The Alexanderson alternator by V. J. Phillips

The Alexanderson alternator was a rotary mechanical generator o f radio carrier-waves atjequencies up to 200 k H z and beyond, developed in the early 1900s. In spite ofthe many technical problems involved in constructing machines ofthis sort they proved to be very successful,

particularlyfor rnzilitary work. One fully operational installation still exists today in southern Sweden.

The background

n its early days, around the turn of the century, the science of rado relied very heavily upon the spark transmitter. The sparks occurred at a low audio rate, and each indwidual spark produced a damped burst

of R F oscillations. With the development of tuning (‘syntonisation’ as it was known in those days), and especially after early attempts at amplitude modulation, it soon became obvious that what was needed was a steady sinusoidal R F carrier. The thermionic valve had not yet been invented, but there were several other possible approaches to the problem of producing such a carrier:

the timed spark. In this method the individual sparks were made to occur at such a rate that the damped pulses of R F blended together at the correct phase so that a more-or-less continuous carrier was produced. the arc. The negative resistance characteristics of the electric arc could be employed to cancel out the damping resistance of a tuned circuit, thereby causing it to oscdlate continuously. The names of Wfiam Duddell and Valdernar Poulsen may be mentioned here as the pioneers of this method. direct generation by rvtav alternabr. In principle this is a very simple idea. To increase the fiequency of output of an alternator it is necessary to raise the speed of rotation of the shaft and/or to increase the number of pole-pairs in the machme. In practice, however, to construct a machine giving an output of ra&o fiequency creates tremendous design problems. For a start, wirewound machme-rotors are out of the question since centrifbgal forces would tear the windings apart at the speeds needed. Very large numbers of poles are dfficult to produce, requiring high-tolerance machining.‘ The air gap in the machme needs to be very small if h g h magnetic

leakage and inefficiency are to be avoided, and t h s in turn creates problems with maintaining the centring and balanee of the machine shaft at lugh speeds. The properties of the magnetic materials at high fiequencies also have to be considered. Excessive energy losses will cause unacceptable temperature rise within the machine and the resulting expansion of the metals d cause misahgnment and changes in the air gap. In short, a nightmare for machine designers.

By about 1900, several experimenters had constructed hgh-fkequency alternators for use with arc lighting systems. The operation of an arc lamp at the usual 50 or 60 Hz caused excessive acoustic noise, and it was hoped to avoid this by operating at fiequencies above the range of the human ear. Tesla, for example, constructed in 1890 a machine capable of giving an output of 12000 Hz, and a later machine with 480 poles, rotating at 3000 rpm, increased ths to 15000 Hz. J. A. Fleming, in his famous book ‘Principles of electric wave telegraphy and telephony’, describes several other ingenious machines, all limited to an upper fiequency of around 10 kHz’. A useful comparative table may also be found in Ernst Ruhmer’s book ‘Wireless telegraphy”. Higher Cequencies were needed for radio work of course. In the early years of t h s century the problems were tackled-and solved- by Alexanderson, worlung in the USA.

Ernst Frederik Werner Alexanderson was born on 25th January, 1878, in Uppsala, Sweden. His father, Professor A. M. Alexanderson, was a professor of Greek at the University ofUppsala, and his mother was a daughter of the noble Swedish f d y of Von Heidenstam. Young Ernst graduated with an engineering degree fkom the Royal Technical University Stockholm in 1900, and t h s was followed by a year of postgraduate training at the Technical University of Berlin. No doubt inheriting slulls in

