Ac Traction and Dc Traction

AC Traction vs. DC Traction thomasnet.com AC TRACTION The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years. While it has been used in locomotives for over two decades (especially in Europe), it has only been recently that the price of the drives has allowed them to be used in most of the new diesel-electric locomotives in the United States. AC traction for locomotives is a major improvement over the old DC systems. The primary advantages of AC traction are adhesion levels up to 100% greater than DC and much higher reliability and reduced maintenance requirements of AC traction motors. The tractive effort of a locomotive (whether AC or DC) is defined by the equations: Tractive effort = Weight on drivers x Adhesion Adhesion = Coefficient of friction x Locomotive adhesion variable The friction coefficient between wheel and rail is usually in the range of .40 to .45 for relatively clean, dry rail in reasonable condition and is essentially the same for all locomotives. The locomotive adhesion variable represents the ability of the locomotive to convert the available friction into usable friction at the wheel rail interface. It varies dramatically from about .45 for old DC units to about .90 for modern AC units. This variable incorporates many factors including electrical design, control systems, truck type and wheel conditions. First generation DC locomotives such as SW1200s, GP9s, SD40s, and GE center cabs typically have adhesion levels of 18% to 20%. More modern units with adhesion control such as SD60s and Dash 8s have adhesion levels of 25% to 27%. The newer AC traction units such as the SD80MAC and the C44AC are usually rated at 37% to 39% adhesion. Thus, the newer locomotives have about twice the adhesion of the older units and the Class I railroads are, in fact, typically replacing two older units with a single new AC unit. There are three primary reasons that AC traction offers so much more adhesion. First, in a standard DC drive, if wheel slip occurs, there is a tendency for the traction motor to speed up and run away, even to the point of mechanical failure if the load is not quickly reduced. As the wheel slippage increases, the coefficient of friction also drops rapidly to a level of 0.10 or less, and because all the motors are connected together, the load to the entire locomotive must be reduced. Therefore, maximum adhesion is obtained by operating at a level with a comfortable margin of safety below the theoretical maximum. More modern DC systems incorporate a wheel slip control which senses the beginning of a slip and automatically modulates the power in order to retain control. This allows the locomotive to operate safely at a point closer to its theoretical maximum. The AC system, however, operates in a very different fashion. The variable frequency drive creates a rotating magnetic field which spins about 1% faster than the motor is turning. Since the rotor cannot exceed the field speed, any wheel slip is minimal (less than 1%) and is quickly detected by the drive which instantly reduces load to the axle. Next, the DC locomotive typically has a number of throttle settings with a set power level for each one. While this sytem is simple and effective, it does not produce a constant motor torque since power is the product of torque and speed. Therefore, the tractive effort varies significantly for each throttle setting depending on speed, making it impossible to obtain maximum adhesion. The AC locomotive, however, can control to a specific motor torque level allowing the tractive effort to be essentially constant at the higher range of available adhesion. Ths fast acting wheel slip control can counteract any wheel slip so that the torque level can be set close to the upper limits. The third way that AC traction provides improved adhesion is through weight transfer

compensation. When a locomotive is pulling a load, weight tends to transfer from the front axle to the rear axle of each truck. At maximum tractive effort, the weight on the lead axle may be reduced by about 20%. Since the tractive effort is proportional to the weight on drivers, then in a DC system where the motors are fed from a common source, the tractive effort will be determined by the lightest axle. Thus, in effect, the equivalent locomotive weight is reduced by about 20%. With an AC system, however, the drive is able to compensate for the weight transfer. When the lead axle goes light, the AC drive system will reduce power to that axle and apply more power to the rear axle without incurring wheelspin. The combination of eliminating wheel slip and compensating for weight transfer gives the AC traction system an adhesion of 37% to 39% versus the 18% to 20% of the older DC systems. Therefore, a locomotive with AC traction can provide the same tractive effort as a DC locomotive weighing twice as much or can give twice as much tractive effort for the same weight. GE and EMD added AC traction to their mainline units and were then able to replace two older DC units with one new AC locomotive. Republic locomotive took a different approach and decided to make a lighter, less costly unit for industrial switching. The DC powered SW9/SW1200, produced in large quantities from 1951 to 1965 and used for heavy yard switching as well as branch line service, was taken as the performance standard. At 230,000 to 240,000 pounds these units are typically rated at about 40,000 pounds tractive effort continuous (somewhat higher intermittent but limited by traction motors and generators). The AC traction RX500 at 144,000 pounds and a conservative 35% adhesion level is rated at 50,400 pounds tractive effort continuous. With AC traction, it is also important to consider braking. As with traction, braking is a function of weight on drivers. Therefore, when using standard friction braking (tread brakes) the braking capability of the locomotive (excluding train braking) is proportional to the locomotive weight. With AC traction, however, the braking can be much higher because the drive system in braking acts just like the drive does in traction thus eliminating wheel slip. The drive converts the motors to generating mode (dynamic braking) and the electricity produced is dissipated in the braking resistors. Thus the motors are slowing the locomotive without using the air brakes. Again, the adhesion levels are much higher so the locomotive can again be significantly lighter for the same amount of braking. The dynamic braking in AC traction locomotives also allows full braking down to zero speed, unlike DC dynamic braking. All in all, the AC traction locomotive offers about twice the amount of adhesion as a DC unit. Therefore, a modern lightweight AC locomotive such as the RX500 can provide as much or more tractive effort than an old style DC unit like the SW1200 which weighs 60% more. © 2012 Republic Locomotive ELECTRIC TRACTION DRIVE railway-technical.com – Electric Traction Glossary Introduction This page describes the way electric motors on locomotives and multiple units drive the axles and wheels. The DC Traction Motor: How it Drives the Axle The traditional DC (Direct Current) electric motor driving a train or locomotive is a simple machine consisting of a case containing a fixed electrical part, the stator (called the stator because it is static and comprising what is called the field coils) and a moving electrical part, the rotor (because it rotates) or armature as it is often called. As the rotor turns, it turns a pinion which drives a gearwheel. The gearwheel is shrunk onto the axle and thus

drives the wheels as shown in the diagram above. The motion of the motor is created by the interaction of the magnetism caused by the currents flowing the the stator and the rotor. This interaction causes the rotor to turn and provide the drive. The stator and the rotor of the DC motor are connected electrically. The connection consists of fixed, carbon brushes which are spring loaded so that they remain in contact with an extension of the armature called the commutator. In this way, the field coils (the stator) are kept in the circuit with the rotor (the armature and commutator). AC and DC Motors Both AC (Alternating Current) and DC motors have the same basic structure but there are differences and, for various reasons, the DC motor was originally the preferred form of motor for railway applications and most systems used it. Nowadays, modern power electronics has allowed the use of AC motors and, for most new equipments built today, the AC motor is the type used. Often, people ask about the differences between AC and DC motors as used in locomotives and multiple-units. In the early days of electric traction at the beginning of this century both types were tried. The limits of the technology at the time favoured the DC motor. It provided the right torque characteristic for railway operation and was reasonably simple to control. By the early 1980s, power electronics had progressed to the stage where the 3-phase AC motor became a serious and more efficient alternative to the DC motor because: 1. They are simpler to construct, they require no mechanical contacts to work (such as brushes) and they are lighter than DC motors for equivalent power. 2. Modern electronics allow AC motors to be controlled effectively to improve both adhesion and traction. 3. AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds. 4. They are more robust and easier to maintain than DC motors. This type of motor is commonly called the Asynchronous Motor and was often referred to as the squirrel cage motor on account of its early design form. The photos below show a DC and an AC motor. The DC motor is similar to look at externally but there are differences in construction, particularly because the DC motor has a commutator and brushes which the AC motor does not. Nose Suspended Motor The following diagram shows the layout of the traditional DC motor mounted in a bogie as a "nose suspended motor". In electric trains or locomotives, the DC motor was traditionally mounted in the bogie frame supported partially by the axle which it drove and partially by the bogie frame. The motor case was provided with a "nose" which rested on a bracket fixed to the transom of the bogie. It was called a "nose suspended motor" (see diagram above) and is still common around the world. Its main disadvantage is that part of the weight rests on the axle and is therefore unsprung. This leads to greater wear on bogie and track. Nowadays, designers try to ensure all the motor weight is sprung by ensuring it is carried entirely by the bogie frame - a frame mounted motor. Quill Drive This is a simplified diagram of a quill drive. A quill is described in the dictionary as, "the hollow stem of a feather" and "a bobbin or spindle", as well as a "feather" and, alternatively, what a porcupine has on its back. In railway traction terms, a quill drive is where a hollow shaft is placed round the driving axle and the motor drives the quill rather than driving the axle as it does with a nose

suspended drive. The quill itself is attached, at one end, to one of the wheels by means of rubber bushed links and, at the other end, to the gearwheel by similar links. The big advantage of such drives is that all the weight of the motor is carried in the bogie frame (so it is a frame mounted motor) instead of it being directly supported by the axle and therefore partially unsprung. An example of a traction motor with quill drive appears in the following photo. Click on it for a full size view and the part names. Various forms of quill drive have been used over the years. Older versions used radially mounted coil steel springs instead of rubber to connect the links to the wheels. Some, like the example shown here, have the motor mounted parallel with the axle. Others have the motor at a right angle to the axle, as in the the UK Class 91 electric locomotives. In German the quill is called "Hohlwelle" (hollow shaft) and used in the ICE1 and ICE2 as well as the electric locomotive Class 101. (Source Tobias Benjamin Koehler 19 Oct 98). Monomotor Bogie As its name implies, the monomotor bogie has a single motor which drives both axles. Click on the thumbnail to see a photo with the part names. The design is much favoured in France, where it was introduced in the 1950s for therubber tyred train concept. The motor is mounted longitudinally in the centre of the bogie and drives each axle through a differential gearbox, similar to a road vehicle. The differential gears are required to compensate for the operation of the rubber tyres round curves. It requires a special bogie frame construction to accommodate the motor. Another version of the monomotor bogie has also been applied to a number of French locomotive designs but here the arrangement is more conventional. Each bogie has a single motor mounted transversely over the centre as shown in the diagram left. The motor is fully suspended in the bogie frame and drives both axles through the gear train, which is contained in a single, large, oil filled gearcase (not shown). This type of drive is referred to in the locomotive wheel arrangement called a B-B, as opposed to a more conventional locomotive with four motors, each driving its own axle, which is called a Bo-Bo. Linear Motor A new form of traction which has appeared in recent years is the linear motor. The principal, compared with a standard motor, is shown here. This simple diagram shows the principal of the linear motor. The conventional DC motor consists of a fixed part (the stator) and a moving part (the rotor). Both parts are contained in a case on the train and the rotor is connected to the axle by a pinion/gear arrangement. When the armature turns, the wheel turns. The two parts of the linear motor are separated and one is placed on the train and the other on the the track. Both parts are unwrapped and they are swapped so that the fixed part of the DC motor becomes the moving part of the linear motor mounted on the train while the former moving part of the DC motor is fixed to the track. The electro-magnetic interaction between the current in the fixed part and that in the moving part causes the train to be drawn along the line. There is a very small air gap (about 10 mm) between the two parts as shown in this photo. The efficiency of the linear motor is about 60% of the conventional motor but it has the advantage of less moving parts and it does not have the reliance on adhesion of the conventional motor. ELECTRONIC POWER FOR TRAINS Introduction This page describes the most recent developments of electric train power equipment including the latest IGBT controlled 3-phase Alternating Current (AC) motors and the new permanent magnet motor. AC and DC Differences

