Electrical engineering was founded on the discoveries of Michael Faraday little more than a hundred years ago. The enormous turbo-generators of the modern power station are striking examples of what the electrical engineer have achieved in a century
30,000-KILOWATTS TURBO-ALTERNATORS generate current at a pressure of 36,000 volts at the Hardingstone Junction Generating Station of the Northampton Electric Light & Power Co Ltd. The great Parsons turbo-alternators have a speed of 3,000 revolutions a minute.
FROM the results of his experience a competent engineer may frequently improve on existing practice; but to originate requires the touch of genius. Once the foundations have been laid, the edifice of knowledge can be built up, slowly and not always surely. Pioneer builders are not favoured with a plan of the complete scheme. They can build only as guided by their own knowledge and by the requirements of the moment. Mistakes are inevitable, but triumphs and failures contribute in almost equal measure to the success of later endeavours.
Michael Faraday laid the foundation of modern electrical engineering practice in 1831. His classic experiment with a copper disk rotating between the poles of a magnet now seems simple — almost obvious. To us it is obvious, because it is impossible for us to blind ourselves to the knowledge that dynamic electricity exists. Our difficulty in ignoring everything that electricity now means is in itself a tribute to the outstanding importance of Faraday’s work.
Faraday had earlier discovered that currents were induced in a conductor when it was moved across the face of a magnet. With his magneto-electric machine he followed up this discovery by arranging that the conductor (the copper disk) should move continuously in the magnetic field. Contacts were made on the axis and perimeter of the disk, connexions being taken to a galvanometer.
When the disk was rotated, the galvanometer recorded a steady current flow, and the first generator had made its appearance.
To consider all the factors leading up to Faraday’s experiment, we must go back a few years to Count Alessandro Volta, who succeeded in producing electricity by chemical action in 1799. Before this, the only artificial manifestations of electricity were the static charges obtained by rubbing a glass rod on cat’s fur, or the high charges obtained from frictional machines. The low-voltage steady current afforded by the voltaic cell stimulated further experiment and research. By noting that a current-carrying conductor exerted a force on a magnet, Hans Christian Oersted discovered that electricity and magnetism were related. This was followed by the production in 1825 by William Sturgeon of the first electro-magnet, which was later improved upon by Joseph Henry, after whom the unit of inductance is named. None of these discoveries provided Faraday with a definite line of research, but they were of great value once he had stated the basic principles of electrical generation.
The only applications of electricity less than a century ago were for the telegraph and the electro-plating industry. This industry had an important bearing on the development of the generator. For plating purposes, a unidirectional current is necessary, so that direct-current generators were favoured.
The logical and easiest form of generation yields an alternating current. Pixii, who followed up Faraday’s work, found this a disadvantage. He rotated (in 1833) a permanent magnet in front of two iron-cored bobbins. This caused an alternating voltage to be set up in them.
An alternating current proceeds from zero to a positive maximum, then reverses and, passing through zero again, continues on to a negative maximum, finally returning once more to zero. This series of events comprises one complete cycle. The problem set Pixii was, somehow, to make this current unidirectional. On A. M. Ampere’s suggestion, he introduced a commutator; that is to say, he tapped off the current from each coil only during the positive portion of each cycle. Commutation in this manner did produce a unidirectional current, but one which was unfortunately still too pulsating for electro-plating purposes.
By increasing the number of armature conductors, or the number of poles, the pulsations can be flattened out. This is clear when it is recalled that each conductor passing under one pole delivers a pulse of current to the external circuit. If the pulsations follow one another with sufficient rapidity, then a close approximation to a steady current flow is obtained.
Several inventors appreciated the possibilities in this direction, and by 1844 a successful direct-current generator for electro-plating had been introduced. Only four years later this achievement was followed by the first use of the generator for lighting. A huge machine, designed by Professor F. H. Holmes, was installed in 1858 at the South Foreland Lighthouse (Kent), its output being applied to a carbon arc. The Holmes machine used 160 coils and sixty compound horseshoe magnets to provide the field. A later machine, designed by Holmes in 1867, was installed in 1871 at Souter Point Lighthouse, Co. Durham.
