摘要:

The phase-margin concept of the behaviour of the Nyquist diagram in the vicinity of the critical point is extended to give a more generalized formula. This relates the damping of the principal oscillatory mode of a closed-loop feedback system to the shape of the Nyquist diagram. All the quantities involved can be obtained from this diagram without further mathematical analysis or graphical construction on the diagram.A theoretical foundation for the formula is given and experimental verification is included.

DOI:10.1049/pi-2.1953.0083

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper describes the principles, operation and performance of an equipment for obtaining in a rapid manner the frequency responses (amplitude and phase) of servo mechanisms, passive networks, amplifiers, etc. The frequency range covered is 0.1–100 c/s, and the phase angle is indicated on a dial for instantaneous reading, thus obviating the tedium of oscillographic methods. A useful characteristic of the equipment is its ability to discriminate in favour of the fundamental when the output signal from the system under test contains harmonics.The most novel feature of the equipment is the employment of an air-cored oil-dashpot-damped dynamometer for indicating phase shift. Preliminary investigation showed that, although air-cored, any dynamometer method would be subject to frequency errors. However, this has been overcome by driving the dynamometer with field-forcing power amplifiers of high linearity. It was also found by mathematical deduction that, provided the fixed-coil system of the dynamometer is excited with pure sinusoidal oscillations, its moving coil will indicate the phase shift of only the fundamental of any distorted signal presented to it. This appeared to be a valuable feature of such a system and has been verified by experiment.The complete equipment includes an electro-mechanical signal generator, providing either modulated carrier-frequency or sinusoidal signals, a special amplifier for coupling the signal generators to test circuits of low input impedance, and a calibrated amplifier for attenuation/frequency measurements. The individual units are designed for maximum flexibility in application, and their particular features are described.

DOI:10.1049/pi-2.1953.0086

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The adoption, in recent years, of layer windings and the introduction of cold-rolled silicon steels have effected a marked increase of impulse strength together with a reduction of size of electromagnetic voltage transformers. The development of synthetic resins as insulants for electromagnetic transformers, although in its early stages, shows promise that they will replace other insulants for voltages up to 33 kV.Graphs are given for estimating the output of capacitor-type transformers, and it is concluded that they are uneconomical below 110 kV. Economically they are very attractive when used in conjunction with high-capacitance-type extra-high-voltage current transformers which have recently been developed.The transient performance of current transformers is examined, and it is concluded that protective systems should either impose a very small burden or permit some saturation. The burden can often be reduced by rating the secondary winding at 1 amp, and this is recommended where the protective apparatus is not adjacent to the transformers. The main objection to such a rating lies in the high voltages which may be induced in the winding; these are discussed.Various power-system operations and protection requirements are considered in order to determine the most suitable location for current transformers and voltage transformers relative to the associated switchgear.

DOI:10.1049/pi-2.1953.0089

出版商:IEE    

年代:1953

数据来源: IET

摘要:

A brief survey is given of the methods of examining opaque specimens in the electron microscope. The principle of the scanning electron microscope is described and mention is made of previous instruments. The design of such microscopes is discussed and the theoretical limit on performance is determined. The optimum scanning-beam voltage, beam current and scanning speeds are given.A recently built electrostatically focused scanning electron microscope for opaque specimens is described. This microscope has several new features, including:(a) The use of high-energy electrons to reduce the effects of surface contamination of the specimen.(b) Oblique scanning of the specimen, so that the image contrasts are formed by the topography of the etched surface of the specimen.(c) Direct amplification of the electron beam with an electron multiplier having beryllium-oxide-coated dynodes.(d) Direct viewing at low magnifications on a cathode-ray tube with a long-persistence screen, and photographic; ecording from the screen at high magnifications.Examples of micrographs of aluminium taken with the instrument are included and these show a resolution of about 500 Å. It is concluded that higher resolutions could be obtained with this instrument if improved magnetic shielding and astigmatism correction were to be incorporated.Some measurements on the reflection of high-energy electrons are described in an Appendix.

