## Power apparatus and electric machines

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- Electrical and electronic engineering [65]
- Power systems and applications [65]
- Power apparatus and electric machines [65]
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- 1964 [65]

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- B.J. Chalmers [4]
- J.L. Dineley [4]
- A.C. Williamson [3]
- B.V. Jayawant [2]
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- J.C. West [2]
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Advances in the performance of internal-combustion engines call for improved electrical equipment. The author describes recent developments in starters, generators and ignition systems.

A systematic investigation has been made of the performance obtained when using a system of negative-reactance compounding for automatically controlling the tap changers of two transformers operating in parallel, either side by side or connected together through a distribution system. A wide range of operating conditions was covered by tests in the field, with a laboratory model and by computer studies. Stable parallel operation of the transformers was proved.

Two forms of reluctance machine with rotors of low inertia, believed to be novel, are described. In the first, the salient poles are detached from the inactive portion of a normal rotor to give what may be regarded as the reluctance equivalent of the drag-cup induction machine. Inertias as low as one fifth of those of conventional machines appear to be attainable, with correspondingly high values of the torque/inertia ratio and the power ratio. The second form of machine is based upon the use of peripheral segments rather than poles. While not permitting the use of values of inertia as low as the first form, it can give higher pull-out torque, power factor and impedance.The construction of experimental versions of both machines is described. Test results for all aspects of their synchronous performance are given, and they are found to be in very satisfactory agreement with computed values, based on previous analytical work by the authors^{3} and certain simple extensions of it. Test results relating to pull-in performance, and theoretical results relating to standstill torque, are also given.It is shown that there is a need for a better understanding of losses caused by harmonic fluxes in the air gap, and of the effects of saturation. The latter leads to nonsinusoidal space variations of reactance and could be of importance in connection with all types of salient-pole synchronous machines.

The analogue simulation of induction machines is considered with the object of providing a convenient general model from which both transient and unbalanced operation of a machine may be predicted. Prior to commencing the transient analysis for a machine, the validity of the mathematical model used is assessed by comparison of practical and computed steady-state torque/speed characteristics.

The demand for circuit breakers of very high rupturing capacity has led to designs which, if tested directly, would require powers greatly in excess of those at present available at proving stations.

Switch regulation is discussed and a full analysis of the circuit using a silicon controlled rectifier given resulting in formulae for the design of this circuit.An example is given of a complete circuit including the control amplifier for a 25 V 4 A supply unit. Performance details are included and the possible variations and applications are considered briefly.

This paper describes a type of double-layer winding known as an interspersed winding, which may be used in a.c. machines to reduce harmonic components of induced e.m.f. or load m.m.f. The principles underlying the design of interspersed windings are described and compared with the standard analysis of conventional windings in terms of harmonic winding factors. Design and performance details are then given for a complete range of 3-phase interspersed windings covering equivalent values of slots/pole per phase from 4 to 9. It is shown that control of the largest low-order harmonics may be achieved with insignificant alteration to manufacturing methods, and this type of winding should therefore find useful application in large generators and motors where the reduction of stray load losses is an important consideration.

The distribution of magnetic flux in the air gap of an electric machine is of fundamental importance in determining such quantities as stray losses, torque, reactances and magnetic noise. The author discusses some of the methods of approach which have been employed in the determination of the basic flux quantities in induction and other doubly slotted machines. By a new form of analytical approach, equations are obtained for some important quantities; these include accurate, but simple, equations for gap permeance and the amplitude of certain flux pulsations. Also developed are some basic equations for air-gap leakage flux, including its variation with rotor movement. Tests made on models in an electric-tank analogue confirm the equations obtained for this component of leakage flux and give some additional information regarding flux pulsations.

If a squirrel-cage induction motor is to be operated efficiently over a wide speed range, it must be supplied from a variable-frequency source whose frequency is adjustable over a range similar to that required for the motor speed. It is now technically possible and economically acceptable to provide power at variable frequency using silicon-controlled-rectifier (or thyristor) invertors. The paper reviews the already well known invertors and concludes that the d.c. link type is preferable for most variable-speed applications. Various methods of artificial commutation are described and applied to invertor circuitry suited to the supply of a squirrel-cage motor. The design flexibility of such an invertor is discussed with reference to component values and losses. Various methods of obtaining the necessary variable voltage are presented. Three different types of control circuits and their influence on drive characteristics are given. Finally, the characteristics of the invertor and motor forming a variable-speed drive are described, and applications are considered where speed, temperature, waterproofing and electrical maintenance are a limitation.

