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PT.8: THE FIRST THREE-PHASE AC ELECTRIC RAILWAY THE EVOLUTION OF ELECTRIC RAILWAYS Since so many of the world's electric railways ore powered by high voltage AC, it is surprising that more countries did not try using induction motors. One country that did recognise the advantages was Italy. By BRYAN MAHER Up to the year 1900, railways were almost entirely steam powered and little heed was given to the few electrified lines then existing. These were all short DCpowered systems, working at voltages in the range 250 to 750 volts. Many were quite small, from the famous Volkes Electric Railway (world's oldest and smallest work- ing electric line) at Brighton, England, to the growing suburban underground or elevated systems of big cities such as London, New York and Chicago. The Union Passenger Railway built at Richmond, Virginia, in 1887 was the first electric line in the USA, and a world-first venture into longer electric systems. Elsewhere LAMINATED STEEL POLES AND MOTOR YOKE vmu SERIES AELD AC SUPPLY 200V-1kV 16.6Hz ~ ARMATURE REVERSING SWITCH Fig.2: the series traction motor is so named because its fields are in series with the armature. To reverse the motor, the connections to the armature (or to the field coils but not both) are swapped by means of the reversing switch. 80 SILICON CHIP Fig.1: AC series motors use a core made of laminated steel sheets, each insulated from the next by an iron oxide scale. This breaks up eddy current paths and reduces power losses. on the world scene, a small difficult section of (otherwise steam) main line might be electrified, such as a world-first at Baltimore, USA where the tunnel district was electrified in 1895. But, for the most part, the railway magnates of the world ignored such ventures into electric traction. Instead, they concentrated on more serious matters, like steam locomotive traction. High voltage AC As we saw last month, the BernLotschberg-Simplon Railway (the famous BLS) of Switzerland came to the notice of the railway world in 1906-1913 when they built the first full-size electric standard gauge heavy-haul main line through extremely difficult mountainous terrain. Their choice of single-phase high voltage alternating current (15kV, 16.6Hz) was innovative, showing that properly designed series motors worked very well on a low frequency AC supply. Furthermore, by carrying a large transformer on Fig.3(a): sectional view of an industrial 3-phase AC squirrel cage induction motor. The 3-phase stator winding produces a rotating magnetic field which is followed by the rotor because of the current induced into the rotor's copper bars. (Photo courtesy General Electric). the locomotive, they could use voltages as high as 15kV (and consequently lower currents) on the overhead contact wire. The onboard transformer then stepped down the high voltages to any convenient lower voltage, between 500 and 1000 volts, for the controllers and motors. of iron-oxide, which breaks the eddy current path and greatly reduces the eddy current problem. Provided the series AC motor is run on low frequency alternating current, the interpoles [described last month) work satisfactorily and the motor's brushes run without commutator-to-brush arcing. Series motors on AC Note that the single-phase AC series traction motor is a straightforward development of the series DC motor. The only real difference between the two types is that in the AC series motor all the iron in the magnetic path is laminated steel, to avoid eddycurrent heating and power loss in the iron. Eddy currents are caused by stray voltages being induced in the iron itself by the presence of alternating current fields in the motor. Because the iron has a very low resistance, very large stray currents flow in "eddies" in the iron, causing heating of the iron and resultant power loss. So, instead of solid iron being used for the cores of the field coils and the magnetic pathways of the frame, a laminated assembly of many sheets of steel is used. Each steel sheet is insulated from the next by the natural scale Fig.3(b): end view of the 3-phase stator coils of a squirrel cage AC induction motor, with rotor removed. (Photo coutesy General Electric). Perhaps you might wonder how the AC series motor runs correctly even though the supply is reversing in polarity (ie, current direction reversing) every 60 milliseconds? Why doesn't the motor rotate backwards-and-forwards on each cycle. The answer is that the motor's direction is determined by the relative direction of currents in both the armature and field coils. During each AC half-cycle the currents in both reverse at the same time, so there is no change in the direction of rotation. When the train driver wishes to reverse the train, his reversing switch swaps the connections to either the armature or fields (but not both). However, in common with its DC counterpart, the AC series motor still has a commutator and brushes, which do become dirty and oily with use, and wear out. Maintenance is a necessity. The DC motor, based on the inventions of