ENGINEERING SCIENCE AND EDUCATION JOURNAL FEBRUARY 1997

37

Fig. 1

languages hom his father, he learned English, German, Bureau. His experimental work had convinced him of French and Latin in addition to his native Swedish. He the need for a continuous carrier wave. At first he tried read, and was greatly impressed by, Charles Steinnietz’s to interest the Westinghouse Co., with whom he had book on ‘Alternating current phenomena’, and this a long-standing connection, in building an alternator fired an ambition to go to the USA to work with him for him, but they were too busy to undertake the at the General Electric Co. at Schenectady, N Y He development of such a specialised and dfficult started work there in 1902 in the fairly humble tasks of machine. He therefore took his proposal to Steinmetz drafting and testing, but he was soon promoted to the at the General Electric Co. in 19003. The first machine engineering staff, working on AC generators under the which Steinmetz eventually produced for him in 1903 direction of Dr. Steinmetz. was of fairly conventional design with a wound

armature on the rotor, and had a maximum hequency Early attempts of operation of only 10 kHz. Fessenden then placed a

second order, but this time specifying a fiequency of The well known American radio pioneer R. A. 150 kHz with an output power of 25 kW or more-a Fessenden was at that time working for the US Weather rather tall order! The job was given to the 26-year-old

(a) The stator and windings and (b) one of the pair of rotor discs of the 1906 alternator

Fig. 2 picture; the drive motor is at the front, with the intermediate pulley shaft at the rear

The 1906 alternator. The alternator itself is in the centre of the

- Alexanderson. He at once abandoned the idea of using a wire-wound rotor, using instead the principle of the so- called ‘inductor-type’ machine. In modern terms, I suppose we would refer to it as a ‘variable-reluctance’ generator. Imagine that a steady DC magnetic field is established between the poles of an electromagnet. If a piece of steel is alternately moved in and out of the gap the flux in the poles wdl alter according to whether or not the steel is in the gap at any time. A ‘search’ coil wound around the poles d thus be subject to a varying flux and will have a voltage induced in it. This concept was not really new; it had been used by several other people, inclulng the Siemens Co., Westinghouse Co., and Duddell in the UK.

Alexanderson was rather constrained in his first design by Fessenden’s insistence that a wooden core be used for the stator. Apparently Fessenden had severe misgivings about the magnetic

ENGINEERING SCIENCE AND EDUCATION JOURNAL FEBRUARY 1997

38

Fig. 3 The second version of the alternator, driven by steam turbine

performance of steel at h g h frequencies, and anticipated high energy losses due to eddy currents and hysteresis. The construction of the machine was completed in 19064. It had a stationary armature (Fig. la) wound on a wooden ring situated between two rotating steel discs furnished with teeth projecting around their circumference (Fig. Ib). The discs were about a foot in diameter and were driven by an electric motor through a system of pulleys (Fig. 2) . Initially a frequency of 50 kHz with an output of 500 W was obtained, but when the machine was equipped with new hard cotton belts, the frequency could be raised to 76 kHz-but the power output was then only 50 W. This was installed and operated at the Brant Rock experimental radio station and it was used in the famous broadcast of speech and music &om Brant Rock in 1906. A second version of the generator seems to have been co- mehately (Fig. 3)5. This one was turned ‘inside out’ in that there was one central double-sided toothed wheel with two wound armatures, one on each side. This version was said to be particularly suitable for use aboard ship, being driven drectly by a De Laval steam turbine (on the right of the Figure).

Further development

These first machines enabled Alexanderson to appreciate fdly the problems involved, so that when he was asked to undertake the design of an alternator for the Telephone Company in 1907 he was able to draw upon his earlier experiences6. The greatest difficulty arose in the design of the high-speed rotor In the new machine, this was driven through gearing from a DC motor at a speed of20 000 rpm. The design ofthe rotor is shown in Fig. 4. It was made of chrome nickel steel, and had 300 slots cut into its periphery. In order to avoid excessive wind resistance the slots were GUed with phosphor bronze and it was carefully finished to provide a smooth surface. The &sc was of tapered cross-section, this being the optimum shape to even

