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Electrical Engineers, Proceedings of the Institution of

Issue 12 • Date December 1968

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Displaying Results 1 - 23 of 23
  • Model for a synchronous p.c.m. transmission complex

    Page(s): 1743 - 1748
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (715 KB)  

    One of the proposed synchronisation systems for a large-scale p.c.m. transmission and switching complex uses independent oscillators at each station, which are phase-locked to the average phase of all the incoming lines. This paper derives a simple analogue to the system which allows a sufficient stability condition to be stated by inspection. The effect of parameter changes upon the equilibrium frequency is discussed, and a detailed discussion of the initial conditions shows that, for a 2-oscillator system, there are two stable-equilibrium frequencies that the system may reach. The Appendix contains rigorous proofs of the more intuitive statements in the paper. View full abstract»

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  • Internal structure and parametric representation of equivalent resistive n-port networks

    Page(s): 1749 - 1754
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    The theory of resistive n-port networks has attracted considerable interest during the last decade. This is due, partly, to the fact that results obtained in analysis and synthesis of resistive networks have been applied to many other engineering problems. The field of applications covers all such systems which can be modelised by nets whose branch characteristics may be expressed by real numbers, e.g. communication and probabilistic nets, switching networks etc. In the paper, general equivalence theory of resistive n-port networks is studied. A parametric representation of the whole class of equivalent resistive n-ports corresponding to a given short-circuit admittance matrix is derived. An algebraic-geometric interpretation of this representation is given and some properties pertinent to its continuity and differentiability and to the connectedness of the domain of realisability are then obtained. A special discussion is devoted to certain distinguished subclasses of equivalent resistive n-ports. The results obtained here reveal the internal symmetry inherent in the whole family of equivalent n-ports. View full abstract»

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  • Experimental tests with orthogonal transmission

    Page(s): 1755 - 1761
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    Propagation tests indicate that the skywave field strength from a medium-frequency broadcasting transmitter can be reduced by 16dB on paths to the north at midlatitudes in the southern hemisphere, when vertically polarised transmission is replaced by orthogonal transmission. The reduction is observed to vary with time and path bearing, the variation with time being such that, during any one night, the median reduction is a maximum when the field strength is high, and a minimum when the field strength is low. Short-term variations are considered to be consistent with a small variation in the limiting polarisation of the ionosphere, and with ray deviations from the most direct path. Long-term variations are considered to be due to changes in the nondeviative absorption from night to night. The observed change in improvement factor with path bearing is in agreement with that expected for the transmitting-aerial configuration. Implementation of this system awaits the development of suitable transmitting aerials. View full abstract»

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  • Application of finite elements to the solution of Helmholtz's equation

    Page(s): 1762 - 1766
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    A novel method, that of finite elements, for the solution of Helmholtz's equation is suggested. Various 2- and 3-dimensional problems are solved using this method, and the results are compared with more conventional techniques, particularly the finite-difference method, which it may be regarded to supersede. The ease with which various boundary conditions may be handled is discussed and illustrated. Nonhomogeneous configurations present no difficulty, nor do they require any special formulation. There is considerable scope for the further development of the technique, which has, until now, been applied mainly to the solution of Laplace or Poisson equations. View full abstract»

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  • Equivalent circuit for nonquadrature, tapped-quadrature and shaded-pole single-phase induction motor

    Page(s): 1767 - 1771
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    It is shown that the nonquadrature, tapped-quadrature and shaded-pole forms of single-phase induction motor can be equivalent to a conventional quadrature motor having a common line impedance. It follows that a single equivalent circuit can be used to determine the performance of each form of motor, provided that appropriate values are employed for the circuit elements in each case. In consequence, the same arrangement of a given type of analogue computer can be used to predict the performance of all three special forms of single-phase motor, as well as the conventional quadrature motor. Further, in contrast to some existing equivalent circuits, the presented circuit has the advantage of direct representation of the actual supply currents taken by each form of motor without any transformation of the supply voltage. View full abstract»

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  • Travelling waves in induction machines: input impedance and equivalent circuits