Michael Faraday of England in 1831, was further developed by Frank Sprague of the USA in 1884. This gave Thomas Edison encouragement to push for DC to be chosen for electric railways, street lighting, and domestic and industrial power. AC induction motor In that same year, 1884, a Hungarian electrical engineer, Nikola Tesla (1856-1943) had migrated to the USA. Four years later he took out a US patent on an electric motor which had no need of a commutator or brushes, because of his clever application of the laws of alternating currents. By using a 3-phase AC supply (rather than single-phase), Nikola Tesla invented a method whereby the AC currents flowing in three sets of coils in the stator (stationary part) produce a rotating magnetic field. A rotor (rotating part) carrying closed-circuit coils will have currents induced in these coils. Such rotor currents interact with the stator magnetic fields , causing the rotor to follow the rotating magnetic field of the stator. Thus the rotor rotates, even though there is no direct electrical connection to the rotor coils. Because it works by induced JUNE 1988 81 TWIN OVERHEAD CATENARY WIRES BONDED \ TOP CATENARY WIRES " +3kVDC OVERHEAD WIRING DROPPERS - - - PHASE A " ' - / OVERHEAD CONTACT WIRES \.: / PHASE A PANTOGRAPH PltASE B - PHASE B PANTOGRAPH BOTH CONTACT WIRES ._,/ +3kVDC ""' ----------◄- - 3-PHASE ELECTRIC LOCOMOTIVE CABIN 3kVDC ELECTRIC LOCOMOTIVE CONTROL NEGATIVE RETURN j FORCED AIR FAN \ INSULATORS CONTROL 3-PHASE CABLES TO TRACTION MOTOR BEARING ONE EXTRA-WIDE PANTOGRAPH TO CONTACT BOTH OVERHEAD WIRES DC 3kV TRACTION MOTORS 3-PHASE AC TRACTION MOTORS SLEEPER BOTH RAILS BONDED MAKE NEGATIVE OF 3kVDC SUPPLY RAILS BONDED TOGETHER FORM PHASE C Fig.4: end view sectional diagram of a 3-phase AC electric locomotive. Two overhead contacts provide phases A and B while the bonded rails supply phase C. The motor is very simple but speed control is difficult. rotor currents, this type of motor is called an AC induction motor. Nikola Tesla sold the rights of his motor patent to George Westinghouse (!846-1914), an American inventor (whose name we have heard before as the inventor of the Westinghouse rail air brake). It seems that Westinghouse and his company advocated AC power reticulation to homes and factories, in competition with Edison's DC systems. Tesla's AC induction motor gave the Westinghouse company a big advantage. Together with the development of the transformer, this led to the success of the Westinghouse Company in the highly competitive electricity business. 82 SILICON CHIP Fig.5: conversion of the old 3-phase AC system to a 3kV DC railway was accomplished by bonding both overhead wires together and connecting them to a 3kV DC supply. The bonded rails form the negative return. High voltage 11kV 3-phase 60Hz AC power lines could run long distances to American suburbs, there to be transformed down to 110 volts for homes and factories. Unfortunately, both DC and AC systems were installed in competition in many cities around the globe. Where AC was installed, various frequency systems were adopted in different cities and countries. Most of the USA uses a 60Hz supply, most of Europe, Australia and many other countries are on ·50Hz, and some like Japan have both 50Hz and 60Hz systems. The shrinking world still suffers from the resulting incompatibilities. But DC system advoc;ates did not give up easily. Hundreds of towns in the USA and elsewhere were wired for DC, before the advantages of AC power systems for general use became widely recognised. (Some readers may even remember shops in York Street, Sydney having DC mains and appliances as late as the 1950s). In some countries a few very forward-thinking people tried experimenting with alternating current quite early. As early as 1899, some railway engineers in Italy wanted to test AC induction motors for traction purposes. The Italian steam locomotives of the day, innovative though they were, needed imported coal supplies, while northern Italy, with its high mountains, lakes and fast snow-fed rivers had the potential for great hydroelectric power systems. Such power stations must be built in the mountains, but the proposed railway electrification was required hundreds of kilometres away, down in the cities. Three phase AC traction motors Therefore the engineers chose 3-phase high voltage AC as their system, with transformers at appropriate locations to step down the voltage to usable levels. That took care of distribution but then there was the problem of using the new 3-phase induction motors for traction. They installed a complex system of overhead wiring above some of their existing rail tracks to supply three phases to a specially built electric locomotive. Two separate overhead wires supplied two of the phases, while the running rails were bonded together to supply the third phase. The roof-top pantograph was in two insulated sections, each separately in contact with one overhead wire, but kept clear of the other. The gyrations of the pantographs in keeping contact with only the correct wire (and not shortcircuiting