To understand the principles of modern traction power control systems, it is worth a look at the basics of DC and AC circuitry. DC is direct current - it travels in one direction only along a conductor. AC is alternating current - so called because it changes direction, flowing first one way along the conductor, then the other. It does this very rapidly. The number of times it changes direction per second is called the frequency and is measured in Hertz (Hz). It used to be called cycles per second, in case you've read of this in historical papers. In a diagrammatic representation, the two types of current appear as shown in the diagram above left. From a transmission point of view, AC is better than DC because it can be distributed at high voltages over a small size conductor wire, whereas DC needs a large, heavy wire or, on many DC railways, an extra rail. DC also needs more frequent feeder substations than AC - the ratio for a railway averages at about 8 to 1. It varies widely from one application to another but this gives a rough idea. See also Electric Traction Pages Power Supplies. Over the hundred years or so since the introduction of electric traction on railways, the rule has generally been that AC is used for longer distances and main lines and DC for shorter, suburban or metro lines. DC gets up to 3000 volts, while AC uses 15,000 - 50,000 volts. Until recently, DC motors have been the preferred type for railways because their characteristics were just right for the job. They were easy to control too. For this reason, even trains powered from AC supplies were usually equipped with DC motors. AC Locomotives with DC Drives This diagram (above) shows a simplified schematic for a 25 kV AC electric locomotive used in the UK from the late 1960s. The 25 kV AC is collected by the pantograph and passed to the transformer. The transformer is needed to step down the voltage to a level which can be managed by the traction motors. The level of current applied to the motors is controlled by a "tap changer", which switches in more sections of the transformer to increase the voltage passing through to the motors. It works in the same way as the resistance controllers used in DC traction, where the resistance contactors are controlled by a camshaft operating under the driver's commands. Before being passed to the motors, the AC has to be changed to DC by passing it through a rectifier. For the last 30 years, rectifiers have used diodes and their derivatives, the continuing development of which has led to the present, state-of-the-art AC traction systems. The Diode A diode is a device with no moving parts, known as a semi-conductor, which allows current to flow through it in one direction only. It will block any current which tries to flow in the opposite direction. Four diodes arranged in a bridge configuration, as shown below, use this property to convert AC into DC or to "rectify" it. It is called a "bridge rectifier". Diodes quickly became popular for railway applications because they represent a low maintenance option. They first appeared in the late 1960s when diode rectifiers were introduced on 25 kV AC electric locomotives. The Thyristor The thyristor is a development of the diode. It acts like a diode in that it allows current to flow in only one direction but differs from the diode in that it will only permit the current to flow after it has been switched on or "gated". Once it has been gated and the current is flowing, the only way it can be turned off is to send current in the opposite direction. This cancels the original gating command. It's simple to achieve on an AC locomotive because the current switches its direction during each cycle. With this development, controllable rectifiers became possible and tap changers quickly became history. A thyristor controlled version of the 25 kV AC electric locomotive traction system looks like the diagram here on the left. A tapping is taken off the transformer for each DC motor and each has its own controlling thyristors and diodes. The AC from the transformer is rectified to DC by chopping the

cycles, so to speak, so that they appear in the raw as half cycles of AC as shown on the left. In reality, a smoothing circuit is added to remove most of the "ripple" and provide a more constant power flow as shown in the diagram (left). Meanwhile, the power level for the motor is controlled by varying the point in each rectified cycle at which the thyristors are fired. The later in the cycle the thyristor is gated, the lower the current available to the motor. As the gating is advanced, so the amount of current increases until the thyristors are "on" for the full cycle. This form of control is known as "phase angle control". SEPEX In more recent thyristor control systems, the motors themselves are wired differently from the old standard DC arrangement. The armatures and fields are no longer wired in series, they are wired separately - separate excitement, or SEPEX. Each field has its own thyristor, which is used to control the individual fields more precisely. Since the motors are separately excited, the acceleration sequence is carried out in two stages. In the first stage, the armature is fed current by its thyristors until it reaches the full voltage. This might give about 25% of the locomotive's full speed. In the second stage, the field thyristors are used to weaken the field current, forcing the motor to speed up to compensate. This technique is known as field weakening and was already used in preelectronic applications. A big advantage of SEPEX is that wheel slip can be detected and corrected quickly, instead of the traditional method of either letting the wheels spin until the driver noticed or using a wheel slip relay to switch off the circuit and then restart it. DC Choppers The traditional resistance control of DC motors wastes current because it is drawn from the line (overhead or third rail) and only some is used to accelerate the train to 20-25 mph when, at last, full voltage is applied. The remainder is consumed in the resistances. Immediately thyristors were shown to work for AC traction, everyone began looking for a way to use them on DC systems. The problem was how to switch the thyristor off once it had been fired, in other words, how to get the reverse voltage to operate on an essentially one-way DC circuit. It is done by adding a "resonant circuit" using an inductor and a capacitor to force current to flow in the opposite direction to normal. This has the effect of switching off the thyristor, or "commutating" it. It is shown as part of the complete DC thyristor control circuit diagram (left). It has its own thyristor to switch it on when required. Two other features of the DC thyristor circuit are the "freewheel diode" and the "line filter". The freewheel diode keeps current circulating through the motor while the thyristor is off, using the motor's own electro magnetic inductance. Without the diode circuit, the current build up for the motor would be slower. Thyristor control can create a lot of electrical interference - with all that chopping, it's bound to. The "line filter" comprises a capacitor and an inductor and, as its name suggests, it is used to prevent interference from the train's power circuit getting into the supply system. The thyristor in DC traction applications controls the current applied to the motor by chopping it into segments, small ones at the beginning of the acceleration process, gradually enlarging as speed increases. This chopping of the circuit gave rise to the nickname "chopper control". It is visually represented by the diagram below, where the "ON" time of the thyristor is regulated to control the average voltage in the motor circuit. If the "ON" time is increased, so does the average voltage and the motor speeds up. The system began to appear on UK EMUs during the 1980s. Dynamic Braking Trains equipped with thyristor control can readily use dynamic braking, where the motors become generators and feed the resulting current into an on-board resistance (rheostatic braking) or back into the supply system (regenerative braking). The circuits are

reconfigured, usually by a "motor/brake switch" operated by a command from the driver, to allow the thyristors to control the current flow as the motors slow down. An advantage of the thyristor control circuitry is its ability to choose either regenerative or rheostatic braking simply by automatically detecting the state of receptivity of the line. So, when the regenerated voltage across the supply connection filter circuit reaches a preset upper limit, a thyristor fires to divert the current to the on-board resistor. The GTO Thyristor By the late 1980s, the thyristor had been developed to a stage where it could be turned off by a control circuit as well as turned on by one. This was the "gate turn off" or GTO thyristor. This meant that the thyristor commutating circuit could be eliminated for DC fed power circuits, a saving on several electronic devices for each circuit. Now thyristors could be turned on and off virtually at will and now a single thyristor could be used to control a DC motor. It is at this point that the conventional DC motor reached its ultimate state in the railway traction industry. Most systems now being built use AC motors. AC Motors There are two types of AC motor, synchronous and asynchronous. The synchronous motor has its field coils mounted on the drive shaft and the armature coils in the housing, the inverse of normal practice. The synchronous motor has been used in electric traction the most well-known application being by the French in their TGV Atlantique train. This used a 25 kV AC supply, rectified to DC and then inverted back to AC for supply to the motor. It was designed before the GTO thyristor had been sufficiently developed for railway use and it used simple thyristors. The advantage for the synchronous motor in this application is that the motor produces the reverse voltages needed to turn off the thyristors. It was a good solution is its day but it was quickly overtaken by the second type of AC motor - the asynchronous motor - when GTO thyristors became available. The Asynchronous Motor The asynchronous motor, also called the induction motor, is an AC motor which comprises a rotor and a stator like the DC motor, but the AC motor does not need current to flow through the armature. The current flowing in the field coils forces the rotor to turn. However, it does have to have a three phase supply, i.e. one where AC has three conductors, each conducting at a point one third into the normal cycle period, as visually represented in the diagram on the left. The two big advantages of the 3-phase design are that, one, the motor has no brushes, since there is no electrical connection between the armature and the fields and, two, the armature can be made of steel laminations, instead of the large number of windings required in other motors. These features make it more robust and cheaper to build than a commutator motor. AC Drive Modern electronics has given us the AC drive. It has only become available with modern electronics because the speed of a 3-phase AC motor is determined by the frequency of its supply but, at the same time, the power has to be varied. The frequency used to be difficult to control and that is why, until the advent of modern electronics, AC motors were almost exclusively used in constant speed applications and were therefore unsuitable for railway operation. A modern railway 3-phase traction motor is controlled by feeding in three AC currents which interact to cause the machine to turn. The three phases are most easily provided by an inverter which supplies the three variable voltage, variable frequency (VVVF) motor inputs. The variations of the voltage and frequency are controlled electronically. The AC motor can be used by either an AC or DC traction supply system. In the case of AC supply (diagram left), the line voltage (say 25kV single phase) is fed into a transformer and a secondary winding is taken off for the rectifier which produces a DC output of say

1500 - 2000 volts depending on the application. This is then passed to the inverter which provides the controlled three phases to the traction motors. The connection between the rectifier and the inverter is called the DC link. This usually also supplies an output for the train's auxiliary circuits. All the thyristors are GTOs, including those in the rectifier, since they are now used to provide a more efficient output than is possible with the older thyristors. In addition, all the facilities of DC motor control are available, including dynamic braking, but are provided more efficiently and with less moving parts. Applied to a DC traction supply, the 3-phase set-up is even more simple, since it doesn't need a transformer or a rectifier. The DC line voltage is applied to the inverter, which provides the 3-phase motor control. Control of these systems is complex but it is all carried out by microprocessors. The control of the voltage pulses and the frequency has to be matched with the motor speed. The changes which occur during this process produce a set of characteristic buzzing noises which sound like the "gear changing" of a road vehicle and which can clearly be heard when riding on the motor car of an AC driven EMU. IGBT Having got AC drive using GTO thyristors universally accepted (well, almost) as the modern traction system to have, power electronics engineers have produced a new development. This is the IGBT or Insulated Gate Bipolar Transistor. The transistor was the forerunner of modern electronics, (remember transistor radios?) and it could be turned on or off like a thyristor but it doesn't need the high currents of the thyristor turn off. However it was, until very recently, only capable of handling very small currents measured in thousanths of amps. Now, the modern device, in the form of the IGBT, can handle thousands of amps and it has appeared in traction applications. A lower current version was first used instead of thyristors in auxiliary supply inverters in the early 1990s but a higher rated version has now entered service in the most recent AC traction drives. Its principle benefit is that it can switch a lot faster (three to four times faster) than GTOs. This reduces the current required and therefore the heat generated, giving smaller and lighter units. The faster switching also reduces the complex "gearing" of GTOs and makes for a much smoother and more even sounding acceleration buzz from under the train. With IGBTs, "gear changing" has gone. Permanent Magnet Motor The next development in electric motor design is the permanent magnet motor. This is a 3phase AC synchronous motor with the usual squirrel cage construction replaced by magnets fixed in the rotor. The motor requires a complex control system system but it can be up to 25% smaller than a conventional 3-phase motor for the same power rating. The design also gives lower operating temperatures so that rotor cooling isn't needed and the stator is a sealed unit with integral liquid cooling. By 2011, a number of different types of trains had been equipped with permanent magnet motors, including 25 AGV high speed train sets, trams in France and Prague and EMUs in Euroe and Japan. The reduced size is particularly attractive for low floor vehicles where hub motors can be an effective way of providing traction in a compact bogie. Development of motor design and the associated control systems continues and it is certain that the permanent magnet motor will be seen on more railways in the future. A good description of the motor by Stuart Hillmansen, Felix Schmid and Thomas Schmid is in Railway Gazette International, February 2011. Introduction Originally derived from lift operation over a hundred years ago, multiple unit (MU) control has become the most common form of train control in use around the world today. This page describes how it started and its development in the century to date. Origins Electric locomotives were originally designed so that the motors were controlled directly by the driver. The traction power circuits passed through a large controller mounted in the

