to include the effects of statistical variations in the number of simultaneous talkers, in the talking volumes, and in the instantaneous voltages from speech at constant volume

Transient Starting Torques in Induction Motors

Abstract: A transient fundamental-frequency torque is shown to occur in the starting of all induction motors. For full-voltage starting on a general-purpose squirrel-cage motor, this torque may be as high as two or three times the pull-out torque. A consideration of such torques is particularly advisable for cases where induction motors are started and stopped frequently or continuously, since under such conditions mechanical failure of the motor or associated parts, gearing, couplings, etc., may occur if the stresses exceed the endurance limit of the material. A method of analysis is given for calculating the transient electrical torques acting in the case of a locked rotor; the results obtained by means of this analysis are compared with test data obtained on a special locked-rotor test setup. A method is also given for calculating shaft torque from the electrical torque in the usual application. Test results also indicate that this method is satisfactory for practical use.

Current Transformers and Relays for High-Speed Differential Protection, With Particular Reference to Offset Transient Currents

Abstract: Up to the present time, the only data generally available to protection engineers for determining the performance of current transformers have been the ratio and phase-angle curves of the current transformers in question. More detailed analysis sis of important high-speed differential protection problems has shown that these data are hopelessly inadequate for the purpose of determining the performance of current transformers during the starting period when the current wave may be fully offset. If the time constant for decay of the d-c component exceeds 0.05 second, large errors of transformation may be expected if standard current transformers are used. While it is possible to make special designs that will not saturate because of the d-c component in the current,1 such designs are usually considerably oversize and expensive. Furthermore, space requirements and economy sometimes dictate that current transformers already installed be used if at all possible, even though it is known that their performance will be far from perfect. The problem then becomes one of determining the actual performance of existing current transformers, so that a suitable relay scheme may be chosen. It is the purpose of this paper to discuss the solution of this problem, and to present approximate methods of calculation not hitherto available. Attention is also given to the proper interpretation of the calculations with respect to relay operations, together with supporting test results.

Harmonic Theory of Noise in Induction Motors

Abstract: This paper presents a theory of noise generation particularly adapted to use with small motors. Since single-phase motors predominate in the small-motor field, a single-phase point of view has been employed. The underlying philosophy of the paper is to reduce the theory to simplest terms and to concentrate attention upon the most probable sources of noise. While the results are shown as a large collection of numbers representing force waves the development steps of the theory are really simple and easily understood. The resultant air-gap flux wave in an induction motor is the difference between the fundamental stator flux plus its harmonics and the fundamental rotor flux plus its harmonics. The difference between the two fundamental fluxes is usually less than the larger one and often less than either one but the harmonics, with few exceptions, are neither of the same velocity nor space order and, therefore, do not add or subtract from each other It follows that the resultant flux wave has more harmonics of much greater per-unit amplitude than does either the stator or rotor flux wave alone. It is the interaction of two large harmonic flux waves of slightly different space order and traveling at high frequency with respect to each other that causes high-frequency magnetic-force waves of long space pitch which are effective in producing noise. The theory has been used for some time in noise investigations and has been found to bear out test results on a large number of motors.

Instruments and Methods of Measuring Radio Noise

Abstract: This paper embodies the relevant agreed recommendations of the Joint Co-ordination Committee on Radio Reception of EEI, NEMA, and RMA, as to the nature, essential characteristics, and performance of an instrument for the measurement of radio-noise voltages. It further gives detailed descriptions of the recommended practices for measuring radio noise directly from low- and high-voltage apparatus, for making noise measurements along overhead lines, for determining broadcast field-strength levels, and methods of collecting data for the establishment of radio-noise standards.

Contact Phenomena in Telephone Switching Circuits

Abstract: The phenomena occurring at the closing and opening of contacts carrying weak currents have been investigated by means which include a study of the high-frequency transient voltages and currents. These influence the erosion in a complex manner which varies with contact materials, surface conditions, and surrounding atmosphere. Three principal classes of effect have been distinguished. These are: (1) Disruptive spark-overs initiating a series of metallic arcs lasting less than a microsecond each. (2) A nitrogen-gas glow discharge at about 300 volts, preceded by a brief group of disruptive spark-overs. (3) High field breakdowns due to cold point discharges which cause transient metallic closures of approaching contacts and similar transient reclosures of separating contacts.