    Page(s): 1772 - 1776
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    The problem of travelling electromagnetic waves in multiregion induction machines at power frequencies is examined. Using the concept of surface impedance, a method is derived whereby equivalent circuits can be established in a systematic manner. A method of determining levitation force from a knowledge of the equivalent circuit is developed. Calculation of input and output power is simplified using the approach outlined. The ferromagnetic portions of the multiregion system are assumed to have constant permeability. View full abstract»

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  • Induction-motor losses due to nonsinusoidal supply waveforms

    Page(s): 1777 - 1782
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    The paper presents an analytical and experimental study of the additional losses which occur in induction motors supplied with nonsinusoidal waveforms. Measurements on an inverted induction motor are used to show the importance of losses due to skew-leakage and end-leakage fluxes, and to verify the methods developed for calculating these components. Computed losses are compared with test results obtained on a variety of machines and supply waveforms, and the agreement is shown to be consistently good. The manner in which the losses are related to the machine design and to the supply harmonic content is described. View full abstract»

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  • Cogging torques in induction machines

    Page(s): 1783 - 1790
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    Depending on the choice of design parameters, an induction machine can produce a cogging torque sufficiently high to give rise to locking at standstill. It is noted that the commonly used method of analysis of these torques, involving permeance waves, is strictly incorrect, and its use in conjunction with the `overlap¿ method is inaccurate. The way in which cogging torques arise is discussed in terms of the energy associated with the magnetic field in the machine. A new method is described by which this torque variation can be determined from simple equations. Experiments on machines confirm the results of the analysis. View full abstract»

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  • Performance of Lorenz-type medium-frequency inductor alternators on load

    Page(s): 1791 - 1800
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    Earlier papers have discussed the air-gap flux-density distribution in inductor alternators and have verified the theory by measurement. It is the aim of this paper to show that, with a knowledge of flux-density distribution, it is possible to predict the load performance of these machines from the open-circuit curve¿an easily calculable starting point. First, the measured open-circuit voltages are compared with those calculated from previous knowledge of the flux-density wave, which in turn has earlier been compared with that calculated from air-gap geometry. By taking the correct components of armature reaction, it is shown that the voltage at any load point can be obtained by combining these with the open-circuit flux distribution, to obtain a resultant flux distribution, and hence voltage. This involves the intersection of generalised curves, expressing the vector-diagram geometry, with the special curve for the particular machine, expressing the open-circuit-voltage/field-current relation. Examples of this method are given both for the experimental machine and for a much larger commercial alternator, and are shown to predict the load curves under widely varying conditions. View full abstract»

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  • Output coefficient of synchronous machines. A new concept

    Page(s): 1801 - 1807
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    In the design of synchronous salient-pole generators and motors, current formulas for the output coefficient express this constant in terms either of the apparent power, speed, length of stator core and gap diameter or of the magnetic and electric loading. The paper develops an analysis in which the output coefficient is expressed as a function of the dimensions of the stator core, the magnetic loading and the electric loading related to the temperature rise of the stator winding, in terms of the thickness of insulation and slot dimensions. The output of the rotor is derived as a function of the pole pitch and height of pole. It is shown that there are two optimum values of the mean flux density in the air gap, and that there is also an optimum value of the electric loading, defined as a function of the width of slot. It is also shown that, for a stator designed in accordance with this analysis, there is one value of the height of rotor pole required to accommodate a field coil to establish the m.m.f. required to balance the armature reaction. The theory is illustrated by a series of curves calculated from the parameters of an existing large hydroelectric generator. It is further shown that the overall dimensions and cost could possibly have been reduced by the application of the principles established in this paper. View full abstract»

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  • Simulation of convertor performance under unbalanced conditions

    Page(s): 1809 - 1817
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    The paper is devoted to the development of mathematical models for the analysis of the behaviour of h.v.d.c. convertors during unbalanced a.c. faults. A current-source simulation is used for the convertor in either 3-phase or sequence components representation. The analysis using these models can be adapted for digital/analogue simulation. Using the digital approach, results were obtained for systems with varying degrees of unbalance, and limits of normal operation were established. For normal operation, the degrees of unbalance of the convertor currents were determined. This provides both the pattern and the limits of variation of these currents, which form a valuable basis for the assessment of overall performance of a.c./d.c. interconnected systems. View full abstract»