driving cab. A handle was rotated by the driver as necessary to change the switches in the circuit to increase or reduce power as required. This arrangement meant that the driver had to remain close to the motors if long and heavy, power-carrying cables were to be avoided. While this arrangement worked well enough, the desire to get rapid turnrounds on city streetcar railways led to the adoption of remote control. The idea was that, if the motors could be remotely controlled, a set of driver's controls could be placed at each end of the train. It would not be necessary to have a locomotive added at the rear of an arriving train to allow it to make the return journey. A cab would be installed at each end of the train and the driver just had to change ends to change direction. Once this idea was established, it was realised that the motors could be placed anywhere along the train, with as many or as few as required to provide the performance desired. With this development, more but smaller motors were scattered along the train instead of building a few large motors in a locomotive. This is how the concept of motor cars and trailer cars evolved. Trailer cars are just passenger carrying vehicles but motor cars are passenger carrying vehicles which have motors and their associated control equipment. Multiple unit trains, as these trains became known, were equipped with control cables called train lines, which connected the driver's controls with the motor controls and power switches on each motor car. The opening and closing of the power switches was achieved by electro-magnetic relays, using principles originally designed for lifts. While the idea was being established on passenger trains, it was also adopted on locomotives. It quickly became the standard method of control. The Relay The diagram below shows how a relay operates. A relay is really a remotely controlled switch. In the diagram left, a power circuit contains a switch which is opened and closed by operation of a relay. The relay is activated by a magnetic core which is energised when a controlling switch is closed. As the core is energised, it lifts and closes a pair of contacts in a second circuit - usually a power circuit. The current required for the relay is usually much lower than that used for the power circuit so it can be provided by a battery. In the diagram, the controlling switch is open, so the relay is de-energised and the power circuit contacts are open. If the controlling switch is closed, as in the right hand diagram, the relay is therefore energised and its core magnet lifts to close the contacts in the power circuit. Applied to a simple lift operating between two levels, a control switch on each landing could use relays to switch on the lift motor to move the lift up or down. On a train, the controlling switch could be located anywhere on the train to activate the relays controlling the power to the motors. The same principles can be used to carry out any other switching e.g. for lights or heating. It represents a safe and simple way of transmitting commands to a number of equipments in a train and it is the foundation upon which multiple unit control was based. On modern rolling stock, the relay is being replaced in many applications by electronic control, which speeds operation, eliminates the mechanical movements required and allows the miniaturisation of control systems. The Contactor As we have seen from the description above, the relay must have a current applied to it all the time it is required to be closed. To avoid having current on to, say, a lighting control relay all the time, a different type of remotely controlled switch is used. This is called a contactor. The contactor is really a latched relay. It can also be called (in the US) a "momentary switch". It only requires current to be on for a "moment" for it to operate. In order to keep the contacts closed once the control current is lost, the power circuit contacts are held in position by a mechanical latch. When it is necessary to open the power circuit, the latch is

released and the contacts drop open. The contactor is operated by two coils, each with their own controlling switch. In this case, the contactor is closed or "set" by pressing the ON button and opened, or "tripped" by pressing the OFF button. Both ON and OFF buttons are sprung so that only a momentary current is used to activate the coil. Contactors are widely used on trains and, for us, are a good example to demonstrate how multiple unit control works in practice. Multiple Unit Control The following diagram illustrates the principal of multiple unit (MU) control as applied to a 3-car train. In the diagram (left), the lighting on each car is switched on and off by a lighting contactor. The contactor is latched closed when its "set" coil is energised or opened by the "trip" coil to unlatch it when required to switch off the lighting. All the lighting contactors on the train are connected to train wires, in this case one for "lights on" (in black) and one for "lights off" (in blue). The ON and OFF buttons are in the driving cabs at each end of the train so, the lighting can be switched on or off from either end of the train. To prevent unauthorised use of the control buttons, most of the important circuits in the cab are isolated by a "control switch" or "cab on switch". This is key operated and keys are only issued to qualified drivers or maintainers. It also means that, in our example, lights can only be switched on or off from one end at a time. The same principle, using contactors or relays, is applied to all other systems on the train - driving controls, braking control, heaters, doors, air conditioning, public address etc. Of course, current for the equipment on each vehicle, as in this case, lighting, comes from a separate source - the auxiliary supplies - in the form of a battery, an alternator, an inverter or a power train line. Forward and Reverse How, one might ask, does one ensure that a number of locomotives or EMUs (or DMUs for that matter), coupled together to work in multiple, perhaps facing in different directions, will all respond to the driver's command to go in the same direction, say forward, from the cab where he is sitting? How do you prevent one locomotive taking off in the wrong direction? Well, it's built in to the wiring and it's simple, as shown in this diagram. Each power unit (whether it be a locomotive or EMU) has a forward wire and a reverse wire connected to a "Forward and Reverse" switch of one sort or another in the cab. Looking at Unit 1, if the driver selects "Forward", the forward wire (in red) is energised and the "forward relay" (the arrow shows the direction of movement obtained for each relay) is energised to make the locomotive move in the forward direction. To ensure the correct direction is achieved by a second locomotive (Unit 2) that is coupled to the first, the forward and reverse wires are crossed over in the jumper cable. If the second locomotive faces in the opposite direction to the first, its reverse wire (shown in black here) will be energised to make the loco run in the same direction as its partner. To make sure this always happens, all multiple unit control jumpers have their forward and reverse wires crossed. But, you might ask, what if the locos both face in the same direction? You don't need the crossed wires in the jumper. The crossed wires in the jumper will make the second loco go the opposite way. No, that's been solved too. So that the same jumper with the crossed wires can be used anywhere, the forward and reverse wires are also crossed ON each locomotive, only at one end, usually near the jumper socket. Now, no matter which way round the locos are coupled to each other, and in what order, the forward command will always make all units drive in the same direction and the reverse command will make all units drive in the opposite direction. One final point. The jumper heads are designed so that they can only be inserted one way

into the coupler socket on the locomotive, rather like a mouse plug on a computer. Modern Control Systems Modern systems use single wires or even fibre optic cables for controls. The system is sometimes referred to as "multiplixing", where a number of control signals are sent along a single wire. Some administrations require hard wired controls for safety systems like train braking but diverse programming can be used to make this redundant. DIRECT CURRENT MOTOR CONTROL Contents DC Motors - DC Resistance Control - Motor Control and Protection - DC Power Circuit - Series-Parallel Control- Regenerative Braking - Rheostatic Braking This page describes the development of electric train power equipment which uses resistance controlled Direct Current (DC) motors. This was the most common form of electric train control for almost 100 years until the advent of power electronics. On other pages you will find Electric Traction Drives, Multiple Unit Operation andElectronic Power Traction described. There is also an Electric Traction Glossary. DC Motors The DC motor was the mainstay of electric traction drives on both electric and dieselelectric for many years. It consists of two parts, a rotating armature and a fixed field. The fixed field consists of tightly wound coils of wire fitted inside the motor case. The armature is another set of coils wound round a central shaft. It is connected to the field through "brushes" which are spring loaded contacts pressing against an extension of the armature called the commutator. The commutator collects all the terminations of the armature coils and distributes them in a circular pattern to allow the correct sequence of current flow. The motor works because, simply put, when a current is passed through the motor circuit, there is a reaction between the current in the field and the current in the armature which causes the armature to turn. The armature and the field are connected in series and the whole motor is referred to as "series wound". A series wound DC motor has a low resistance field and armature circuit. Because of this, when voltage is applied to it, the current is high. (Ohms Law: current = voltage/resistance). The advantage of high current is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting a train. The disadvantage is that the current flowing into the motor has to be limited somehow, otherwise the supply could be overloaded and/or the motor and its cabling could be damaged. At best, the torque would exceed the adhesion and the driving wheels would slip. Traditionally, resistors were used to limit the initial current. DC Resistance Control As the DC motor starts to turn, the interaction of the magnetic fields inside it causes it to generate a voltage internally. This "back voltage" opposes the applied voltage and the current that flows is governed by the difference between the two. So, as the motor speeds up, the internally generated voltage rises, the effective voltage falls, less current is forced through the motor and thus the torque falls. The motor naturally stops accelerating when the drag of the train matches the torque produced by the motors. To continue accelerating the train, resistors are switched out in steps, each step increasing the effective voltage and thus the current and torque for a little bit longer until the motor catches up. This can be heard and felt in older DC trains as a series of clunks under the floor, each accompanied by a jerk of acceleration as the torque suddenly increases in response to the new surge of current. When no resistor is left in the circuit, the full line voltage is applied directly to the motor. The train's speed remains constant at the point where the torque of the motor, governed by the effective voltage, equals the drag - sometimes referred to as balancing speed. If the train starts to climb a grade, the speed reduces because drag is greater than torque. But the reduction in speed causes the back voltage to decline and thus the

effective voltage rises - until the current forced through the motor produces enough torque to match the new drag. On an electric train, the driver originally had to control the cutting out of resistance manually but, by the beginning of the First World War in 1914, automatic acceleration was being used in the UK on multiple-unit trains. This was achieved by an accelerating relay (often called a "notching relay") in the motor circuit (see next diagram below) which monitored the fall of current as each step of resistance was cut out. All the driver had to do was select low, medium or full speed (called "shunt", "series" and "parallel" from the way the motors were connected in the resistance circuit) and the equipment would do the rest. Motor Control and Protection As we have seen, DC motors are controlled by a "notching relay" set into the power circuit. But there are other relays provided for motor protection. Sharp spikes of current will quickly damage a DC motor so protective equipment is provided in the form of an "overload relay", which detects excessive current in the circuit and, when it occurs, switches off the power to avoid damage to the motors. Power is switched off by means of Line Breakers, one or two heavy-duty switches similar to circuit breakers which are remotely controlled. They would normally be opened or closed by the action of the driver's controller but they can also be opened automatically by the action of the overload relay. On a historical note, early equipment (pre-1905) had a huge fuse instead of an overload relay. Some of these lasted into the 1970s and I recall the complications of changing one, which involved inserting a wooden board (called a "paddle") between the shoes and the current rail. This was to isolate the current from the locomotive while you changed the fuse. A further protective device is also provided in the classic DC motor control circuit. This is the "no-volt" relay, which detects power lost for any reason and makes sure that the control sequence is returned to the starting point (i.e. all the resistances are restored to the power circuit) before power could be re-applied. This is necessary to ensure that too much current is not applied to a motor which lost speed while current was off. DC Power Circuit This diagram shows a simple traction motor power control circuit. Most DC motor circuits are arranged to control two or four motors. The control range is enhanced by changing the connections to the motors as the train accelerates. The system is known as "seriesparallel control". Series-Parallel Control This diagram shows the principle of series-parallel control. There are three stages, "series", "transition" and "parallel", which operate in that order. The connections are changed automatically as the train accelerates. Upon starting, the motors are in series with each other and with all the resistance. The resistance is cut out in steps and the train accelerates to "full series" when all the resistance is out of circuit. The train may be running at about 30 km/h now. Field Weakening The DC motor can be made to run faster than the basic "balancing speed" achieved whilst in the full parallel configuration without any resistance in circuit. This is done by "field shunting". An additional circuit is provided in the motor field to weaken the current flowing through the field. The weakening is achieved by placing a resistance in parallel with the field. This has the effect of forcing the armature to speed up to restore the balance between its magnetic filed and that being produced in the field coils. It makes the train go faster. Various stages of field weakening can be employed, according to the design of the motor and the intended purpose. Some locomotives used as many as six steps of field weakening.

