## Power apparatus and electric machines

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- Electrical and electronic engineering [66]
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- 1966 [66]

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- M.R. Harris [3]
- P.J. Lawrenson [3]
- A. Wright [2]
- B. Adkins [2]
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- B.M. Bird [2]
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The voltage transformation, corresponding to a current transformation *i* = *C*.*i′*, is usually taken as *v′*=*C*_{t},.*v*, for invariance of total instantaneous power. It is shown that this solution is not unique, but is one of an infinite family, having the general form*v′*=[*I*+*Z*_{s}.*Z′*_{0}^{−1}].*C*_{t}*v*where *Z*_{s} is any skew-symmetric nonreactive impedance matrix, and *Z′*_{0} is the usual transformed impedance matrix. The theory of this matter is outlined, and its topological significance (corresponding to interconnection with gyrators) is discussed. All the usual properties of invariance apply to this more general solution. Whilst this treatment clarifies a mathematical difficulty, the extended solutions are not apparently of great physical importance.

The paper describes the development of a commutatorless diesel-electric drive system which uses an alternator and induction motors as the drive elements. The system provides a set of discrete final motor drive speeds at a fixed alternator drive speed, and so may be regarded as an electrical ‘gearbox’, a feature of the scheme being that, during normal operation, gear selection is carried out by pole changing the alternator excitation winding.Pole changing of the alternator output winding and the induction-motor stator winding is also used, but is a consequence of changing the alternator excitation pole number, and switching at full power level between the alternator and motors is thereby entirely avoided.The paper includes a sample design for a traction system having a continuous rating of 3600hp. The prediction of the performance of the sample design is substantiated by experimental results taken on a simplified laboratory model of the system.

This theoretical and experimental analysis of the magnitudes and distribution of stator-flux harmonic components in a Lorenz-type inductor alternator is part of a broad investigation into this class of machine. The test alternator is a specially designed 30kW model of the standard 300kW industrial unit. Current designs employ field damping to reduce second-harmonic flux modulations expected by analogy with the Guy-type inductor alternator on load. The paper shows that no modulation of the field flux could exist in an ideal Lorenz-type alternator. However, the practical machine, with a.c.-slot openings interrupting the stator surface between field slots, will have harmonic components of flux, both on open circuit and on load, whose magnitudes depend on the width of the a.c.-slot opening. Damping is shown to be sucessful in reducing second- (and other even-) harmonic components, whilst being totally ineffective against fundamental variations in field flux and odd-order-harmonic fluxes. A theory is given that explains these effects. Measurements on the experimental model verify this theory. The paper introduces a technique for relating fluxes to the geometry of the airgap surfaces, and has shown the distribution for inductor alternators to be somewhat more complex than was presumed. It is expected that this analysis will solve the problem of accurate loss calculation in these machines.

The paper describes two mathematical models for the simulation of laminated-and solid-rotor synchronous machines. Employing these models, machine behaviour during asynchronous operation and the process of resynchronisation have been studied in detail using a general-purpose analogue computer. Results obtained on the analogue computer have been verified by comparison with results obtained on large alternators and on laboratory micromachines. Investigations have been made into the process of pulling into step from the state of steady asynchronous operation, using an entirely different approach, and the process is explained in simple physical terms. The effects of various auxiliary equipments, different machine parameters and system conditions on out-of-synchronous operation and resynchronisation have been studied.

The paper describes the development of windings to produce 2_{N} pole settings, the windings being fully energised on each setting. The theoretical shapes for the windings are developed initially, and the paper then goes on to show how acceptable approximations to these shapes may be formed which yield good utilisation of the slot space with an acceptable harmonic content. The work is supported by results from an experimental machine.

The paper outlines criteria for the resynchronisation of a synchronous machine following system disturbances. Based on these criteria and using static relaying techniques, a trasistor relay is proposed for automatic resynchronisation. The relay has been constructed, and its effectiveness has been demonstrated on a laboratory micromachine. The proposed scheme for automatic resynchronisation is particularly adaptable to the controls of the future, and is suited to online computer control.