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  • Nonlinear transformer-winding synthesis for improved surge-voltage distribution

    Page(s): 1821 - 1827
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    The paper shows that the capacitance and inductance distributions within a power transformer can be so synthetised that an acceptable transient surge-voltage distribution can be obtained without necessarily requiring the reinforcement of end turns. This process can furthermore be carried out without contravening practical requirements, in particular without increasing a given transformer frame size. Linear, exponential and hyperbolic-sine distributions are considered in the paper, although the method, in fact, extends to any arbitrarily chosen function of voltage distribution. The effect of a particular distribution on the main geometric parameters of the transformer windings is then established in such a way that they can be incorporated in the transformer design. It is shown that arrangements giving initial and final voltage distributions of exponential and hyperbolic-sine form, respectively, are likely to be satisfactory to the designer. View full abstract»

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  • Control-system stability in multiterminal h.v. d.c. systems

    Page(s): 1828 - 1834
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    A linearised analysis of a multiterminal h.v. d.c. system leads to a stability criterion in a convenient form for assessing the results of changes in controller properties at one terminal. Constant-current, constant-power and constant-extinction-angle control are considered in detail, and the stability of various control configurations is examined. The predictions of the small-signal analysis are verified by digital simulation for these cases and for changes in the smoothing-reactor size, the length of a branch of the d.c. line, and the controller transfer functions. View full abstract»

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  • Magnetic properties of grain-oriented silicon iron. Part 1: Use of ceramic displacement transducers for the measurement of magnetostriction

    Page(s): 1835 - 1839
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    One basic problem in the measurement of magnetostriction in laminations is that samples of silicon-iron sheet are rarely perfectly flat when tested. This can sometimes lead to spurious values for the longitudinal magnetostriction, with ceramic displacement transducers. A method has been developed whereby, using three transducers in turn, the true longitudinal component of magnetostrictive motion can be determined. The use of an oil film between the sample and the bed plate, to reduce or eliminate the vertical movement, has been shown to be unreliable. To measure the magnetostriction directly over a small region (say, from grain to grain), a special pickup has been developed from the basic parts of two transducers. Each part is calibrated independently, and the two outputs are connected in opposition. By varying the apparent sensitivity of one part of the double pickup in predetermined steps, the true longitudinal magnetostriction over the small region can be evaluated. View full abstract»

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  • Magnetic properties of grain-oriented silicon iron. Part 2: Basic experiments on the nature of the anomalous loss using an individual grain

    Page(s): 1840 - 1845
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    Measurements of the position of the parallel-bar-domain walls at various points on the magnetising cycle have been made over the frequency range 25¿100 Hz in an individual grain of transformer steel, using the Kerr magneto-optic technique. A single-turn search coil of very thin wire wound around the test grain, through two 0.004 in holes, enabled the flux density to be measured. By plotting the degree of magnetic saturation, as indicated by the position of the surface walls, against the degree of saturation, as measured in the volume by the search coil, the amount of domain-wall bowing could be noted at various frequencies. Even at 25 Hz, there exists considerable bowing. In addition, the total power loss in the same grain has been measured at various flux densities in the frequency range 10¿100 Hz, using the rate-of-temperature-rise technique reported earlier. The importance of carrying out this basic study on a single grain is noted, and the experimental results are discussed in terms of various domain models, and of the various concepts that have been introduced to explain the nature of the anomalous loss. View full abstract»

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  • Dielectric loss of poly(4-methyl pentene-1) at cryogenic temperatures

    Page(s): 1846 - 1848
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    The loss-tangent of aged samples of poly(4-methyl pentene-1) have been measured within the temperature range 2.5¿295° K and frequency range 229Hz¿21.5kHz. These results have been compared with published results made on a new sample of the polymer. The nature of the loss tangent and the number of absorption regions depended on the age of the polymer. The aged samples exhibited absorption regions at temperatures of about 250, 150 and 4°K; the maximum loss tangent in each region was about 1¿5 × 10¿4. The highest temperature region was not observed in the new sample and appeared to be due to oxidation. The other regions were exhibited by the new sample, and both appeared to be due to the side group of the polymer. The loss tangent in these regions decreased with increasing age; an effect which seemed to be due to increased crystallinity. During 18 months aging, the increased crystallinity increased the activation energy of the lowest-temperature loss region from 2.2 to 10.3cal/mol, increased the most probable relaxation time from 9 ¿s to 27 ¿s but did not affect the distribution of relaxation times. View full abstract»