Fig. 4 The high-speed rotor of the later machine

The alternator is on &e left, and it will be seen that two adjusting knobs were provided to enable the armatures to be moved relative to each other in order to achieve optimum phasing of their outputs. The item in the middle is a speed reduction gear to enable a small dynamo to be driven from the main shaft for purposes of steam regulation and speed control. According to Fessenden, this machine set, which was about 30 cm high and 91 cm long, had an output of 2.5 kW at a frequency of around 75 kHz.

out the stresses and to minimise the alteration in the diameter due to centrifugal forces. It was mounted on a long flexible shaft. As a rotor of this sort is gradually increased in speed, it passes through several critical speeds (1700 rpm and 9000 rpm in this particular case) where, due to resonances, bad lateral vibrations occur. Above these speeds it runs smoothly in a stable well-

centred manner. The shaft was supported at its ends, but extra side thrust bearings, normally inoperative, were provided near the &sc in order to restrain its motion as it passed through the critical speeds. It has been calculated that the speed of travel of the rim in this machine was nearly twelve d e s per minute. It has also been calculated that if the disc were released at full speed, and if this speed were maintained, it would roll from the USA to Europe in just under three hours- that’s also assuming it &dn’t sink en route of course!

ENGINEERING SCIENCE AND EDUCATION JOURNAL FEBRUARY 1997

39

D

E

Fig. 5 C, stator B, DC field coils A, and laminated pole pieces E

Cross-section of the alternator showing the rotor

Fig. 6 was installed at Radio Central, Long Island, New York in 1921

Dr. Alexanderson with a 200 kW machine which

Fig. 5 shows how the rotor was positioned in the stator. The field coils A are part of a system of windings which create a constant magnetic field around the frame, and this field concentrates into the pole pieces E made of very thm sheets of iron. ,The non-magnetic phosphor-bronze inserts in the rotor move through the gap creating the changes of flux which induce high- kequency voltage into the winding whch can be seen in the notches above and below the laminations E. These coils on all the pole pieces are connected together to form the stator winding. In some of the later, larger machmes the coils were connected into 64 subwindmgs whch were individually connected to 64 primary windmgs of a combining output transformer7. The air gap in the machine could be made as small as 0.01 cm, although for normal operation a gap of 0.038 cm was used.

Various versions of thx machine were built in the years that followed. The earliest version was driven from an electric motor via belts and pulleys, which enabled the rotor to revolve at 15000 rpm, giving a frequency output of 75 kHz. This method of driving proved impracticable for normal use, and so later versions were driven through a 10:l gearing. These were rated at an output of 2 kW By about 1917, the machmes had increased in size considerably; Fig. 6 shows Dr. Alexanderson standmg alongside a 200 kW machme, one of a pair installed at Ram0 Central, Long Island, New York. These very large machines usually operated at rather lower frequencies, from about 15 kHz to 30 kHz.

Several problems associated with the operation of such alternators must be briefly mentioned.

Precise control oj3equency: This was important, since sharply tuned transmitting antennas were used, and especially if narrow tuned circuits were to be employed at the receiver. Control was effected using a tuned circuit as a frequency &scriminator7. A series tuned circuit was connected across the output ofthe machine, and tuned to slightly above the desired output frequency. If the kequency wandered, the output across the inductor of the tuned circuit rose and fell. As shown in Fig. 7, a magnetically coupled output voltage was rectified using a mercury arc device, and the resulting DC was used in the control coils of a magnetic variable reactor in order to regulate the AC current in the driving motor, thereby achieving quite precise speed control. The circuit used in practice was actually a little more complicated than this. The rectified D C was fed to the field-coils of a DC generator, the output of which supplied the control current to two saturable reactors in the three-phase AC line supplying the driving motor. In other words, the subsidiary DC generator was used as a DC amplifier giving gain in the feedback circuit. (In some sets, two-phase motors were used.) In one version, it was claimed that this control system was capable of maintaining the driving motor speed to within one part in 900 (0.111%) for a transmitted output &om zero to full load. Alternatively expressed, this represents a variation of 19 Hz in a