DOI:10.1049/pi-2.1953.0095

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper describes a new permanent-magnet moving-coil relay which overcomes the non-linearities associated with electromagnetic differential relays.Its application to balanced-current protective systems is described, and the factors affecting the use and interpretation of through-current bias are discussed.Other applications of the relay to impedance, pilot-wire and busbar protective systems are described as well as a new scheme of instantaneous differential protection for power transformers.The appendices indicate that the performance of protective gear has been reduced to a calculable basis.

DOI:10.1049/pi-2.1953.0099

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper describes the uses to which an additional conductor on an overhead transmission line can be put if the conductor is lightly insulated at each support or embedded in insulation within the earth wire and earthed at the ends. One such use is to provide a cheap and reliable communication channel; another is to employ the induced currents from the main conductors in new types of discriminating protection, and in the measurement of fault locations on straight or teed feeders. Formulae are derived for calculating the current in the signal conductor under various conditions of operation and fault, and the calculated values are compared with those observed.

DOI:10.1049/pi-2.1953.0102

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The most common form of static frequency tripler consists of a 3-phase transformer operated at a fairly high flux density, with an open-delta secondary connected to the triple-frequency load. The nonlinear property of theB/Hcharacteristic of the core material is used to obtain the frequency conversion, and the unwanted fundamental component is removed by the method of connection. There is no theoretical reason why such a frequency tripler should not be designed starting only with the magnetization curve of the core material, but to do this is exceedingly tedious. A more practical approach is to abstract the required information experimentally and present it in a manner convenient to the designer.A series of tests were performed to obtain the information necessary; the cases investigated included pure resistive loads, pure inductive loads and capacitive ballast added to a resistive load to improve the performance of the tripler. The main information obtained is: (a) the relation between the triple-frequency output voltage and current with the input voltage and current at all values and for each type of load; (b) the phase of the output; (c) the iron losses. In addition, an important ferro-resonance phenomenon which occurs when the tripler is connected to a capacitive load is described.A method of deducing the relevant characteristics of the tripler starting from the magnetization curve is included mainly to throw light on the mechanism of tripling under differing load conditions. Reasonable agreement was obtained between experimental and predicted values.

DOI:10.1049/pi-2.1953.0108

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The regulation curve of a double 3-phase mercury-arc rectifier is characterized by a sharp fall in the output voltage as the load current increases from zero to a value about twice that of the peak magnetizing current of the inter-phase reactor. Over this current range the mode of operation of the rectifier changes; at no load each anode fires in turn, in a manner typical of a 6-phase rectifier, but when the load current is just sufficient to magnetize the inter-phase reactor, double 3-phase operation of the rectifier commences. To reduce the no-load voltage rise, which in some rectifier applications cannot be tolerated, one of the more satisfactory methods is to excite the inter-phase reactor from an auxiliary source. It is shown in the paper that, by using a frequency tripler as a source of auxiliary excitation, a tenfold reduction is obtained both in the no-load voltage rise and in the critical load current at which double 3-phase operation commences. The variation of the rectifier no-load voltage rise with the phase and magnitude of the frequency-tripler output voltage and with the alternating supply voltage is also deduced.

DOI:10.1049/pi-2.1953.0109

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The influence of polarity, polarity reversal, repeated stressing, waveshape and conductor size on the impulse puncture voltage for three sizes of single-conductor mass-impregnated 33-kV cable has been investigated. Experimental results are discussed and conclusions drawn from which a test for incorporation in a cable specification and the revision of B.S. 923: 1940 for cables has been suggested.

DOI:10.1049/pi-2.1953.0110

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The basic design principles for resistance standards having a resistance differing from the nominal value by less than 0.01% and a phase angle of less than 0.0001 radn at frequencies up to about 20 kc/s are described. The new nickel-chromium-aluminium alloys, on account of their small change of resistance with temperature, appear to be suitable for standards which must be operated with a considerable temperature rise owing to self-heating. Formulae are given for calculating the residual inductance of standards of various types. A one-ohm standard having a calculated phase angle of less than 0.0001 radn at frequencies up to 30 kc/s is described.

DOI:10.1049/pi-2.1953.0111

出版商:IEE    

年代:1953

数据来源: IET