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  • Platinum-based permanent-magnet alloys

    Page(s): 1849 - 1852
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    Studies of the changes in magnetic properties of PtCo and PtFe consequent on replacing part or whole of the platinum by palladium, rhodium, iridium and ruthenium, and on preparative methods and heat treatments for these alloys, have shown that the pseudobinary alloys do not appear to offer improvements in magnetic properties. By careful heat treatment of PtCo, made by sintering and working or by vacuum melting, energy products of 9.5 and 10.0 × 106Gs-Oe, combined with coercive forces of 5.0 and 4.5 × 103Oe respectively, can be obtained. View full abstract»

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  • Evaluation of some aspects of relativity

    Page(s): 1853 - 1856
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (628 KB)  

    If the relativity theory is to predict experimental results about the rates of clocks, the time intervals must be defined in such a way that they are capable of measurement. When this is done the confusion which has been associated with the theory is explained. The theory is shown to be self-consistent up to a point, but an error is then made in the course of a `thought experiment¿. The result given does not follow from the `experiment¿ but from an additional assumption made implicitly. The same kind of error is made in the `thought experiments¿ in Einstein's later papers. The results again follow from implicit assumptions and not from the `experiments¿. His final assumptions that time dilatation is a consequence of acceleration or gravitational potential are supported by the results of recent experiments. View full abstract»

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  • Application of a computerised alarm-analysis system to a nuclear power station

    Page(s): 1858 - 1864
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    The paper describes the alarm analysis by digital-computer system which has been installed and is now operating at Oldbury nuclear power station. This is the first such plant in which alarm information has been presented to the operators by c.r.t. displays. The paper also shows how the predicted alarm fault patterns, on which the analysis is based, were prepared for programming. Some details of operational experience are also given and conclusions drawn. View full abstract»

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  • Dynamic performance of induction motors in control systems

    Page(s): 1865 - 1870
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    The paper analyses the behaviour of induction motors in control systems in response to sinusoidally varying torque distrurbances by using a 2-coil rotor model. It is found that induction motors exhibit resonance and possibilities of instability in response to torque disturbances, and transient torques substantially greater than the steady-state peak torque of the motor may be generated in the region of resonance. It is also observed that, in certain frequency ranges, the speed may be considered as leading the torque disturbances. Extension of previous frequency-response analysis in response to excitation-frequency variations and applicability of the present analysis to constant-net-flux operation are also given. View full abstract»

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  • Design of an accurate sampled-data simulator using field-effect transistors

    Page(s): 1871 - 1877
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    The paper describes the design of an analogue simulator suitable for sampled-data systems. The basic unit, the zero-order sample and hold, uses field-effect transistors and employs methods which allow a single value of the holding capacitor to be chosen to cover a variation in sample frequencies of 1:6 × 105, consistent with a sample time ts¿ 1 ¿s and a hold/sample ratio not greater than 3 × 107, while maintaining an accuracy of within 0.1%. The minimum sample time of the technique described is about 30ns, which compares favourably with a gallium-arsenide-diode bridge technique of greater complexity and having an inferior holding performance. The simulator uses a parallel sequential method of delay and is capable of operating continuously over the sample frequency range 20kHz to 30s per cycle, maintaining an overall accuracy of about 0.37% without recourse to any form of switching. A method of selecting and measuring output levels from the simulator is described which, when used in conjunction with a pen recorder, could produce a problem-solution time of the same order as that produced by a digital-computer program with a graphical output routine. Practical tests are described which show clearly the extent and nature of the small errors which occur during a general simulation. View full abstract»

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Aims & Scope

The Proceedings of the Institution of Electrical Engineers was published by the IET between 1963 and 1979.

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