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Swiss Railways’ fast new locomotives Recently, the Swiss Railways introduced a new series of locomotives which are compact, very powerful and equally suited to pulling fast passenger trains or heavy freights. This was made possible by comprehensive use of electronics in the drive system. By LEO SIMPSON Intended mainly for use on the Gotthard line, the new locomotive, designated Re4/4 460, has 3-phase induction motors, very efficient regenerative braking and produces minimal wear and tear on its equipment. Locomotives designed for a variety of duties clearly offer advantages over locomotives built for just one type of duty. The work schedule for multi-pur4  Silicon Chip pose units can be drawn up to take advantage of their versatility, making down-times shorter. Also, the training of the drivers and maintenance staff is easier and spare parts inventories can be kept smaller. The Re4/4 460 locomotive is designed to operate from a single-phase 15kV AC catenary at 162/3Hz. It has a BoBo wheel arrangement (ie, two bogies with two motors each) and its adhesion mass is 84 tonnes. The maximum power at the wheel rim is 6100 kilowatts. This is a very high power for any locomotive, regardless of its design, and amounts to over 2000 horsepower per axle. In typical locomotives with series DC motors, tractive effort drops off at high speed. But in these new locos, high speed and high tractive effort are both achieved. This is made possible by the variable frequency drive system for the induction motors. The starting tractive effort is 275kN (27.5 tonnes) which is very high considering the mass of the locomotive. This maximum tractive effort is available up to a speed of 80km/h. Even at its maximum speed of 230km/h, the locomotive can still develop a tractive effort of 83kN. At the top operational speed of 200km/h, a tractive effort of about 110kN is available. This is enough to pull an inter-city train with seven passenger cars over relatively flat routes with gradients of up to 1% at a speed of 200km/h. Because of the locomotive’s tractive power and the permitted temperature rise in the traction motors, two of these locos can accelerate a train weighing 1300 tonnes to 80km/h on a 2.7% (1 in 37) gradient and then maintain this speed, at which the draw-bar power limit on the Gotthard line is reached. The experience gained with the propulsion system and the control electronics on previous Swiss locomotives (Re 4/4 and Re 4/4 450 series) proved to be very valuable. However, the higher power output and top speed called for the very latest technology. The maximum loco speed of 230km/h means that aerodynamic design is most important even though the unit is quite boxy to look at. The fact that the locomotive is used to push or pull trains made a symmetrical design necessary, with a driver’s cab at each end. Furthermore, it was important that the slipstream over the roof did not cause underpressure, especially when the train passed through tunnels, as this could impair cooling of the traction motors and converters. New bogie design The special bogie suspension allows the locomotive to travel through curves 30 percent faster than before without exceeding structural clearances. Since at this speed the lateral acceleration can reach 1.8m/ s2, passenger comfort then depends on carriages having active tilting. These are not yet in use but are being considered in Switzerland. The complete bogie weighs just 16 tonnes, including the two motors. Forces are transmitted between the body and bogie by push/pull rods, which enable the transmission point Facing page: One of Swiss Railways’ Re4/4 460 locomotives crosses the ‘Kander’ viaduct in the Bernese Overland on the occasion of the inauguration of the Berne-LotschbergSimplon Railway’s double track. The bogies for the Re4/4 460 locomotive employ two high speed 3-phase induction motors each continuously rated at 1200 kilowatts. The very short wheelbase of the bogies is made possible by the small size of the motors. on the bogie to be kept as low as possible. The load difference between each bogie’s wheelsets are therefore small. Lateral forces acting between the wheels and rails are reduced by ‘soft’ suspension of the wheelsets in the bogie frame, allowing the wheelsets to adjust radially when the train runs through curves. Another factor promoting good running in curves is the short wheelbase of only 2.8 metres. This was made possible mainly by the compact traction motors. In any electric locomotive such as this, operating from a high voltage catenary supply (ie, 15kV AC), the heaviest item of equipment is the main transformer which has to supply the full load power of more than 6 megawatts. In this case the designers have gone to special lengths to get the weight down. For example, they replaced the metal core clamps by a far lighter, non-metallic material, plywood, which also has the benefit of eliminating eddy-current losses. The aluminium transformer tank also saves weight and damps stray magnetic fields occurring at harmonic frequencies. The traction motors are four-pole, high-speed squirrel cage induction motors with a maximum speed of 4180 rev/min for an input frequency of 143Hz, and a continuous rating