ENGINEERING SCIENCE AND EDUCATION JOURNAL FEBRUARY 1997

40

Fig. 7 The frequency control circuit

17 kHz output signal. Mouse keying OY amplitude modulation. In his original

Brant Rock experiments, Fessenden had modulated the output by means of a carbon microphone drectly connected in the aerial circuit. This gave very variable results as the carbon granules soon packed down and became ineffective. The problem was solved in the Alexanderson machines by another application of the saturable reactor principle8. A variable reactor was shunted across the output of the alternator, thus dwerting current from the antenna. A D C current through the control limb of the reactor was switched on and off for Morse, or else an auhon valve was used to ampli+ the output from a microphone and this was used as the control current to produce amplitude modulation. A change in DC control current of two or three amp2res was sufficient to modulate the amplitude from full output to virtually zero.

Later developments

An Alexanderson 50 k q 50 kHz machine was installed in the Marconi station at New Brunswick in 1917, and proved to be of great strategic value for communication with war-time Europe. A further 200 kW machine put in in 1918 proved to be even better and was used during negotiations between Germany and the USA for the cessation of hostilities. A special company, the Ra&o Corporation of America, was set up to exploit the system, and about twenty ofthe 200 kW machines, in pairs, were installed throughout the world in the years immediately following World War I, their long wavelengths of operation providing reliable communications, day and night’. Two machines (21.26 kHz and 31.27 kHz) were supplied to the well-known Marconi station at Caernarfon in North Wales. Valve technology eventually began to take over, and most of these

alternators were scrapped during the 1930s. The Caernarfon machines went in 1939. However a few survived to see service in World War 11, two sets installed in Hawaii (25.8 kHz and 19.23 kHz) being used for naval communications in the Pacific Ocean. The US continued to operate two sets in Massachusetts untd 1957 (22.35 kHz and 18.8 kHz), using them for reliable communication to the Arctic bases. These were finally scrapped in 1961, one set going to the Smithsonian Institution.

One Alexanderson machine remains in workmg order to this day, situated in the d a g e of Grimeton, near Varberg, in southern Sweden (call sign SAQ). The station was located here in 1924 because the great circle propagation path to New York lay entirely over sea, apart from a few kdometres over the very flat land of the northern tip of Denmark. The whole transmitter has been maintained in its original state; modern technology has been allowed to intrude only to the extent that the mercury arc rectifier in the fiequency- control circuit has been replaced by two half-ampkre rating silicon diodes. It is interesting to note that one small Morse key on the control panel can key the whole of the 200 kW, 17.2 kHz output of the generator. At the conference held at Savoy Place in September 1995 to celebrate ‘100 years of radio’, a lecture on the Alexanderson alternator by the present author was followed by the reception in the lecture .theatre of a Morse message of greetings transmitted ‘live’ from Grimeton. This was picked up at the BBC receiving station at Crowsley Park and sent via Caversham Park by land-line to the IEE.

Summary

The Alexanderson alternator was a novel and innovative machine which was of great value in radio communications for thirty years or so. Its place in

ENGINEERING SCIENCE AND EDUCATION JOURNAL FEBRUARY 1997

41

history was recopsed in 1992 when it was accorded the status ‘Engineering milestone’ by the American Institute of Electrical and Electronics Engineers.