Regenerative Braking Since the DC motor and a DC generator are virtually the same machine mechanically, it was immediately realised that a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy. The idea formed that if the power could be returned to the source, other trains could use it. Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains. Various schemes were tried over many years with more or less success but it was not until the adoption of modern electronics that reliable schemes have been available. Rheostatic Braking The major drawback with the regenerative braking system is that the line is not always able to accept the regenerated current. Some railways had substations fitted with giant resistors to absorb regenerated current not used by trains but this was a complex and not always reliable solution. As each train already had resistors, it was a logical step to use these to dispose of the generated current. The result was rheostatic braking. When the driver calls for brake, the power circuit connections to the motors are changed from their power configuration to a brake configuration and the resistors inserted into the motor circuit. As the motor generated energy is dispersed in the resistors and the train speed slows, the resistors are switched out in steps, just as they are during acceleration. Rheostatic braking on a DC motored train can be continued down to less than 20 mph when the friction brakes are used to bring the train to a stop. Before the advent of power electronics, there were some attempts to combine the two forms of what we now call "dynamic braking" so that the generated current would go to the power supply overhead line or third rail if it could be absorbed by other trains but diverted to on-board resistors if not. In the case of diesel-electric locomotives, dynamic braking is restricted to the rheostatic type. Racks of resistors can often be seen on the roofs of heavy-haul locomotives for which dynamic braking is a big advantage on long downhill grades where speed must be maintained at a restricted level for long periods. ELECTRIC TRACTION POWER SUPPLIES There is a wide variety of electric traction systems around the world, which have been built according to the type of railway, its location and the technology available at the time of the installation. Many installations seen today were first built up to 100 years ago, some when electric traction was barely out its diapers, so to speak, and this has had a great influence on what is seen today. In the last 20 years there has been a gigantic acceleration in railway traction development. This has run in parallel with the development of power electronics and microprocessors. What have been the accepted norms for the industry for, sometimes, 80 years, have suddenly been thrown out and replaced by fundamental changes in design, manufacture and operation. Many of these developments are highly technical and complex, the details of which are therefore beyond the scope of these texts. Because these changes have been so rapid, there are still plenty of examples of the original technology around and in regular use, so I have covered these in my articles. This is useful, since it helps the reader to get to grips with the modern stuff. Power Supply To begin with, the electric railway needs a power supply that the trains can access at all times. It must be safe, economical and user friendly. It can use either DC (direct current) or AC (alternating current), the former being, for many years, simpler for railway traction purposes, the latter being better over long distances and cheaper to install but, until recently, more complicated to control at train level. Transmission of power is always along the track by means of an overhead wire or at ground level, using an extra, third rail laid close to the running rails. AC systems always

use overhead wires, DC can use either an overhead wire or a third rail; both are common. Both overhead systems require at least one collector attached to the train so it can always be in contact with the power. Overhead current collectors use a "pantograph", so called because that was the shape of most of them until about 30 years ago. The return circuit is via the running rails back to the substation. The running rails are at earth potential and are connected to the substation. Third Rail This diagram shows a DC 3-Rail Traction System with the location of the current rail in relation to the running rails. The third rail system uses a "shoe" to collect the current on the train, perhaps because it was first called a "slipper" by the pioneers of the industry (it slipped along the rail, OK?) but it was not very pretty to look at, so perhaps someone thought shoe was a better description. Whatever the origin, shoe has stuck to this day. Shoes and Shoegear The diagram above shows a top contact third rail system but there are other types as shown in this diagram. Third rail current collection comes in a variety of designs. The simplest is what is called "top contact" because that’s the part of the rail upon which the pick-up shoe slides. Being the simplest, it has drawbacks, not the least of which is that it is exposed to anyone or any thing which might come into contact with it. It also suffers during bad weather, the smallest amount of ice or snow rendering top contact third rail systems almost unworkable unless expensive remedies are carried out. Side contact is not much better but at least it is less exposed. Bottom contact is best - you can cover effectively most of the rail and it is protected from the worst of the cold weather. This DC 3rd rail Top Contact Collector Shoe (London Underground - Central Line) has remote lifting facilities. All shoes need some way of being moved clear of the current rail, usually for emergency purposes. The most common reason is when a shoe breaks off and its connecting lead to the electrical equipment on the train has to be secured safely. The other shoes on the same circuit must be isolated while this is done, unless the current is switched off from the whole section - perhaps disabling several other trains. Isolation used to involve inserting a wooden "paddle" between the shoe and the current rail and then tying the shoe up with a strap or rope. More recently, mechanical or pneumatic systems have been devised to make it possible to lift shoes from inside the train remotely from the driving cab. Most types of top contact shoes simply hang from a beam suspended between the axleboxes of the bogie. The suspension method was originally just a couple of slotted links to compensate for movement which allowed gravity to provide the necessary pressure. Later systems had radially mounted shoes to provide more stable contact through lever action. Top contact systems with protective covers over them, like the New York Subway (photo left), needed radially mounted shoes anyway to allow them to fit under the cover. Side and bottom contact shoes are spring loaded to provide the necessary contact force. An example of a bottom contact shoe as used on a German metro line is shown in the photo (left). Some top contact systems have also used spring loading but they are mechanically more difficult to control because of the hunting action of the bogie and the risk that the shoes will get trapped under the head of the rail and turn it over. Gaps You will often see trains with only one pantograph but, on trains which use shoes, there are always several shoes. The contact with the overhead wire is not normally broken but the third rail must be broken at junctions to allow for the continuity of running rails. These third rail breaks, or "gaps", as they are called, can lead to loss of power on the train. The

power losses can be reduced by locating shoes along the train and connecting them together by a cable known as a busline. In spite of this, there can be problems. Woe betide the driver who stops his train with all the shoes "off juice" or "gapped". Yes, it happens more often than you think and yes, before you ask, it's happened to me. It is an embarrassing nuisance only solved by being pushed onto the third rail by another train or by obtaining special long leads with a plug at one end for the train and shoes at the other end for the third rail. Of course, it does cause a long delay. Current rail gaps are also provided where the substations feed the line (diagram, left). Normally, each track is fed in each direction towards the next substation. This allows for some over supply and provides for continuity if one substation fails. These substation gaps are usually marked by a sign or a light which indicates if the current is on in the section ahead. A train must stop before entering the dead section. Since the current may have been switched off to stop an arc or because of a short circuit, it is important that the train does not connect the dead section to the live section by passing over the gap and allowing its busline to bridge the gap. Some of the more sophisticated systems in use today now link the traction current status to the signalling so that a train will not be allowed to proceed onto a dead section. At various points along the line, there will be places where trains can be temporarily isolated electrically from the supply system. At such places, like terminal stations, "section switches" are provided. When opened, they prevent part of the line for being fed by the substation. They are used when it is necessary to isolate a train with an electrical fault in its current collection system. 3rd Rail Uses Although 3rd rail is considered a suburban or metro railway system, 750 volt DC third rail supply has been used extensively over southern England and trains using it run regularly up to 145 km/h. This is about its limit for speed and has only spread over such a large area for historical reasons. Return What about the electrical return? There has to be a complete circuit, from the source of the energy out to the consuming item (light bulb, cooking stove or train) and back to the source, so a return conductor is needed for our railway. Simple – use the steel rails the wheels run on. Provided precautions are taken to prevent the voltage getting too high above the zero of the ground, it works very well and has done so for the last century. Of course, as many railways use the running rails for signalling circuits as well, special precautions have to be taken to protect them from interference. The power circuit on the train is completed by connecting the return to brushes rubbing on the axle ends. The wheels, being steel, take it to the running rails. These are wired into the substation supplying the power and that does the job. The same technique is used for DC or AC overhead line supplies. AC or DC traction It doesn’t really matter whether you have AC or DC motors, nowadays either can work with an AC or DC supply. You just need to put the right sort of control system between the supply and the motor and it will work. However, the choice of AC or DC power transmission system along the line is important. Generally, it’s a question of what sort of railway you have. It can be summarised simply as AC for long distance and DC for short distance. Of course there are exceptions and we will see some of them later. It is easier to boost the voltage of AC than that of DC, so it is easier to send more power over transmission lines with AC. This is why national electrical supplies are distributed at up to 765,000 volts AC . As AC is easier to transmit over long distances, it is an ideal medium for electric railways. Only the problems of converting it on the train to run DC motors restricted its widespread adoption until the 1960s. DC, on the other hand was the preferred option for shorter lines, urban systems and

tramways. However, it was also used on a number of main line railway systems, and still is in some parts of continental Europe, for example. Apart from only requiring a simple control system for the motors, the smaller size of urban operations meant that trains were usually lighter and needed less power. Of course, it needed a heavier transmission medium, a third rail or a thick wire, to carry the power and it lost a fair amount of voltage as the distance between supply connections increased. This was overcome by placing substations at close intervals – every three or four kilometres at first, nowadays two or three on a 750 volt system – compared with every 20 kilometres or so for a 25 kV AC line. It should be mentioned at this point that corrosion is always a factor to be considered in electric supply systems, particularly DC systems. The tendency of return currents to wander away from the running rails into the ground can set up electrolysis with water pipes and similar metallics. This was well understood in the late 19th Century and was one of the reasons why London’s Underground railways adopted a fully insulated DC system with a separate negative return rail as well as a positive rail - the four-rail system. Nevertheless, some embarrassing incidents in Asia with disintegrating manhole covers near a metro line as recently as the early 1980s means that the problem still exists and isn’t always properly understood. Careful preparation of earthing protection in structures and tunnels is an essential part of the railway design process and is neglected at one’s peril. Overhead Line (Catenary) The mechanics of power supply wiring is not as simple as it looks (diagram, left). Hanging a wire over the track, providing it with current and running trains under it is not that easy if it is to do the job properly and last long enough to justify the expense of installing it. The wire must be able to carry the current (several thousand amps), remain in line with the route, withstand wind (in Hong Kong typhoon winds can reach 200 km/h), extreme cold and heat and other hostile weather conditions. Overhead catenary systems, called "catenary" from the curve formed by the supporting cable, have a complex geometry, nowadays usually designed by computer. The contact wire has to be held in tension horizontally and pulled laterally to negotiate curves in the track. The contact wire tension will be in the region of 2 tonnes. The wire length is usually between 1000 and 1500 metres long, depending on the temperature ranges. The wire is zigzagged relative to the centre line of the track to even the wear on the train's pantograph as it runs underneath. The contact wire is grooved to allow a clip to be fixed on the top side. The clip is used to attach the dropper wire. The tension of the wire is maintained by weights suspended at each end of its length. Each length is overlapped by its neighbour to ensure a smooth passage for the "pan". Incorrect tension, combined with the wrong speed of a train, will cause the pantograph head to start bouncing. An electric arc occurs with each bounce and a pan and wire will soon both become worn through under such conditions. More than one pantograph on a train can cause a similar problem when the leading pantograph head sets up a wave in the wire and the rear head can’t stay in contact. High speeds worsen the problem. The French TGV (High Speed Train) formation has a power car at each end of the train but only runs with one pantograph raised under the high speed 25 kV AC lines. The rear car is supplied through a 25 kV cable running the length of the train. This would be prohibited in Britain due to the inflexible safety approach there. A waving wire will cause another problem. It can cause the dropper wires, from which the contact wire is hung, to "kink" and form little loops. The contact wire then becomes too high and aggravates the poor contact. Overhead lines are normally fed in sections like 3rd rail systems, but AC overhead sections are usually much longer. Each subsection is isolated from its neighbour by a section insulator in the overhead contact as shown in this picture below. The subsections can be joined through special high speed section switches. To reduce the arcing at a neutral section in the overhead catenary, some systems use