both overhead wires at crossovers and points) was an example of ingenious Italian engineering. BEARING/ THREE COILS/ ON ROTOR I ____ STATOR THREE COPPER SLIP RINGS ON INSULATED CENTRES __, Fig.6: a 3-phase wound rotor induction motor has three coils wound on the rotor which are connected via sliprings to three stationary speed control resistances. When all resistances are in circuit, the motor runs at low speed and develops greatest torque. Constant speed motors The induction motor does have one big problem though - its "synchronous speed' '. Synchronous speed is the constant speed of rotation of the motor's magnetic field, and is fixed by the frequency and the number of motor poles. For instance, a 2-pole motor on a frequency of 50Hz has a synchronous speed of 3000 RPM (ie, 50 revolutions per second). Some possible supply frequencies and corresponding motor speeds are shown in Table 2. When on full load the rotor always wants to rotate at about 96% of the synchronous speed. The fact that an induction motor rotor tends to rotate at the one fixed speed is excellent for driving factory machines such as lathes, grinders, planers, air compressors Fig.7: a DC industrial motor with the top half lifted to show the brushes, two main poles and one interpole. The armature and remaining two poles are in the lower half. (Photo courstesy General Electric). etc. But this one-speed property is not much good for trains. Also the starting torque of a simple induction motor is not very high, maximum torque being attained after accelerating to about half the synchronous speed. Nevertheless, the attraction of simple low-maintenance traction motors and cheap trackside substaJUNE 1988 83 Table 1: Notable Electric Railway Dates Date State/ Country Railway System 1842 Scotland E&G DC 120V (battery) Robert Davidson built the first electrically powered railway vehicle 1879 Germany Siemens DC First electric railway to carry paying passengers (demonstration only) 1880 England Volkes DC World's oldest and smallest permanent electric railway 1890 England London DC 750V third rail London tube, underground and Southern, suburban electric railway system 1900-28 Italy FS AC 3-phase Largest ever 3-phase railway 1906-13 Switzerland BLS AC 16.6Hz 15kV World's first full size electric main line railway 1912 Great Northern AC 25Hz 5000 HP loco, 214km of electrified track in Rockies USA Details 1914-20 USA Milwaukee DC 3kV First US railroad to electrify 400km main line 1915-22 Sweden Lappland AC 15kV 15 & 16Hz 9500 HP rod drive locos pulling 5000 tonne iron ore trains 1918-25 Switzerland SBB AC 15kV 16.6Hz Swiss Federal Railway all main lines electrified 1'919-23 Norway Lappland AC 15kV Norwegian end of Lappland line electrified 1919 VA DC 1.5kY First electric train in Australia 1920-34 France Midi DC 1.5kV Electrified all the south-west of France 1920-22 Germany DB & DR AC 15kV 16.6Hz First German main line to be electrified 1922-70 Norway NSB AC 15kV 16.6Hz All Norwegian lines except Bodo line electrified 1923 Victoria VA DC 1.5kV First electric locomotive in Australia (coal lines) 1926 NSW SRA DC 1.5kV Sydney suburban electric railway and underground 1929 USSR USSR Railway DC 3kV & AC 25kV Russian railways commence electrification (mixed AC and DC) 1980 Switzerland to Austria AC 15kV 16.6Hz International system still in use 1930 USA Virginia AC 11kV 25Hz Virginia Railway electrified mountain coal lines, strongest ever locomotives 1930 USA Penn AC 11kV 25ttz Pennsylvania Railroad commenced electrification :1932-81 USA Penn AC 11kV 25Hz General Electric "GG-1 ", the first and most longlived high speed electric express locomotive ,1934 USSR USSR Railway DC 850V Moscow metro underground electric railway commenced 1934 France Midi DC 1.5kV Completed electrification all Midi lines, SW France 1950 France SNCF AC 25kV Construction commenced for North and East of France 1979 Old QR AC 25kV 50Hz Brisbane suburban electric railway (first high voltage AC railway in Australia) 1986 Old OR AC 25kV 50Hz Delivery of first 25kV AC locomotive in Australia (July 1986) 1986 Old Seaworld AC 415V 50Hz First monorail in Australia '1986 Old OR AC 25kV 50Hz Opening of Gladstone-Rockhampton-Blackwater electrification (6th September 1986); first long distance AC high-voltage electric railway in Australia Victoria ' 84 SILICON CHIP tions (consisting of simply a 3-phase transformer and protection) was considerable. Thus, the Italian 3-phase electric railway began in a small way in the year 1900. Wound rotor induction motors An alternative construction for a 3-phase induction motor is to install three windings on the rotor, connected to three shaft-mounted sliprings, which have stationary carbon brushes. These brushes carry the rotor currents out from the motor to three external variable resistances. Shorting out these three resistances results in the motor running at 95 % of synchronous speed, but with some resistance in circuit the motor runs at a slower speed. More resistance still results in still lower motor speed and more shaft torque. When a value of external resistance