Despite the undoubted success of these early magneto-electric machines, and although they represented an important advance on Faraday’s first generator, their designers were wrong in placing their reliance in a permanent magnet field. Such a method is unwieldy and inefficient. Further, Sturgeon had demonstrated more than twenty years before the capabilities of the electro-magnet.
Once it is realized that the passing of a current through a coil of wire surrounding an iron core will create the required field magnetism, it is only a further short step forward to the conclusion that the generator itself can supply this current. The machine, in brief, will be self-exciting.
IN THE HUGE POWER STATION at Dunston-on-Tyne the Parsons turbo-generating plant has an output of 150,000 kilowatts. There are three 50,000-kilowatts turbo-generators running at 1,500 revolutions a minute.
The limitations of the permanent magnet were fully appreciated by 1850, and in the ensuing ten years many workers missed the principle of self-excitation when it was almost within their grasp. One man (Brett) led current off from his machine through a coil surrounding the permanent field magnet so as to increase the magnetic strength. It is surprising that he should have realized that the machine was at least capable of contributing to its own field, and then have been unable to assess the importance of his discovery. Neither did anyone else, but the movement away from the permanent magnet continued.
Wilde’s separately excited dynamo (1863) served the useful purpose of ending the claims of the permanent magnet. In this design there was a field winding fed by a separate machine or battery. During 1864 the principle of self-excitation, which had proved so evasive, was grasped by several workers simultaneously. There began almost a race, with S. A. Varley, Werner von Siemens and Sir Charles Wheatstone taking the lead. Varley first took out a patent for a self-exciting machine in which the residual magnetism in the iron of the field system was used to generate a weak current in the armature conductors. This current was fed through the field winding, increasing the magnetism, and giving rise to an increased armature current, thus cumulatively building up. Here we have the full principle stated at last, exactly as in modern machines. For some reason, Varley allowed his patent to lapse and the major credit for the development of the self-exciting generator must go to Siemens.
The machines of Wheatstone and Siemens, which were basically similar to Varley’s, were both described before scientific bodies on the same evening in February 1867, the former in London, before the Royal Society, and the latter in Berlin, before the Academy of Science. The interest attaching to this striking coincidence is somewhat minimized by the knowledge that Dr. von Siemens had communicated the principle of his machine to the Academy the month before. Moreover , he was much more active in its subsequent exploitation than Wheatstone.
At this stage attention was given to the improvement of armatures and commutation. The wasteful ring armature, after a brief predominance, gave place to the slotted drum armature which, with relatively minor modifications, has remained standard practice to this day.
As its name denotes, the ring armature is made up of an iron ring on which is wound a continuous spiral of wire. The conductor is divided into several sections from which connexions are taken to the commutator. The active conductors are those on the surface of the ring, those on the underside doing no useful work. Nearly half of the copper used is therefore wasted. In the drum armature all the conductors are arranged outside. Efficient methods of armature winding, including lap and wave, were evolved about this time, and the relative merits of series, shunt and compound running were investigated.
Early Efficiency
In the later years of the nineteenth century, generators were still in a crude state of development. Yet efficiency tests conducted by the back-to-back method evolved by Dr. John Hopkinson disclosed that some machines had an efficiency in excess of 90 per cent. This figure exceeds that generally met with in modern machines of equivalent size. The reason was that crudity of construction was offset by the unnecessarily large copper and iron sections. From a modern production point of view the generators were uneconomical.
It is difficult to consider the development of one branch of electrical engineering divorced from the whole. Some of the reasons which contributed to the adoption of direct current generation have been considered. Similarly, the reintroduction of the alternator was preceded by inventions to which alternating current was better suited. Further, it was linked up with developments in transformation and distribution. In the 1880s the alternator again received serious attention and its progress was rapid.