track magnets to automatically switch off the power on the train on the approach to the neutral section. A second set of magnets restores the power immediately after the neutral section has been passed. The next photo shows a set of track magnets. Track Magnets. Catenary Suspension Systems Various forms of catenary suspension are used (see diagram below), depending on the system, its age, its location and the speed of trains using it. Broadly speaking, the higher speeds, the more complex the "stitching", although a simple catenary will usually suffice if the support posts are close enough together on a high speed route. Modern installations often use the simple catenary, slightly sagged to provide a good contact. It has been found to perform well at speeds up to 125 m/hr (200 km/hr). At the other end of the scale, a tram depot may have just a single wire hung directly from insulated supports. As a pantograph passes along it, the wire can be seen to rise and fall. This is all that is necessary in a slow speed depot environment. I haven’t yet mentioned trolley poles as a method of current collection. These were used for current collection on low speed overhead systems and were common on trams or streetcars but they are now obsolete. DC overhead wires are usually thicker and, in extreme load cases, double wires are used, as in Hong Kong Mass Transit’s 1500 v DC supply system. Up to 3000 volts overhead is used by DC main line systems (e.g. parts of France, Belgium and Italy) but below 1500 volts, a third rail can be used. In operating terms, the third rail is awkward because of the greater risk of it being touched at ground level. It also means that, if trains are stopped and have to be evacuated, the current has to be turned off before passengers can be allowed to wander the track. Third rail routes need special protection to be completely safe. On the other hand, some people consider the overhead catenary system a visual intrusion. Singapore, for example, has banned its use outside of tunnels. Booster Transformers On lines equipped with AC overhead wires, special precautions are taken to reduce interference in communications cables. If a communications cable is laid alongside rails carrying the return current of the overhead line supply, it can have unequal voltages induced in it. Over long distances the unequal voltages can represent a safety hazard. To overcome this problem, booster transformers are provided. These are positioned on masts at intervals along the route. They are connected to the feeder station by a return conductor cable hung from the masts so that it is roughly the same distance from the track as the overhead line. The return conductor is connected to the running rail at intervals to parallel the return cable and rails. The effect of this arrangement is to reduce the noise levels in the communications cable and ensure the voltages remain at a safe level. Pantographs Current is collected from overhead lines by pantographs. Pantographs are easy in terms of isolation - you just lower the pan to lose the power supply to the vehicle. However, they do provide some complications in other ways. Since the pantograph is usually the single point power contact for the locomotive or power car, it must maintain good contact under all running conditions. The higher the speed, the more difficult the maintenance of good contact. We have already mentioned the problem (above) of a wave being formed in the wire by a pantograph moving at high speed. Pantograph contact is maintained either by spring or air pressure. Compressed air pressure is preferred for high speed operation. The pantograph is connected to a piston in a cylinder and air pressure in the cylinder maintains the pantograph in the raised condition. Originally, pantographs were just that, a diamond-shaped "pantograph" with the contact head at the top. Two contact faces are normally provided. More modern systems use a

single arm pantograph - really just half of the original shape - a neater looking design (photo above). The contact strips of the pantograph are supported by a lightweight transverse frame which has "horns" at each end. These are turned downwards to reduce the risk of the pantograph being hooked over the top of the contact wire as the train moves along. This is one of the most common causes of wires "being down". A train moving at speed with its pantograph hooked over the wire can bring down several kilometres of line before it is detected and the train stopped. The most sophisticated pantographs have horns which are designed to break off when struck hard, for example, by a dropper or catenary support arm. These special horns have a small air pressure tube attached which, if the pressure is lost, will cause the pan to lower automatically and so reduce the possible wire damage. Dual Voltage Some train services operate over lines using more than one type of current. In cities such as London, New York City and Boston, the same trains run under overhead wires for part of the journey and use third rail for the remainder. In Europe, some locomotives are equipped to operate under four voltages - 25 kV AC, 15kV AC, 3,000 V DC and 1,500 V DC. Modern electronics makes this possible with relative ease and cross voltage travel is now possible without changing locomotives. Replace DC traction with AC traction motors in locomotives AC traction systems can replace conventional DC traction motors to provide improved levels of wheel to rail traction. This can enable less powerful locomotives, or a smaller number of locomotives, to perform the same task.1 SCT Logistics used AC Traction to achieve 30% more loading, and was able to replace four DC traction locomotives for two or three locomotives.2 Other benefits of AC traction motors include reduced maintenance requirements due to the smaller number of locomotives performing the same tasks and quicker servicing turnaround times. For more information see AC traction. AC traction AC traction systems replace conventional DC traction motors in a locomotive. They provide higher levels of wheel to rail adhesion and enable less powerful locomotives, or a smaller number of locomotives, to be used for a specific task. Application relevance Australian operators have demonstrated benefits on certain routes that were originally operated by conventional DC locomotives. The available benefits will be dependent on grade severity and other specific factors. Case studies indicate that technology adoption is likely to require a long procurement process. Part of the reason for this is that specific design adaptations for Australia must account for a smaller kinematic envelope compared to some overseas applications and requirements. Potential benefits Although AC traction can reduce fuel consumption, the potential benefits may depend on the mixture of performance of other locomotives on the same track (i.e. may be reduced to slowest track running time). Other benefits for the operator include reduced maintenance requirements due to the fewer number of locomotives performing the same tasks, and quicker servicing turnaround times. Supporting this argument, a Halcrow study (on behalf of SCT Logistics) indicated that 20% of maintenance issues related solely to the specific technology associated with DC traction.

Key implementation considerations Available US data suggests that the premium over a DC locomotive has reduced from 40% when AC traction was first introduced in the mid-1990s, to about 10% in recent years The primary drawback of AC traction systems is the cost. However, other considerations that have required resolution include size, weight and noise. Examples of implementation Introducing the next generation locomotive to the Australian rail network This detailed case study discusses Australia’s first AC traction locomotive certified to operate across the Interstate Rail Network (specifically Melbourne to Perth). Detailed planning and modelling is illustrated using cost benefit analysis to explain the decision for choosing AC traction. SCT Logistics achieved up to 30% more loading with AC traction and this enabled two or three locomotives to replace four DC traction locomotives (Ramsey et al 2008). Advantages and benefits of modern AC traction technology This Queensland Rail case study describes the advantages of AC Traction Technology, notably the ability to substitute five DC locomotives with three AC locomotives (Latour et al. 2008). Eternal War between Alternating and Direct Current continuing for Rail Transport Mahesh Kumar Jain February 6, 2014 AC versus DC is a subject of debate over the century for different applications of power. DC arrived first at the scene of illuminating the house in 1850’s with many patents to the credit of Thomas Edition. The use of AC for power supply distribution taken up by entrepreneur cum engineer George Westinghouse in 1880s and the war started between General Electric and Westinghouse because it was the business empire of DC equipment manufacturers and many patents losing its sheen. Source: http://en.wikipedia.org/wiki/War_of_Currents Finally, it was AC which won the war because of technical superiority and dominating the scene of the power supply distribution till today. Debate of AC versus DC continues for Rail Transport The DC series motor characteristics is the most suitable for traction purpose, and therefore, DC dominated early traction schemes and DC traction motorcontinued to dominate in Rail transportation. With the development of on-board Ignitron, Excitron tube and later solid state rectifier, AC arrived at the scene as the feeding voltage. Transfer of power at 25kV single phase to locomotive and then converting it into DC for traction motor was demonstrated by SNCF and Indian Railways also decided to follow 25 kV AC power supply standard in its Railway Electrification policy. But it also passed through a debate on technology benefits among Railway Electrical Engineers as narrated by Ex Member Electrical in the posts Finally, Indian Railway decided to go for 25kV AC with the intervention of the then Minister of Railways Jagjivan Ram. Before deciding for conversion of DC system to AC system on Mumbai Sub-urban network, similar war broke out with few advocating against the investment for the conversion. The most important factors which have gone in favour of deciding for 25kV AC traction in Mumbai Suburban network were Uni-gauze network without the need for a change of locomotive thus saving on running time The current level at 1500V DC is very high and difficult to enhance passenger transportation capacity. The current level reduces by 1/20 thtimes at 25kV making it easier to enhance capacities. Energy saving due to less traction distribution losses. There were no debatable issues while deciding in favour of 3 phase traction motor in lieu of DC series Traction Motor and it went through smoothly

Discussion of Traction voltage for Metro System This subject has again opened up debate while deciding for the suitable supply voltage for Metro Transport System. National Manufacturing Competitiveness Council (NMCC) organized a meeting on May 03, 2012 and one of the Agenda item was “Promotion of Manufacturing for Metro System in India as well as formation of Standards for the same”. In view of this, Ministry of Urban Development has taken initiative to form a committee for “Standardization and Indigenization of Metro Rail Systems” in May 2012. Subcommittees were formed to standardize system in Traction system, rolling Stock, Signalling System, Fare Collection System, Operation and Maintenance, Track Structure and Simulation Tools. The committee on Traction System is headed by Shri Satish Kumar Ex..Director/Elect/DMRC Convener, with experts from DMRC, MOUD, RDSO, CMRL, BMRCL, Industries etc. Terms of Reference for Traction Supply Sub-committee were 1.Study of traction systems adopted by various metros around the world, including year of commissioning of these metros 2.Study current trends, i.e. traction system being adopted by newly built metros (say last five years) and metros being built 3.Establishing a relation between type of traction system and maximum PHPDT that can be catered to by 750V DC/1500V DC third rail and 25kV AC OCS 4.Analysis of Capital cost of various types of traction systems for different levels of traffic for a sample corridor. Such analysis shall include Direct cost of traction power system The direct cost of rolling stock Weight reduction of rolling stock and consequent energy savings and impact on operating cost The cost impact of regenerative braking e.g. DC system may rewire additional investment in inverters for utilizing the regenerative energy Civil infrastructure cost (e.g. Cost impact of increased tunnel diameter) Study of Capital cost of electrification of the DMRC, BMRCL, CMRL, KMRC, Indian Railways, Kolkata Metro etc. and compare operating costs on both types of traction An analysis of energy savings on account of regenerative braking in DMRC, BMRCL, Mumbai Suburban and other relevant systems Thorough analysis of regenerated energy during braking in Mumbai Sub-urban with 1500 DC and 25kV AC system Identifying constraints in process of indigenous development and evolving strategy for placing development orders for assemblies/ systems/subsystems Prepare report covering above, including cost benefit analysis and recommendations of traction system. Salient features of the Report Kolkata, Kochi and Bangalore Metro adopted 750 V DC, whereas DMRC, Chennai, Jaipur and Hyderabad adopted 25 KV AC. With two traction system existing in India, debate DC versus AC for traction is likely to emerge with each one proving his point. The most significant result of the study is finally the acceptance of the suitability of any of the two depending from case to case. The issues debated thereon are: Advantage 25 KV AC 1.Higher voltage has the benefit of handling lower current. Lower current means 1.Reduce losses 2.Improved protection system 3.Increased inter-Substation distance – an important factor in urban transport due to high land cost. 4.Improved current collection at pantograph level