Effect of Load Factor on Operation of Power Transformers by Temperature

Abstract: The practice of loading transformers by temperature is growing among operators who desire to take advantage of the additional capacity offered by this method of loading. The standards now recognize continuous overloads in cool ambients and short-time overloads under recurrent and emergency conditions. The paper discusses the effects of different shapes of system load factors on the aging of the insulation. Then the ones that result in the greatest amount of aging are used for determining the permissible peak loads that can be carried safely with the same deterioration of its insulation as results from continuous operation at rated kilovolt-amperes. A rational method of calculating the hot-spot temperature is shown by means of examples using a general average ``time constant''. The hot-spot temperature curve is then integrated to compare the aging with a given constant temperature. From these results it is concluded that a normal power transformer can be overloaded 3 per cent for each 10 per cent that the system load factor is below 100 per cent. That is, for a 50 per cent load factor the permissible peak load is 15 per cent over the transformer rating in a standard ambient. For low ambients where 1 per cent overload for each degree that the ambient is below 30 degrees is now permitted, the permissible peak loads will vary depending on ambient and on load factor.

Special Problems of Two-Pole Turbine Generators

Abstract: Double-frequency rotor vibration due to different rigidity constants for the two major axes in two-pole 60-cycle generators can be eliminated by equalizing the rigidity constants on the two axes. This can be accomplished without introducing any reduction in flux capacity by machining narrow transverse slots in the body of the poles. The double-frequency vibratory forces inherent in two-pole generator stators cannot be eliminated. It is further believed that acceptable results cannot be economically obtained by reducing the air-gap magnetic densities and increasing the rigidity of the stator core over that normally obtained when magnetic conditions are adequately satisfied. Also the stator vibration is not of an amplitude considered to be harmful; consequently, the most practical solution is to isolate the stator core by means of flexible supports, and thus materially reduce the magnitude of the disturbing forces transmitted to the stator frame, foundation supports, and associated apparatus.

Arc-Furnace Loads on Long Transmission Lines

Abstract: LARGE electric arc furnaces are loads which frequently introduce special technical problems when served from a central-station system. The fluctuations in the load taken by the furnaces are violent, and this characteristic combined with the inherently low power factor of this type of load results in a serious flicker problem. The low power factor also may require power-factor correction, in order to maintain a reasonable voltage level and reduce transmission losses. A further complication is that the arc is an excellent source of higher-harmonic currents and voltages. Where the load or the supply lines are close to telephone circuits, this may necessitate special measures to prevent telephone interference.

What Wood May Add to Primary Insulation for Withstanding Lightning

Abstract: ON power-transmission lines, wood, serving certain structural functions, simultaneously may provide supplementary insulation against lightning if properly chosen and applied. However, to sustain the power-frequency voltage, adequate primary insulation in the form of porcelain insulators or the like is essential.

``Magne-Blast'' Air Circuit Breaker for 5,000-Volt Service

Abstract: Fundamental principles of magnetic action and thermal reaction have been united in an original manner to create a new circuit interrupter, the ``Magne-Blast'' air circuit breaker. Intended for general applications in switching, controlling, and protecting 2,300- to 5,000-volt circuits, this breaker, throughout a comprehensive test program, has proved satisfactory in every requirement. An open presentation of test results shows how an efficient arc chute functions thermally to decay and extinguish the arc with a minimum of voltage disturbance. The breaker is immune to circuit recovery characteristics. Principles of performance are discussed using the familiar terms of current, voltage, resistance, and reactance in showing their effect on the instantaneous values of phase angle, current reduction, and the attendant rates of rise of recovery voltage. The coordinated electrical and mechanical design is described in some detail and a typical arrangement is shown as used in a metalclad equipment.