MAGNETO-ELECTRIC GENERATOR designed by Professor F. H. Holmes in 1867. Four years later it was installed in Souter Point Lighthouse (Co. Durham). The armature shaft carries ninety-six coils. Arranged radially round the armature are fifty-six compound permanent magnets, whose poles project into the spaces between the rings carrying the coils. At 400 revolutions a minute the machine, which weighs three tons, gave a mean light of 1,520 candle-power.
Some particularly valuable experience had been gained with direct current machines and in Great Britain, the United States and Germany many investigators were concentrating on the production of reliable alternators. Several useful single-phase machines, the largest of which was of 600 horse-power, were introduced between 1880 and 1890. Two of the outstanding men in Great Britain devoting their energies to alternator design were A. Thompson and S. Z. de Ferranti.
In 1889 Ferranti embarked on a scheme which was amazing for its enterprise, foresight and courage. Hopkinson had already stated the principles of the parallel running of alternators, but at that time the system was unreliable. Ferranti then determined on large single machines for the plant which was to be installed at Deptford for the London Electric Supply Corporation. His design provided for separate alternators delivering one hundred amperes at a pressure of 10,000 volts.
Many factors militated against the success of the scheme. To use a mathematical metaphor, Ferranti had set himself an equation containing a mass of unknowns. These problems he had to solve as they arose. One of the major troubles was caused by lack of suitable insulating materials, and the insulated current was first carried on rubber overhead cables. Later, this arrangement was abandoned in favour of short lengths of paper-insulated cable run underground from the power station into London.
Breakdowns were fairly frequent in the early stages of the scheme, and it did not at first justify itself as a commercial venture. It was, however, everywhere recognized as an outstanding technical achievement and one which provided most valuable experience for later designs.
About this time experiment was devoted to the production of polyphase alternators. The method first used was to couple, mechanically and electrically, two or more single-phase machines. It was then discovered that an arrangement of independent windings on the same armature would achieve a similar object, and a power station delivering a three-phase supply was operating in Germany as early as 1892.
Supply frequency is dependent upon the rotor speed and the number of poles. For a given frequency, if the speed can be doubled, the number of poles can be halved, thus causing simplification of the design.
The turbo-alternator provides this high-frequency high-speed generation, and its adoption may be said to be due to the work of Sir Charles Parsons in developing a high-speed power unit. His first patents for the now well-known turbine were. taken out in 1884 and provided for a turbine speed of 18,000 revolutions a minute. Enormous difficulties were met with in the early stages, in the design of the turbine and the dynamo. Only the infinite patience and ingenuity of Parsons made the success of the turbo-alternator possible.
Because of the great centrifugal force exerted on armatures revolving at high speed and the difficulty of leading off high-tension currents, the rotating field was introduced in the first few years of this century. The largest turbo-alternator built up to 1907 was rated at 4,000 kW. Five years later one of 25,000 kW rating was built for the United States, and subsequent development has been rapid. Almost all large electricity supply undertakings rely on the turbo-alternator.
Force of Repulsion
Two of the alternator sets at the famous power station at Battersea (London) are each designed for an output of 69,000 kW, and are direct-driven by a triple-expansion turbine running at 1,500 revolutions a minute. The third set is even larger, as its output is 105,000 kW. All three sets generate current at a pressure of 11,000 volts. Though these machines are large, even by modern standards, still larger alternator sets have been built in the United States.
The development of the electric motor has proceeded concurrently with the generator, as it was realized in 1838 that a generator could be run as a motor. When an armature is rotated in a magnetic field an electromotive force is produced in the conductors wound on it, due to the cutting of “lines of force”, which are assumed to flow from a north to a south pole. The current through the conductors also sets up a field which ungratefully acts in opposition to the field which gave rise to it.