5.Higher voltage helps in higher DC link and TM voltage level. This benefits achieving higher HP/Weight ratio. 2.With the completion of Phase 3 and Phase 4 network of DMRC project, the traffic density of the existing network will swell, requiring running of the trains at an interval of 90 seconds. With such the traffic density, it has been simulated that 2x25kV system will be more suitable mainly due to 1.Energy efficiency 2.Avoiding installing additional sub-station 3.Mitigating EMC/EMI problem and avoiding booster transformer 3.The 2.25 KV system has been adopted by Seoul Metro on their Sin Bundang Line. Advantage DC Traction System Advantage AC Traction System Advantage 750/1500 V DC 1.Avoiding carrying dead weight of the transformer and front end converter all along resulting 1.Reduced passenger transportation capacity due to dead weight of transformer and front end converter 2.Increased cost of coach due to addition of transformer and front end converter 3.Increased energy cost due to transport of unnecessary dead weight 2.Higher reliability of Third Rail Current Collection due to less number of component and less time to attend breakdown. 3.Lower capital cost of traction system as compared to 25kV AC 4.Higher tunnel bore diameter, thus requiring higher capital cost 5.With no OHE mast, better aesthetic of skyline Discussion on disadvantages of 25kV AC Traction DMRC has gone for 25kV AC Traction in 2002 after a lot of debate and studies by RITES and approval by Railway Board. Let us discuss the disadvantages and how it accounts at Macro level and actual field conditions. 1.Cost of 25kV AC Motor coach is higher than 750V DC Motor Coach Voltage

750 V DC

25kV AC

Name of Metro

Cost Scenario I Cost Scenario II

Remarks

BMRCL

9.99

9.41

2.88m wide, 1.0 m/s2, 67% motored axles

KMRCL

9.29

8.05

-do- + flexible PVC clauses for 66% components without any clamping

CMRL

8.74

8.28

2.9m wide, 0.82 m/s2 , 50% motored axles, flexible PVC clauses for 66% components without any clamping

DMRC RS2

9.26

8.73

3.2m wide, 0.82 m/s2 , 50% motored axles

DMRC RS3

10.09

9.97

2.9m wide, 0.82 m/s2 ,

50% motored axles RS10

8.58

7.91

3.2m wide, 1.0 m/s2, 67% motored axle and more energy efficient

Scenario I is with taxes and including export benefits and scenario includes cost of spares but without tax benefits. From the above it is clear that the cost of 25kV AC rolling stock is now comparable with 750V DC rolling stock. It is in view of the large many factors accounting into costing such as Number of coaches involved in the procurement Specification i.e. acceleration, de-acceleration, scheduled speed, DC Link voltage Commercial Conditions like defect liability period, indigenisation clauses, price variation, delivery period, time frame for completion, ambient conditions etc. Risk factor as perceived by the contractor 1.Capital Cost of the 25kV AC system when compared with 750V DC as per actual tendering process found to be favouring 25kV OHE system by as much as 15-35% under different scenario. This is mainly due to higher cost of Third rail and limited competition. 2.750 V DC third rail system is certainly reliable in view of less number of components, but over the period the reliability issues of the 25kV OHE system have been suitability addressed through redundancies, reliability measures etc. 3.Regeneration is the most important feature of urban transport for its sustainability. The actual records of DMRC reveal an energy saving of 25% in 25kV AC traction system operating with acceleration of 0.82 m/s2 and maximum speed of 75 Kmph. Simulation studies indicate that the energy saving in 25kV AC system may go up to 35% with the use of higher acceleration of 1 m/s2 using 4M+2T rake. 4.A 25kV AC system for underground system will need higher tunnel bore diameter up to 5.8m but in actual practice it is observed that M&P facilities required from 5.2 to 5.8m is similar and only makes a slight difference in the cost of tunnel boring for the 25kV traction system. This cost is also offset by reducing in cost due to lesser number of substation and other associated benefits of larger tunnel diameter. In fact, the optimum tunnel diameter is worked out not only on Traction system, but also on many other factors such as a) Dimensions of the coach b) Number of coaches in trains and length of train c) Minimum curvatures d) Type of evacuati0n (side or front) e) Traction Voltage f) OCS or Third Rail g) Soil Temperature 5.750V/1500 V DC third rail gives better aesthetics in urban city as compared to 25kV Traction system and while deciding 750 V DC to Bangalore, this was one of the major factors. Bangalore is blessed with large size tree right in the heart of the city and certainly 25kV system will call for cutting or trimming a few ones. Conclusion The Major advantage to AC system is ease of transformation to different voltage level and has always won the debate in its favor. Energy saving due to low-level of current is the long-term benefit and has always weighed in favor of AC traction. With technological advancements every day, it cannot be said that it will be AC only in the future. DC will find wide range application if voltage transformation could be made possible with efficiency and cost advantage in future. There is already a thought to provide DC grid supply at Information Technology Centre to meet the power supply need of Hardware thus saving energy to the tune of 7-8% towards UPS.

What is special of the report The committee has given enough of insight material for the new metros to decide Traction system based on merits and demerits enumerated in the report. Every advantage and dis-advantage of a system has been discussed and debated with factual data, and that makes the report a wonderful worth reading document for every electrical engineer and traction planner. AC vs. DC traction Posted by Anonymous on Wednesday, September 21, 2005 12:14 PM Can someone explain to me what are the pros and cons of each of the traction systems, and for what kind of jobs would it be preferable to use one over the other? In general terms: DC drive Good : Cheaper (currently), simpler, well understood, generates less interference with signalling etc Bad : Not as good performance as AC for really heavy-haul freight or for high-speed passenger use. Motors have brushgear which needs maintenance and is a weak point if abused. AC drive Good : Better wheelslip control than DC so ideal for really heavy-haul freight (also better dynamic braking), and AC traction motors can withstand abuse that would wreck DC motors. AC motors are lighter and smaller than DC so it's possible to get more power in the same space (which is probably why the GE/EMD 6000hp units are AC drive only) - this is also important for for high-speed passenger trains. Bad : More expensive than DC (but getting cheaper) due to complex electronics, can create interference problems with signalling etc. The railway world in general is going AC drive, so as soon as the cost of AC drives matches that of DC the DC drive will become obsolete for new rolling stock (my prediction [:)]). We've pretty much reached that stage in Europe, certainly for electric locomotives/multiple units. Tony Good answer. I'll just add: AC: better wheelslip = higher adhesion = more tractive effort, which is the stuff you need to haul heavy trains such as unit trains The reason you don't see 6000 HP DC locomotives is that about the most the frt RRs ever need from a locomotive with 100,000 to 120,000# TE is 4000 - 4500 HP. More than this and you're just wasting fuel. A six axle AC and do 150,000 to 175,000# TE which is a good match for 5000 - 6000 HP. I read an account in TRAINS some time ago that said that an AC was in run 8 at about a half mile per hour, pulling a sizeable train up a grade. Can't do that with DC. (Would have been interesting to see the result when they broke a drawbar, which I believe the story said they did) Good explanations Yea, thank you all, good readings. So then, if you had a bunch of AC's and bunch of DC's in your fleet, how would you assign jobs to them? I mean it seems to me like AC's would get all the job, cause they are good for both high speed trains and for slow heavy crawls. Godd Answers! Question: Can AC & DC be MU'd? Yes they can but when they are mue'd use lose they low speed advantage of the ac units. Always at war with those that think OTR trucking is EASY. I have seen the BN burn up alot of DC motors on pushers at Crawford Hill.

Allan. The AC's go to coal and grain where slow speed heavy haul is important. The DC's go to manifest, intermodal and local service. Dave H. QUOTE: I read an account in TRAINS some time ago that said that an AC was in run 8 at about a half mile per hour, pulling a sizeable train up a grade. Can't do that with DC. It was that story in Trains that promted my "AC traction motors can withstand abuse that would wreck DC motors" comment above - one of those 'wish I could have been there to see the show' stories [:)] Incidentally, does anyone know what the AC/DC price differential is for the latest generation EMD/GE diesels ? Tony I've been told the DC's are about half a million dollars cheaper but can't verify. Any takers? A half a million sounds right. I suppose the next question is how much does an SD70ACe etc cost anyway ? (I'd guess 2-3 million USD but I could be wildly out [:)]) Tony afair in large orders a ACe loco is about 1.2-1.5 million a piece. The reason for the higher traction ratings for AC motors is that they do not seem to have an amperage limit rating. Amperage (current flow rate) heats up circuit wires. At a given horsepower setting in a diesel or electric locomotive, the slower the engine moves (when held back by a load) the higher the current flow (amperage) experienced by the traction motors. The tractive effort rating for a locomotive is based on the maximum current that the dc motors will tolerate on a continuous basis. Electric Locomotives can accelerate faster than diesels because they can temporarily overload their motors to twice their steady state rating for short periods of time. AC motors, having no amperage limit, are rated by the absolute maximum tractive effort they can generate, even with slightly slipping wheels. Comparing a 6000 hp AC engine to a 4400 hp AC engine, the 6000 hp engine will be able to pull a given train at a higher speed only if you are traveling above at 28 mph. Below that speed, assuming both engines weigh the same, adhesion limits prevent the stronger engine from generating more tractive effort that the 4400 hp locomotive. Consequently, both engines have the same start-up train weight limits (i.e. if the 4400 hp engine can start a 44 car train, the 6000 hp will not be able to handle a longer train). I could see where DC locomotives are well worth having. Given the data here, a DC loco goes for between 60 percent and two-thirds the cost of the comparable AC model. But I do have some dumb questions (I'll start a new thread if you like but these are still in topic): 1. I know it may be impossible to compare since AC and DC's have comparative advantages, but "all things being equal" which gets the better fuel economy? 2. Who assigns locomotives, the number, electrical mode and tractive power? The yardmaster? Regardless of whom, are there computer formulas that list the most efficient use with variables set for speed, length of train and tare weight? If computer designs aren't used or aren't always the most helpful, can somone like a yardmaster chose AC or DC, etc., from his experience and seat-of-the-pants reckoning? 3. Finally, I notice that CP's 2004 Fact Book is proud of the fact that more than half of its locomotives are now AC and gave the distinct impression that DC was going to be attritioned out. Of course, a lot of CP's traffic is in heavy commodities, but proportionately the road hauls more intermodal than CN. SOOooo, would it be a good idea to keep DC's since that power mode has been specifically mentioned here as good for fast freight and intermodals? 4. A lot has been written about burning out DC's. Do DC locomotives have a shorter useful life than their AC brothers?