The Hydrogen-Cooled Turbine Generator

Abstract: HYDROGEN cooling for electrical machinery was first proposed to the industry through an AIEE paper presented in 1925.1 The first hydrogen-cooled machine to be placed in commercial service was a 12,500-kva synchronous condenser, installed in 1928.2 This was followed by other machines of this type, and later, the hydrogen-cooled frequency-converter was introduced. There are now in operation in the United States 20 of these two types of hydrogen-cooled machines, with a combined output of over one-half million kva, and their records of performance have been highly satisfactory. Prior to 1937, the application of hydrogen cooling to turbine generators had been confined to developmental machines in the manufacturers' plants, although several air-cooled generators had been built with provision for later adaptation to hydrogen cooling. In October 1937 the first hydrogen-cooled generator built for commercial service was placed in operation at Dayton, Ohio. This was a 3,600-rpm unit, of General Electric manufacture, rated at 31,250 kva at 0.8 power factor. Since then, a total of 10 hydrogen-cooled generators have been placed in service in various parts of the country, and 27 others are in the course of installation or construction. The total capacity of these 37 generators is over 2,000,000 kva, and their sizes range from 17,000 kva to 81,250 kva at 3,600 rpm, and from 75,000 kva to 176,470 kva at 1,800 rpm.

Wave Shape of 30- and 60-Phase Rectifier Groups

Abstract: AS pointed out in a number of recent papers,1,2 a mercury-arc rectifier draws a distorted current from the a-c supply system, even though power to the system is supplied by a sine-wave source. The harmonics in the current wave react on the supply system to cause distortion in the voltage wave, the degree of distortion being a function of the rating of the rectifier, the number of phases in which power is supplied to the anodes, the impedance at harmonic frequencies of the supply system, and the rectifier load. It is well known that, in theory at least, reductions in the wave-shape distortion may be effected by increasing the number of rectifier phases and thus reducing the number of harmonic components present in the supply-circuit current.1,2 Up to 1937, however, because of practical difficulties, largely in the design of the complex transformer arrangement required, no attempts were made to produce rectifiers commercially having more than 12 phases.

Overcurrent Performance of Bushing-Type Current Transformers

Abstract: Conventional methods used for defining and comparing current-transformer performances are commonly based on their ratio and phase-angle characteristics. For years, these characteristics have been continuously improved, particularly with respect to the requirements for metering service. However, from the standpoint of circuit protection, current-transformer application becomes more involved and as a result, increased attention has been given it. A better knowledge of transformer performance is necessitated by the requirements of new relays and protective schemes. Attention to the influence of the d-c component in short-circuit transients is especially necessary when using high-speed relays. Also, in differential connections, the necessity for equivalent matching of saturation characteristics is essential. These are illustrations of the types of problems which ratio and phase- angle characteristics alone do not solve. The analysis of open-circuit saturation curves has proved of immense value in the solution of these problems. This paper gives results of investigations on bushing-type transformers that indicate the proper choice of analysis procedure and the limitations of the various methods for determining performance under overcurrent operating conditions.

A Proposed Method for the Determination of Current-Transformer Errors

Abstract: CURRENT TRANSFORMERS for relay service must be able to function with reasonable accuracy under very high overload conditions, and it is therefore necessary that their accuracy for such high currents be verified. However, in performing the usual accuracy test at very high currents, certain difficulties are encountered — the principal ones being:

Basic Impulse Insulation Levels

Abstract: ONE of the early steps taken by the electrical industry, in an attempt to help solve the insulation-co-ordination problem was the formation of the National Electric Light Association (now Edison Electric Institute)-National Electrical Manufacturers Association Joint Committee on System Insulation Co-ordination in 1931. Before relating the work done by this committee, to acquaint the industry with what has been done to date and the present status of the work, it may be well to comment briefly, on what is meant by “insulation co-ordination.”

Sensitive Ground Protection for Radial Distribution Feeders

Abstract: THERE are three reasons for isolating or de-energizing grounded feeders: first, danger to life; second, danger to property; and third, to permit joint use of pole lines with the telephone companies. The first two are very difficult to evaluate, but in certain territories a means of isolation is very important. The third reason can be given a definite value, and the economies resulting from joint use of pole lines can more than justify this proposed scheme of sensitive ground protection for radial feeders.