of 1200kW. Their short term capacity is 1560kW, equivalent to 2090 horsepower. High speed squirrel cage induction motors are used because they are lighter and more compact than equivalent series DC motors used for traction. As well, they have no brushes, commutator or slip rings and thus their long term maintenance is minimal. But the really big advantage of these induction motors is their excellent speed control and resistance to wheel slip. This comes about because of the drive system. Induction motors operating from a fixed frequency AC supply are notoriously difficult to speed control. In fact, their more or less constant speed regardless of load is normally a virtue but for traction, where trains need to run over a wide range of speeds, it is a big drawback. This is why series DC motors have been “king” for traction for so long. However, by providing a continuously variable frequency AC supply to the induction motors, speed control is achieved. Not only that, wheel slip under acceleration is virtually eliminated and full regenerative braking, almost down to a complete stop, is achieved. The two motors of each bogie are connected electrically in parallel and September 1993  5 The driver’s cab has the speedo in the centre and a diagnostics screen to the right. as in the Re4/4 and Re4/4 450 locomotives, the two bogie drive units operate completely independently of each other. Even if a fault occurs in one of the drive systems or its control units and auxiliaries, the train can continue its journey on half power. Fig.1 shows the schematic circuit of the new Re 4/4 460 locomotive and remember that this operates at powers up to 6 megawatts and beyond. At the top of the circuit is the 15kV AC catenary wire and this is fed down to the main transformer which has seven secondary windings. Three of these, marked A, B and C provide auxiliary supplies for the loco. The other four each drive four quadrant controllers. These employ gate turn-off (GTO) thyristors with an off-state voltage rating of 4.5kV and turn-off current of 2500 amps. The output of the four quadrant controllers is the so-called converter’s DC link which has a nominal voltage of 3.5kV. Such a high DC link voltage is desirable as it keeps the currents at acceptable levels. In addition, it allows the same circuit to be used in dual-voltage locomotives which are designed to run on the rail networks of neighbouring countries operating with a 3000V DC catenary. 6  Silicon Chip The DC link then supplies the variable frequency inverters which drive the three phase induction motors. These inverters are based on the same GTO thyristors as used in the four quadrant controllers. The frequency output of the inverters ranges from below 1Hz to 143Hz, at which the motors run at 4180 RPM. Regenerative braking An induction motor can be used as a powerful regenerative brake. All that needs to be done is to drive it at faster than its “synchronous speed”. With a variable frequency drive in a locomotive, this is easily achieved simply by reducing the frequency. The motor then acts as a generator and the power is then fed back via the four quadrant controllers of the inverters and DC link to the transformer and thence back to the 15kV AC catenary supply. This brake is applied continuously on downhill runs and is also used to brake the trains almost to a standstill. On the Gotthard route, for example, the locomotive’s electrical brake has to be capable of braking loads of up to 650 tonnes to a constant speed of 80km/h on gradients of about 1 in 40. GTO thyristor-controlled resistors built into the DC link provide protection from transient over-voltages caused by unexpected disconnections of the catenary supply. The resistors are connected into circuit whenever there is a power supply failure or system disturbance. The regenerative brake’s large range of action allowed a reduction in the power of the locomotive’s mechanical brakes (ie, the shoe brakes and the magnetic rail brake), despite the fact that the locomotive’s speed has been increased. The magnetic rail brake, equipped with permanent magnets, performs safety functions and serves as the parking brake. Microprocessor control The MICAS S2 traction control system used in the Re4/4 460 locomotive uses a fibre optics serial bus with data signalling rate of 1.1 Mbit/s. It can be used to link up to 256 unit addresses. Commands entered by the driver in his cab are transmitted via the locomotive bus to the locomotive control unit in the electronics cabinet. After processing, the signals are transmitted over the bus to the relevant stations. Fibre optics has special advantages for locomotives with converter-fed propulsion because of 15kV 16.66Hz 1 3 2 5 4 29 21 6 13 M 3 17 30 25 DG1 7 26 8 35 15 31 18 36 M 3 22 9 32 23 10 14 M 3 19 33 27 DG2 11 28 12 37 16 34 20 A B C RAIL 38 M 3 24 DG1 DG2 A B C 1 2 3 4 5-12 13,14 15,16 17-20 21-24 25-28 29-34 35-38 Bogie 1 Bogie 2 Converter for auxiliaries 220VAC for auxiliaries 1000VAC train busbar Pantograph (catenary) Grounding switch Main circuit breaker Main transformer Four quadrant controllers Series resonant reactors Series resonant capacitors DC link capacitors Voltage limiters Voltage limiter resistors AC drive inverter 3-phase induction motors