5. Does CP's inclination toward AC echo that of the other large carriers? My, he asks a lot of questions. [8D] Thanks, Al Smalling 1> if all things are equal then so would the mileage?? 2> We had a power desk, and the guy there knew which locos were in the redy track, which way they were facing, which were mued to what, etc. He also had a bit of paper which gave the loco ratings over each subdivision depending on the ruling grade. When a train was to be run he'd look at the total tons and and assign enough tractive effort to pull that train. This was before AC traction and I suppose it's all computer based now though. 3>No, DC traction is good at this stuff,, but I'd say AC is better. 4> Not necessarily,, some parts have a shorter service lfe, but this stuff can (and is) replaced when it goes wrong beyond economic repair. Lots of 30 yar old DC locos about still,, in 30 years ther'll be lots of AC locos about still. 5> Yes,, CP has always been a forward lookin railway, and this basically means that they are less risk averse than the competition. It could have gone all wrong for them (can you say Edsel). As the price of the locos come down and the reliability improves as the technology matures more railroads will move to AC traction. In a few years DC locos will go the way of the steam engine. Nowt wrong with asking questions. Generally a lurker by nature Generally speaking, AC drive locomotives are 5%-7% more efficient in converting the cankshift HP of the diesel to drawbar horsepower. The current question is it worth spending $500,000 more on a locomotive for this efficiency increase. When diesel fuel was very cheap (in relative terms) a few years ago, some RRs, like the NS, didn't feel it was. With the manufacturing costs of AC going down, and fuel prices going up, we very well may see new DC traction go the way of steam locomotives. If AC units are only for Coal Trains. Then why are AC units on "other" Trains as well? Allan. That's not true. AC locomotives are used for everything from coal to high speed intermodal. Thanks, guys, I'm enjoying the answers and learning. allen smalling AC locomotive's greatest advantage is in low speed drag service, pusher service, mountain railroading, but they have an advantage in all kinds of service as well because of generally lower maintenance costs and greater fuel economy. In fact, today, probably if you figure total costs over the life of the engine, the AC would come out ahead despite greater first costs. For all you engineers, is AC easier to operate in a locomotive, or DC, or no diff?? Say, would an engineer who is used to DC have to learn a new style to operate AC? Is it true DC has the drawback that it can be burned out? Is the ES44DC well thought of? My best friend has as his wallpaper a photo of a BNSF locomotive, and the shot is so close-up the stenciling with ES44DC can be seen. Inquiring minds, etc. Thanks allen It seems to me that the inverter on an AC unit would put out a modified sine wave, which would have a short interval of no voltage between half cycles. If the power factor is reasonably close to unity, the current would die out quickly enough that a resistance measurement could be made on the motor windings so as to indicate the instantaneous temperature. This would allow an "intelligent" inverter to apply the maximum power that could be used at the moment without damaging the motor.

It never fails to surprise me that the engine-generator set puts out AC, which is then rectified to DC, then is inverted back to AC for the traction motors, and finally winds up more efficient than the old-time straight DC rigs! Ah, the wonders of modern solid-state electronics! If the economics were such that the price of AC locomotives would plummet like computers, we would all wonder why DC was ever considered. Another thought should be taken into consideration: what AC means. Alternating Current switches polarity several times per second. Most table saws run the same speed because they all face 60 cycle per second household current. On an AC locomotive, the power plant generates power with an AC cycle based on the RPM of the alternator. This is then converted to DC which is then converted back to AC, only at a different frequency. Control of an AC motor requires setting both the freqency AND the voltage. The rule is: 105% acceleration. That means the inverters respond to an acceleration command from the throttle by supplying 5% more than what is being used at the moment. The result is that both the power and the frequency are set to that optimum setting. On a DC locomotive, a Run 8 throttle setting could be supply 200% or more of the power that's needed to just go faster. This extra power is wasted off as heat and the destruction of the traction motors. AC traction motors only ever get about 5% more than what they are already handling, never mind what the engineer is asking for. The speed increases and so does the 5% figure until either the locomotive goes the speed the engineer wants or something breaks. The computer that tends to all of this also looks at how fast each axle is going. On a DC loco, an axle that goes overspeed (wheel slip) is either cut back or cut out, depending on the wheel slip management. On an AC locomotive, that axle won't ever speed up beyond 5% more than the others because of the power applied. Then the computer looks at this overspeed and pulls it back to exactly the same as the other five axles. If all six can't speed up, the unit lays down, just as a DC unit would do, only without spinning wheels or overheated motors. Thanks to the computerize inverters, money saving from AC unit not only comes from nominal fuel savings but also lesser maintenance expenses, greater locomotive availability (in theory), and more flexible assignment. John All of the above is true, but the Transit industry has come up with an alternative. New York's MTA has placed orders for large fleets of Commuter and Rapid Transit cars that are powered by DC, but utilize the power as AC. That means that the investment in DC transmission is kept in place, while the trains get the benefits of AC power. One of the additional benefits of this AC/DC arrangement has been the restoration of Dual Power locomotives to the American rail industry. Historians here will remember the EMD FL-9 locomotive on the New Haven, and earlier Dual Power locomotives from GE in the formative days of the Diesel. Thanks to the research done by New York's MTA and the manufacturers who had large potential orders as incentive, AMTRAK now has Dual Power locomotives they use on the New York - Albany route as well. As before, the Dual Power status is Diesel prime movers, this time producing AC Power, and DC Third Rail Shoes, and the switching equipment necessary to select AC Power from the two sources. For the first time in a long time, AMTRAK and the two commuter roads can run trains from deep in Diesel territory direct into Penn Station and Grand Central Terminal without stopping to change locomotives. Regarding MU'g AC and DC locos, I believe the earlier mentioned article in Trains discussed that they had to be careful MU'g the two types, trying to keep the DC loco as the lead loco. The reason for the concern was that the AC could be redlining all day long without burning out the traction motors while the trailing DC unit's TM's would be fried after like 30-60 minutes. That was back when AC was new on the freight scene, before the train crews understood AC very well or were trained to understand that what was OK/going on

in the front AC unit may not be so OK for any trailing DC unit(s). Early on several DC locos got fried in such mixed consists. I don't know if it's still an issue or not. The crews now have a lot more experience with and a far better understanding of AC locos now that there are so many running in the fleets, so they may be trained to factor for that. Perhaps even the newer DC locos have software programming or hardware to prevent redlining them beyond certain limits, causing them to automatically cut out? Does anybody know? Thanks, Greg Well, I have heard that all of the above is true and that the engineers can put the loco's in notch one or two to help hold a train on a hill and not stall burn the motors like DC. All I know is that I changed out a lot more DC motors than AC. Good thread. Couple of thoughts… I broke some old time hoggers in on AC while I was an EMD field engineer; it was not easy convincing them that they would not burn up the motors. In one case, a road foreman had to run. Depending on the hill, you sometimes needed th. 4, 5, or 6 to hold a heavy train. If you goof, the train slides back and the brakes apply. After waiting for another try, we had to notch out to 6 before releasing the brakes. THAT was really hard for them to grasp (at first). Imagine being at almost full throttle before leasing the brakes! Its all about tractive effort, AC does more but comes with a $500k-$700k premium. [DC’s were around $1.5M and AC $2 to $2.2M]. Just look at two 5,000 HP SD80MAC’s replacing four C30’s for Conrail on the B&A, or three SD70MAC’s doing the work of five SD40-2’s for BN. It pays for itself but in the right application. HP equals speed, but AC can pull more (low end torque). Put both together and you have 6000 HP AC’s that can do it all. BTW, DC’s could not put 6000 HP to the rails, they would slip. Its simple (well sort of…) physics. The locomotives would need to weigh too much. AC invertors are like traction control in a car, which is why you have to take AC from a constant speed engine/alternator set, clean it up (DC), and convert it back to AC with a computer fast enough to make the adjustments. It is like a fly-by-wire jet. Only a computer can get that much HP to the rails. With all the advantages of AC over DC, It must be simple economics that keeps NS from switching to AC traction. Any comments? Joe A. NS has long been very conservative when it comes to motive power: high noses, AAR control stands, etc. They may be trying to keep parts inventories reasonably simple and other similar ways of controlling costs. Paul The commute to work may be part of the daily grind, but I get two train rides a day out of it. NS believes a 4000 HP DC locomotive is the "one size fits all" solution them. No doubt there are applications where AC would be more economical, but they believe the utilization penalty would outweigh the benefit. Once upon a time, I was talking with an NS mech dept guy (back when the mech dept ruled the roost at NS w.r.t. loco purchases). He said ACs were a poor choice because nearly all NS trains could be handled by two DC units. That may have been true then, but is certainly isn't the case now. Someday, NS will purchase AC units, but it doesn't look like it'll be real soon. If NS believes that the C40-9W/ES40DC is pretty much a one-size-fits-all road locomotive, it would be similar to the motive power policy of pre-1964 N&W, which was overwhelmingly GP9/18's and some RS11/36's. Paul The commute to work may be part of the daily grind, but I get two train rides a day out of it. It's funny how this transition in technology doesn't trade anything but money. Usually all new technologies lacked something that old ones had. Like transition from records to CD's (records had a different warm texture), transition from technicolor to eastmancolor, transition

from film photography to digital photography (film is still alive though), transition from steamers to diesel etc. All of these transitions left someone unhappy, but this time there is nothing to complain about. it seems. About mixing AC & DC locos. It's my understanding (not always correct) is that the reason the AC4400CTE is designated CTE (controlled tractive effort) is that those units can automatically derate themselves when MUed to DC units. Maybe NS sees a benefit in keeping the number of loco. models to a minimum. Recall that Southwest Airlines uses only 737's, even though there would be times (vacations, Xmas, etc.) when a bigger plane would fill to capacity. But using only one type of airliner means the mechanics are well familiar with them, also that they're easier--or at least more convenient--to repair. Possibly true for NS, or am I just speculating? What is the advantage and disadvantage of both? Do you think AC will be the only thing available in the future? Will Other than AC has better low speed control, and has no copper windings to 'bird cage'? DC traction motors do not cost as much and for high speed service, they work just fine. As AC costs go down, I would suspect we will see more AC traction motor locomotives. GE offered a AC 'A-1-A' trucked solution to BNSF last year for the same price as C-C trucked DC units. I have heard nothing about the results of the locomotives BNSF bought, but I have heard nothing bad from the field. My understanding is that the units were wired for 6 AC traction motors, but only have 4 invertors/traction motors, and GE claims they can be upgraded to full C-C units at some later point if desired. Jim Bernier Cost of AC motors and control equipment will come down as more are built. Advantages for the AC motors are lighter for an equivalent power, and AC motor adhesion control is much more effective, and no commutators to wear and need replacing. The following are the advantages: 1. No commutator or brushes to need maintenance. 2. If the pinion breaks or something, the motor will essentially continue on at its current speed instead of over revving and blowing apart (birdcaging). 3. Easier to seal and cool the motor. 4. More horsepower and torque is possible from a given volume of motor. Traction motor space is very limited. 5. More precise control of drive wheel speed which can increase available tractive effort. 6. The motors themselves are actually simpler and less expensive with only two bearings and one moving part. The main disadvantages are: 1. More complex electrical and electronics gear with inverters and so on. 2. More cost because of the extra cost of the drives. IMHO, DC drive technology is very mature without much room to grow. AC drive technology is much more open to expansion. As the cost of the drives comes down it is likely that AC drives will become more predominant, especially on heavy mainline locomotives. Current high horsepower variable frequency drives (inverters) are about 1/10 the cost they were 15 years ago. I suspect that the cost of high power inverters will continue to decline. Current state of the art are IGBT inverters with induction motors, 5 to 10 years down the line we may be seeing GaN or SiC FET's driving permanent magnet synchronous motors. The new FET's should result is smaller inverters due to faster switching times leading to less heat being dissipated in the active devices. A permanent magnet synchronous motor can produce at least 25% more torque for a given frame size than an induction motor, as well as being more efficient at low speed high tractive effort conditions. Cooling will be even simpler than