Neglecting friction, windage and other relatively minor losses, this opposition field is the only reason why mechanical work has to be done to rotate the armature. In other words, the two fields exert a force of repulsion. As the field proper is fixed, if an opposition field is set up in the armature, the armature will rotate.
THE FIRST GENERATOR was invented by Michael Faraday in 1831. A copper disk was arranged to rotate in a magnetic field. Connexions at the axis and at the perimeter of the disk led to a galvanometer which recorded a steady flow of electricity when the disk was rotated.
This is the familiar principle of the direct-current electric motor. By feeding in current to the generator output terminals the armature can be made to rotate, thus translating electrical to mechanical energy. Because of its high degree of flexibility the direct-current motor is often to be preferred to one operating on alternating current. It is easy to start, and simple adjustment of the field current lends it an extremely wide speed range. According to the method of connecting the field a variety of requirements can be met. The ability to start under load, as in traction work, is afforded by the series winding. Constant speed under all loads is the merit of the shunt winding, and a combination of the two, in the form of the compound winding, provides the advantages of both.
Single-phase alternating current motors are not normally self-starting, which is one of the reasons for the introduction of the polyphase machine referred to above. A self-starting commutator motor was developed in 1884, but simpler machines had first to be run up to speed before they would run from an alternating supply.
This is the principle of operation of the synchronous motor. Instead of mechanically running them up to speed, starting windings have been introduced. By this means, the machine will start as an induction motor, but on reaching synchronous speed, the starting winding is cut out by a centrifugal switch, and the machine runs synchronously.
Even before Faraday’s experiments, it had been shown that a magnet, when rotated, would carry a conductor round with it. The magnet in a motor or generator is the field, so that if the field can be made to rotate, so will the armature. This requirement was met in 1885, and Tesla had produced several practical designs for industrial machines by 1890. A rotating field is produced electrically by arranging that a number of coils shall be excited in a given phase relationship. This is not readily obtainable with a single-phase supply.
D C. Motor on A.C. Supply
In 1884 two important discoveries were made which assisted the development of the alternating commutator motor. Fleming hit upon the basic principle of the repulsion motor, and Alexander Siemens showed- that a direct-current series motor would run on an alternating current supply. The characteristics of the alternating current series machine are similar to those of its direct current counterpart.
After Fleming’s discovery, Elihu Thomson (see the chapter “The Craft of the Welder”), produced a repulsion motor with alternating current supplied to the field windings and the brushes short-circuited. The rise-and-fall of current in the field sets up a correspondingly varying flux. This “cuts” the armature conductors, thus setting up currents which give rise to a voltage out of phase with the supply. These two voltages combine to set up r rotating field which causes the armature to rotate. Both repulsion and series alternating-current motors find a wide application for traction purposes.
Modern domestic equipment using an electric motor frequently employs a series wound machine because of its ability to run on either alternating or direct current, and because of its good starting torque. Fan-using devices, such as vacuum cleaners and hair dryers, involve a considerable starting load for a tiny motor. The universal machine’s disadvantage for domestic usage is that sparking at the commutater sets up radio interference. The alternating current induction machine is therefore generally preferred.
Fundamentally, the design of generators and motors has remained unchanged for many years. Great improvement has been made in the design of individual features with a view to obtaining maximum efficiency and reliability, coupled with low production costs. Important advances have been made, too, with insulating materials, magnetic alloys, commutation and so on. It is improvement of this nature, rather than change, which must be anticipated in the future development of the generator.
ELECTRIC POWER AT 36,000 VOLTS is generated by three 20,000-kilowatts turbo-alternators in the Salt River Power Station, at Capetown, South Africa. The turbo-alternators, which are of the Parsons type, run at a speed of 3,000 revolutions a minute.
Neglecting friction, windage and other relatively minor losses, this opposition field is the only reason why mechanical work has to be done to rotate the armature. In other words, the two fields exert a force of repulsion. As the field proper is fixed, if an opposition field is set up in the armature, the armature will rotate.