The load losses of an induction motor, defined as the difference between the actual losses and the conventional segregated losses under normal running conditions, have, in the past been difficult either to measure or to calculate accurately. An earlier paper gave methods of determining similar losses at larger slips and at reduced voltage, in terms of calculations and measurements of the torque/speed curve. The calculations allowed for saturation in the tooth tips, but not for saturation due to the full main flux which is present at normal voltage. The measurements were not made at full voltage because of heating considerations, and no useful measurements at normal full load were obtained because the load losses are a very small fraction of the input. In this paper, the method of calculation is modified, and improved testing methods are used to overcome these limitations, and thus to derive more reliable methods of measuring and calculating the load losses. Apparatus is described for the automatic recording of speed/torque and power-input curves at full voltage, from which the harmonic torques caused by the load losses can immediately be measured. The paper also describes a new back-to-back test for induction motors, which constitutes the most accurate and reliable method of measuring load losses under full-load conditions to date. Ten different rotors with carefully controlled parameters, used in conjunction with two identical stators, were tested in the back-to-back arrangement, and the results are compared with the theoretical computations and with the reverse-rotation test carried out on the same machines.

The method of commutation is of fundamental importance in an invertor, and good commutation makes for successful operation of the circuit. This paper describes an improved method of commutation for the parallel-type invertor, using two separate capacitors to store energy instead of one as in the conventional circuit. These capacitors are also used to bypass voltage transients which might go through the thyristors. An analysis of a single-phase parallel invertor is given, and the operation of a 3-phase invertor formed by three single-phase circuits is also described.

The paper describes a method of rapidly reversing a large current in a highly inductive load, which has been satisfactorily applied to the paralleled field windings of a pair of 2MW d.c. generators. The technique discussed avoids disconnection of the inductive load whilst current is still flowing, and hence the problem of dissipating the stored energy. The power unit evolved consists of two 3-phase fully controlled thyristor bridges arranged in a figure-of-eight formation. The means of avoiding large circulating currents by controlled methods is discussed in detail.

The volume of a superconducting transformer is calculated by investigating the behaviour of an idealised, lossfree, interleaved transformer. Because of the limitation of leakage magnetic fields with superconducting windings, it is found that the volume is large compared with a conventional transformer of the same power rating. It is shown how the volume of an idealised transformer can be reduced. When account is taken of the additional factors involved in designing a real superconducting transformer, it seems very probable that its size will be so much larger than that of an equivalent conventional transformer as to be unacceptable.

The paper deals with the application of pulsed voltage waveforms to the field windings of separately excited d.c. machines, and describes the development of an unusual form of control scheme, which holds the field current and the armature current in a fixed relationship. A wide range of torque/speed characteristics are shown to be possible, using simple and inexpensive control apparatus. One arrangement, which allows the separately excited machine to behave as a series machine in the motoring region, and as a constant-torque machine in the braking region, is described in detail, and its possible application as a battery electric traction drive is discussed.

It has been shown recently that asymmetrical fractional-slot windings can be devised to draw balanced currents. The leakage reactance for the three phases of these windings may, however, no longer be equal. The paper discusses the effects of the asymmetry of the windings upon one component of leakage, namely the slot leakage. It is shown that, by careful selection of the coil pitch for the winding, equal slot leakages are possible for the three phases. The paper also suggests how the slot leakage reactance may be determined for other coil pitches.

A method of calculating the magnetic field in the end region of a turbogenerator is described. Allowance is made for nonmagnetic stator-core clamping plates and for the magnetic shaft and casing of the machine. The effect of a magnetic rotor-retaining ring is studied, the ring being regarded as a tube of finite permeability separating the stator and rotor end windings. Calculations are compared with experimental results. The field calculated at the surface of the stator-core end plates is used to estimate the eddy-current loss in these plates, and the effect upon this loss of the retaining-ring permeability and the stator-winding cone angle is considered.

In certain circumstances, the force on a secondary conducting body carrying induced eddy currents can be calculated from the gradient of effective inductance, measured in the primary circuit at the operating frequency. Uncertainty as to the exact circumstances is resolved by an approach through matrix algebra. Either the self parameters of the secondary impedance matrix must be constant, or the secondary circuits must be effectively resistanceless. In the resistanceless case, the theory extends to cover multiple primary circuits, but not otherwise. The theory is verified by application to the single-phase induction motor, and other physical implications are discussed.

An exponentially tapered distributed *RC* notch filter loaded by a source and a load resistor is shown to have maximum notch steepness for a finite taper degree.

A novel method of controlling speed and direction of rotation of an electric motor energised by microwaves in a waveguide is accomplished by parametric operation of the waveguide motor. The modulation frequency of microwaves determines the speed of the motor. The direction of rotation is controlled by the frequency of the microwave carrier.