References

FLEMING, J. A.: ‘Principles of electric wave telegraphy and Telephony’ (Longmans Green, London, 1916, 3rd edn.), Chap. 1 RUHMER, E.: ‘Wireless telegraphy’ (translated by J. Erskine-Murray) (Cmsby Lockwood, London, 1908), pp. 132-134 AITKEN, H. G. J.: ‘The continuous wave‘ (Princeton University Press, New Jersey, 1985), pp.51 ff. FESSENDEN, R. A.: ‘Long &stance wireless telegraphy’, The Electrician, 1907, 59, pp.985-989

5 FESSENDEN, R. A.: ‘Portable type of high fkquency alternator’, The Electrician, 1908, 61, pp.441-444

6 ALEXANDERSON, E. F W ‘Alternator for one hundred thousand cycles’, The Electrician, 1909, 63, pp.541-542

7 ALEXANDERSON, E. F W. ‘Transatlantic radio communication’, Proc AIEE, 1919, pp.1077-1094

8 ALEXANDERSON, E. F W ‘A magnetic amplifier for radio telephony’, Proc. IRE, 1916, pp.101-120, and discussion pp. 121-129

9 MAYES, T L.: ‘The Alexanderson 200 kW high-frequency alternator transmitters’, Society of Wireless Pioneers (USA), Historical Papers ‘Ports 0’ call’, 1975, 4, pp.31-40

0 IEE: 1997 Dr. Phdhps has recently retired from the Department of Electrical and Electronic Engineering, University of Wales, Swansea, UK. He is an IEE Fellow.

One hundred years of maritime radio W D. Goodwin

Brow, Son G Feipson Ltd., 1995, 259pp., A38, ISBN 0 85174 632 2

Radio apparatus was first installed in a merchant ship in 1900. Implementation of the Global Maritime Distress and Safety System (GMDSS) will remove the need for radio watch-keeping in ships after 1999; the marine radio operator’s profession (and with it the commercial use of the Morse code) will then disappear after a century of dedicated service.

Mr. Goodwin, as an experienced ship’s Radio Officer and a Fellow of the IEE, is well-qualified to tell the story of marine radio over ths past century. He states in the foreword to this book that his intention was to promde ‘a permanent record of the contribution of radio and radio officers to the well-being and safety of all who travel by sea’; he says further that this story was to be set in the context of the various Safety of Life at Sea conventions (SOLAS) and associated legislation. This is an appropriate time to publish such a study, and it meets a real need. No previous book has provided a single comprehensive account of either topic.

The title imphes rather greater coverage than the book actually achieves. It concentrates on the mercantile marine (including fisheries and coastal rescue services), providing little account of the development and use of radio in warships. Accepting this limitation, the story is told as a chronological narrative which includes brief technical descriptions of the more important marime radio systems. There are no line drawings of any kind, but plenty of well-chosen photographs. The various clauses of the SOLAS conventions are spelled out compre- hensively wherever they have influenced radio design or operating practice. All of this factual information is enlivened by accounts of dramatic rescues at sea and w t h personal remifiiscences fiom radio officers. Mr. Goodwin writes well and his story should be accessible even to those with little technical knowledge.

This book meets its stated aims admirably. Radio

telegraphy evolved originally to meet the communi- cations needs of ships at sea; in conjunction with Sir Arthur Hezlet’s book ‘The electron and sea power’, to provide the additional details of military developments, this book completes the story of rad0 in this most important application during its first century. It is a most interesting account of the development of a major technology for its present readers. With its comprrhensive photographic coverage and lists of references it will, I am sure, be an essential source-book for future generations.

R. E POCOCK

Gleanings from academe Dear Sir-V. J. Phillips’s article (ESEJ October 1996, pp.206-210) was interesting to those who have seen similar contributions, but not all humour is unintended. When I was a senior examiner, examination rubrics for a number of years carried the strange instruction ‘Candidates are expected to display a minimum standard of Enghsh’. One bright student finished his examination early enough to comply. He added a final paragraph of two sentences which went something like:

‘I done this exam great but the questions this time was esier and I took fewer time and was learned good by my teacher. Itrust the examiner will accept the previous sentence as my display of a minimum standard of Enghsh.’

It made me wonder whether students were alone in needing instruction on how to present decent Enghsh.

C. PARTON (MIEE) Uddingston, Glasgow

ENGINEERING SCIENCE AND EDUCATION JOURNAL FEBRUARY 1997

42