Fig.1: schematic diagram of the Re4/4 460 locomotive. All the circuitry is controlled by a complex microprocessor system employing fibre optic links to avoid problems of electromagnetic interference. its immunity to the strong electromagnetic interference throughout the locomotive. It is anticipated that multiple control will be used very often, particularly on the Gotthard route. It is possible to operate up to four locomotives in this mode. In such cases, the locomotive bus systems will be linked to the train bus, over which the commands and messages to and from the leading locomotive are transmitted. Since the locomotive bus is a fibre optic link and the train bus uses copper conductors (two cores of the electropneumatic brake control cable) operating in TDM mode (with telegram exchange), each locomotive is coupled to the train bus by a time multiplexer multiple-control coupler. It is due to this system that locomotives can September 1993  7 regulation in the case of motors. The power is provided by four identical converters which also feature GTO thyristors. Two converter modules supply power to the traction motor and oil-cooler blowers, the third to the compressor motor, and the fourth to the oil circulation pumps of the main transformer and converter, the air-conditioning system in the driver’s cab, and the battery charging system. Mounted in the same frame is the electronics equipment for controlling the onboard system converters and auxiliaries. Driver’s cab This photo shows the four quadrant controller and other equipment asseociated with the frequency converter for a bogie drive. All the power electronics are housed in oil-filled tanks for efficient cooling. The main transformer is situated underneath the locomotive. also be placed at some intermediate position in the train, the only proviso being that the cars have to be equipped with the electropneumatic brake control cable. Diagnostics No microprocessor control system for a locomotive would be complete without a diagnostics facility and the one in the Re4/4 460 locomotive is comprehensive. Its task is to collect information needed by the train driver and the maintenance crew, without intervening itself in the process sequences. Automatic measures are initiated at the locomotive control level as they become necessary. All failure symptoms and their corresponding signals are programmed in the distributed microprocessors of the control system. These detect deviations from the setpoint behaviour in their respective areas, and transmit the information to the locomotive’s central diagnostics processor. This has a non-volatile memory with a capacity for storing up to 2500 events. The evaluation of the fault signals takes place at three levels. At level 1, a fault is announced by an alarm lamp lighting up within the driver’s field of vision, followed by short messages 8  Silicon Chip being displayed on the diagnostics screen. These messages give the nature of the fault and instructions on how to proceed. Under fault-free conditions, nothing is displayed. The driver can isolate failed equipment by pressing a fault-clearing button on his console. Level 2 is for minor maintenance. The driver can request a list of the stored faults from the diagnostics messages on the monitor. Level 3 is for detailed investigation of the failure and for obtaining a statistical evaluation of the relevant events. The diagnostic data is transferred, with all related data and fault-clearing instructions, to a portable personal computer, from where they are loaded into a central database. Although with multiple control the individual diagnostics systems represent stand-alone units, fault data is transmitted over the train bus to the driver’s cab. Provision has also been made for diagnostics data from the passenger cars to be displayed in the driver’s cab. Auxiliaries All the locomotive’s auxiliaries are fed with three-phase AC, at variable frequency and voltage to allow speed The driver’s cab incorporates basic ergonomic features which are to be found in all modern Swiss locomotives: • Controls and instruments for traction and electrical braking are on the right. • Controls and instruments for pneumatic braking are placed on the left. • The speedometer is in the centre of the driver’s field of vision. The driver’s cabs are soundproofed and fully air-conditioned. The design of the air-conditioning system overcomes the problem of presssure changes in the cabs when trains cross in tunnels. Fresh air enters from the roof chamber, above the machine compartment. All 99 of these locomotives for the Swiss Railways will have been delivered by mid-1994, as planned. They represent the very latest in traction technology and they illustrate the fact that electronics and computerisation is now vital to the efficient functioning of locomotives. In fact, without electronics and computers, today’s modern electric locomotives would simply be a dream. SC Acknowledgement The background material and photographs for this article came from the October 1992 issue of ABB Review. Other articles on modern electric locos and 3-phase propulsion were published in the series entitled “The Evolution of Electric Railways”, in the June 1989 and August 1989 issues of Silicon Chip.