The paper begins by considering the reliability of two low-power methods of measuring stray load losses: the first of these methods is the well known reverse-rotation test and the second a relatively unknown test called the d.c. excitation test. The development of a new and accurate full-load method of measuring stray load losses in squirrel-cage induction motors is then discussed, and the results obtained from this test are compared with the low-power test results. Fifteen motors in all were tested.The paper also deals with the effect on stray load losses of speed, current and the m.m.f. harmonic contents of the stator winding. The magnitudes of the measured stray load losses are compared with the amounts allowed by British Standard 269: 1927.

Use of the high-voltage direct-current method for power transmission necessitates conversion of a.c. into h.v.d.c. for the line, and conversion of the d.c. back to a.c. again. The critical component of such convertors is the controllable electric valve. The basic problem of extending the voltage range of mercury-arc valves to these high levels is outlined, as well as the nature of the underlying development work. Some features of present-day valves are quoted, as well as some considerations which have to be taken into account when designing the convertor circuits, owing to the properties and limitations of actual valves.

It is shown that the accelerating or braking time of a polyphase induction motor after a step change of applied voltage can be estimated by using the reciprocal-accelerating-torque/speed curve. For some transitions, the time required is shown to vary inversely with the area under the steady-state-torque/speed curve. Balanced applied voltages are shown to be preferable to unbalanced voltages for plugging purposes. Equations developed for speed-response times include load-torque terms. The use of Saturistors is shown to be advantageous in giving low speed-response times without excessive motor currents.

A ferromagnetic wheel may be made to do mechanical work if the rim is passed through a magnetic field and a source of heat. The theory of this device is developed, first when the wheel is paramagnetic, and then for the ferromagnetic case. Expressions for the efficiency, work done per revolution and mean torque are derived for the general case. Relatively high torques are possible. For an iron wheel the efficiency would be only about 0.05%, but much higher efficiencies could be attained at low temperatures in the region of 20°K using materials of low Curie points. The maximum possible efficiency is calculated to be 27.6%.

A study of the literature reveals that there is still considerable confusion on various aspects of stray losses, particularly with regard to their definition and origin. Accordingly the theoretical information is presented in a rationalised and systematic way to obtain a fully comprehensive approach, based on classifying stray losses into two groups, namely those due to main flux variations (stray no-load losses) and those due to leakage fluxes (stray load losses). In this way three and six main basic types of stray losses can be recognised under the two headings, respectively. Having identified the physical origin of these stray losses, their effect on motor design parameters can be ascertained qualitatively, and available methods of quantitative calculation, which are mostly semiempirical, can then be judged.A critical appraisal of two synthetic test methods for stray load losses, the reverse-rotation and d.c./a.c.-short-circuit tests, is made in the light of basic theory, and the fundamental deficiencies of these tests, particularly the reverse-rotation test, are demonstrated theoretically and by tests carried out on a number of medium-to-large induction motors ranging from 400hp, 4pole, to 1700hp, 8pole. The importance of insulating squirrel cages to avoid circulating-current losses deduced theoretically is confirmed by experiment.Additionally, a direct investigation into squirrel-cage circulating-current losses on a 1500hp 4pole motor shows how such losses can be controlled by a suitable current-displacement rotor design; these additional losses being directly measurable at no load on this machine with open stator slots.Consideration is also given to the fact that modern, high-efficiency (particularly totally enclosed) machine designs cannot cater thermally for substantially increased losses, e.g. excessive stray losses of any kind. There is therefore already an overriding and definite safeguard that the declaration of efficiency to BS 269 is substantially correct, or, more specifically, that there cannot be any major error in the value of the declared losses. However, a modification of the fixed allowance for stray load losses is suggested.From the theoretical and practical investigations carried out, it is concluded that making a nominal allowance for stray load losses is at present the only reasonable and practicable method of declaring efficiency.