is selected such that all the resistance in the rotor circuit (rotor winding plus external resistance) is numerically equal to the inductive reactance of the rotor winding at line frequency, then the induction motor develops its maximum starting torque. For electric locomotive applications, this condition results in maximum starting drawbar-pull; ie, highest locomotive pulling force. This, of course, is the best choice for starting a heavy train. So effective did the Italian 3-phase AC railway become that the system was continually extended. It replaced steam locomotives on many lines and became the world's largest and most successful 3-phase electric railway system, lasting until 1971. However, the difficulty of building the double overhead contact wire installation, especially over the complex trackwork at the approaches to large city terminal stations, produced much engineering opposition, and for good reason. Also all induction motors always have a fixed top speed, as shown in Table 2, which restricted the running speed of trains. Italy adopts 3kV DC Because of these objections, all new Italian railway electrification Table 2: Induction Motor Synchronous Speeds Frequency Number of Poles 50Hz 50Hz 50Hz 50Hz 50Hz 25Hz 25Hz 25Hz 25Hz 25Hz 16.6Hz 16.6Hz 16.6Hz 16.6Hz 16.6Hz 2 4 6 8 12 2 Synchronous Speed 3000 1500 1000 750 500 1500 750 500 375 250 1000 500 333.3 250 166.7 4 6 8 12 2 4 6 8 12 undertaken after 1928 used the 3kV DC system, despite the extra maintenance necessitated by the DC series motors with their commutators and brush gear. The 3kV DC system gradually took over lines previously constructed as 3-phase AC systems, beginning at the city of Genoa in 1928, until all AC lines were converted to 3kV DC by 1971. The conversion from 3-phase AC to 3kV DC was done initially by removing the 3-phase AC supply, and bridging the two previouslyseparate overhead wires together (without physically moving them) to become a common positive 3kV overhead conductor. Then each DC electric locomotive was equipped with a rooftop pantograph wide enough to run in contact with both overhead wires, as shown in Fig. 4. The DC return current flows (as usual) via the running wheels and rails. Of course all new installa- . , ~ ~ 0 RPM RPM RPM RPM RPM RPM RPM RPM RPM RPM RPM RPM RPM RPM RPM tions used a single overhead contact wire for the positive 3kV DC conductor. In the early part of this century the electric railways of the world were poised to proliferate, but as Table 1 shows, many and varied were the voltages and frequencies adopted by different countries and systems. AC and DC systems will continue to have their devotees throughout this century, and maybe well into next. Next month, we will again consider high-voltage AC single-phase low frequency railway systems. Acknowledgements Thanks to ASEA/Brown Boveri, SBB (Swiss Federal Railway), BLS (Bern-Lots c h be r g-Simpl on Railway), SJ (Swedish Railways), FS (Italian State Railway), and GE (General Electric Company, USA and Aust.) for data, photos and permission to publish. ~ J)~ ·-✓• g ·$ g ~~ ....~&.nn11fl.■111~lli111 RCS Radio Pty Ltd is the only company which manufa.ctures and sells every PCB & front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207 Phone (02) 587 3491 for instant prices 4-HOUR TURNAROUND SERVICE Bookshelf continued from page 51 Specifications listed include supply and temperature maximums, gain, offset voltage, input current, slew rate, input impedance, CMRR (common mode rejection ratio) and PSRR (power supply rejection ratio). Finally, there are several appendices which include a glossary of op amp terms, abbreviation codes, manufacturers' lettering designations and case outline and pin-out diagrams. In summary, a very useful book for checking out unknown or obscure devices, or when a device is unavailable and an equivalent is wanted. Our review copy came from Dick Smith Electronics. Copies are available from Dick Smith stores. Digital IC selector handbook Towers' International Digital IC Selector, by T.D. Towers. Published 1987 by Manish Jain for B. P. B. Publications, 376 Old Lajpat Rai Market, Delhi, India. Soft Covers 246 pages, 175 x 243mm. This selector for digital ICs provides information on 10,000 separate digital devices. It covers devices from the USA, UK, East and West Europe, and Japan. The ICs are listed in alphabetical order with descriptions of IC operation and control specifications. Information is given for the type of IC (CMOS, TTL, ECL, etc), its use and description, the type of casing, supply voltage, temperature, speed, pin-out, manufacturer and substitute device. Diagrams are shown for pin-outs of each device in the appendices . The usefulness of this reference can be limited, particularly when information is required for design purposes. However as a source of information on IC function and operation, the book is excellent. For more detailed information on particular devices, the reader should refer to manufacturers' data. Our review copy came from Dick Smith Electronics. Copies are available from Dick Smith stores.~ JUNE 1988 85