with the induction motor as very little energy will be dissipated in the rotor - it might be simplest just to use water cooling. - Erik Some advantages noted are a comparsion of AEM-7DCs vs AEM-7ACs. To meet AMTRAK acceleration specifications for the NEC; they usually limit DCs to 7 - 8 cars and ACs to 10 cars. But as mention in the above post the abilty to seal the motor is the most important one. The thread about the CAL Z problems of running in the snow and having ground faults on the DC locos is very important. As a CEO of any RR that operates in snow country I would want all my trains to operate in snow without grounding out. The severe winterwe are having maybe many RRs will change their orders to AC and order a much higher percentage of ACs. The reduction of inverter costs may also be a factor? I think the only Class 1s that are ordering DC 4000-4400hp 6-axle GE and EMD locos are CN, NS, and BNSF, though BNSF has a large AC-traction fleet too. UP, CSX, KCS and CP have seem to gone exclusively with AC with recent orders. Mostly DC-traction NS has a handful of former-CR SD80MACs and 24 ES44ACs, while CN has no AC-traction whatsoever. Even regional Class 2s IAIS and MRL have AC-traction on their rosters. And the Indiana RR now has SD9043MACs too. The Chicago Transit Authority is also beginning to go with AC traction motors, some other transit operations may be doing the same thing. Paul The commute to work may be part of the daily grind, but I get two train rides a day out of it. I think they generally only need to power one truck when they do this. There's enough adhesion provided to achieve the acceleration they need for service. That seems to be a good idea for CTA since they do have some winters with a lot of snow. Do you think AC will be the only thing available in the future? Yes - it's already happened in Europe - as far as I know, the big rolling stock suppliers on this side of the pond (e.g. Bombardier/Alstom/Siemens) only offer AC traction on their light rail/EMU/locomotive/HSR products. I'm sure EMD & GE are only continuing to build DC equipment because some US/Canadian railroads still want to buy it, not because they believe it has a long-term sales future. Tony I'm sure EMD & GE are only continuing to build DC equipment because some US/Canadian railroads still want to buy it, not because they believe it has a long-term sales future. Tony Could it be that the tax situation in the US and Canada (CNRR esp) is different than Europe.? Probably ACs will increase in price less than DCs. There also could be a sudden closing the price gap if some component(s) becomes much cheaper . Maybe when the price of AC locomotives gets to a certain point closer to DC locos then all RRs will order all AC lcomotives? At that point the remaining DCs will slowly become orphans with the DC specific item costs gradually esculating faster than the other loco costs. Have no idea what this year's awful winter snows will do to future equipment decisions? May come to the point where in winter DCs will be sent to south away from snow storms. GE has been implying of late that they would like to only offer AC power, the new A-1-A 4 motored ES44ACs for BNSF are being touted as a replacement for 6 motored DC units.. I think the only Class 1s that are ordering DC 4000-4400hp 6-axle GE and EMD locos are CN, NS, and BNSF, though BNSF has a large AC-traction fleet too. UP, CSX, KCS and CP have seem to gone exclusively with AC with recent orders. Mostly DC-traction NS has a handful of former-CR SD80MACs and 24 ES44ACs, while CN has no AC-traction whatsoever. Even regional Class 2s IAIS and MRL have AC-traction on their rosters. And the Indiana RR now has SD9043MACs too.

BNSF has covert their ES44AC to 4,000HP from 4,400. Thank you all for the great responses. Will After searching Google for AC traction motor- images, I'm surprised. What i saw was nothing more than a squirrel cage induction motor, although with a rather massive shaft. I expected a wound rotor motor, which I had been told was in use in Europe. Has the wound rotor and its rotor current limiting device been made superfluous by solid state electronic controls? Has the wound rotor and its rotor current limiting device been made superfluous by solid state electronic controls? Yes. The advantage of a wound rotor motor was that it would give both high starting torque and high running efficiency for operating at a fixed line frequency. The disadvantages are that it is heavier, more expensive and less rugged than a squirrel cage motor. With a variable voltage variable frequency drive, the drive frequency can be set low enough to ensure good starting torque (peak torque is a function of slip frequency for a given motor design). My understanding is that most ac traction motors currently in use are not true synchronous motors completely dependent on the input electrical frequency for speed control. The dissadvantage of such motors is that an instant overload where there is insufficient torque means loss of all power. What I have been told (and correct me if I am wrong) is that they are "hysterises non-synchronous" motors. The operating principle is pretty much the same, but the difference in construction is that the rotor bars are not perfectly horizontal but are slanted, all parallel of course, but not at right angles to the direction of motion and not parallel with the rotor shaft. The great advantage over wound rotors is huge ability for high thermal overload allowing short-time overload ratings to be longer in time and larger. This and the absence of brushes associated with both dc motors and ac-commutator motors (GG-1, EF-3, etc.) drastically reduces maintenance requirements. Awesome! Lyon_Wonder I think the only Class 1s that are ordering DC 4000-4400hp 6-axle GE and EMD locos are CN, NS, and BNSF, though BNSF has a large AC-traction fleet too. UP, CSX, KCS and CP have seem to gone exclusively with AC with recent orders. Mostly DC-traction NS has a handful of former-CR SD80MACs and 24 ES44ACs, while CN has no AC-traction whatsoever. Even regional Class 2s IAIS and MRL have AC-traction on their rosters. And the Indiana RR now has SD9043MACs too. BNSF has covert their ES44AC to 4,000HP from 4,400. Really? I'm aware that some of UP's GEVO fleet has a software system that reduces tractive effort when the units are used in DPU mode but I didn't think they had derated the whole fleet... Thank you all for the responses. I didn't know this kind of traction system could be so interesting. Will Did someone in another thread mention that DC traction motors were limited to about 1000HP per axel? ACs do not have that limitation? daveklepper My understanding is that most ac traction motors currently in use are not true synchronous motors completely dependent on the input electrical frequency for speed control. As of 2005, GE was using induction motors for their AC diesel electric locomotives (from a talk given at our company shortly after we were bought by GE, we're now a joint venture with Safran Group) and I would be very surprised if EMD was not using induction motors in

their AC diesel electrics. These motors are presumably squirrel cage motors, quite possibly with the slanted rotor bars. Anyway, one of the subjects of the talk was GE's research into permanent magnet synchronous motors which would give about 25% more torque for a given motor frame than an induction motor and better efficiency at high tractive efforts (no waste from the eddy currents in the rotor bars). One problem with squirrel cage motors was that, assuming a constant line frequency, they could be designed for high efficiency and low starting torque (low resistance bars) or high starting torque and reduced efficiency (high resistance bars). The wound rotor was a way of having the best of both worlds, using high resistance to get the starting torque and low resistance once the motor came up to running speed. Another way of putting it is that peak torque occurs at a high slip speed with high resistance and a low slip speed with low resistance. With a variable frequency drive, the motor can be designed with a low resistance motor, and to get good starting torque, the drive operates at a low enough frequency that the slip speed is less than the slip speed that gives peak torque (you always want some torque headroom). This removes the need for the variable rotor resistance provided by the wound rotor induction motor. One other advantage of the low resistance rotor design is that the resulting motor torque changes very rapidly as a function of speed for a given drive frequency. Any wheelslip will cause the motor to speed up and the torque will drop. - Erik erikem Another way of putting it is that peak torque occurs at a high slip speed with high resistance and a low slip speed with low resistance. With a variable frequency drive, the motor can be designed with a low resistance motor, and to get good starting torque, the drive operates at a low enough frequency that the slip speed is less than the slip speed that gives peak torque (you always want some torque headroom). Could you please define "slip speed"? Thank you. The only Class one I was aware of that Reduced the HP on the ES44 was NS theirs are considered ES40's I asked a engineer for the BNSF helps I live right next to the BNSF connection track with the NS and be amazed what information you can get for a big cup of hot Coffee. He flat out told me that the BNSF Evos are all rated for 4400 HP and will not be reduced in power. Also he has heard that they are getting 100 more of the ES44AC4's evidently they love them and will be using them on any and all trains except coal. Always at war with those that think OTR trucking is EASY. edbenton The only Class one I was aware of that Reduced the HP on the ES44 was NS theirs are considered ES40's I asked a engineer for the BNSF helps I live right next to the BNSF connection track with the NS and be amazed what information you can get for a big cup of hot Coffee. He flat out told me that the BNSF Evos are all rated for 4400 HP and will not be reduced in power. Also he has heard that they are getting 100 more of the ES44AC4's evidently they love them and will be using them on any and all trains except coal. IIRC, NS ES44ACs are being delivered without the electronic derating feature and operate at the full 4400 HP. They are primarily for coal drag service and NS tests showed that having full HP was a plus for that application when using AC traction. NS fleet of ES40DCs and C40-9s have a switchable engine output control. They normally operate at 4000 HP but supervisors have a key which switches the locomotive to 4400 HP.... CP's AC4400CWs and ES44ACs have software that limits power output at full throttle to 4000hp. at speeds above 28 mph. Below that speed throttle position 8 will cause the diesel

to produce 4400hp. (well 4390hp. if you want to be precise). JayPotter Could you please define "slip speed"? Slip speed is the difference between the synchronous speed and the actual rotational speed of an induction motor. For example, a four pole motor running on 60 Hz will have a synchronous speed of 1800 rpm, but will be typically spec'ed at 1725 rpm, for a difference of 75 rpm. This slip of 75 rpm is what causes current to be induced (hence "induction") in the rotor bars and the interaction of the induced current and the rotating magnetic field produced by the stator windings is what produces the torque. If you try spinning the shaft of an induction motor faster than the synchronous speed, the "motor" will generate an opposing torque and become an induction generator. A decent, but not perfect, analogy is a fluid clutch or torque converter (without the torque multiplication). Hope this helps. - Erik carnej1 IIRC, NS ES44ACs are being delivered without the electronic derating feature and operate at the full 4400 HP. They are primarily for coal drag service and NS tests showed that having full HP was a plus for that application when using AC traction. NS fleet of ES40DCs and C40-9s have a switchable engine output control. They normally operate at 4000 HP but supervisors have a key which switches the locomotive to 4400 HP.... I HIGHLY doubt that. even if the railroad supervisors had the ability to increase the HP there would never be one rated at 4000 HP if nore was available. I would believe that GE had the software necessary to uprate the units should the railroad decide to. By the way... as a forum newbie, what does IIRC stand for? Tugboat Tony carnej1 IIRC, NS ES44ACs are being delivered without the electronic derating feature and operate at the full 4400 HP. They are primarily for coal drag service and NS tests showed that having full HP was a plus for that application when using AC traction. NS fleet of ES40DCs and C40-9s have a switchable engine output control. They normally operate at 4000 HP but supervisors have a key which switches the locomotive to 4400 HP.... I HIGHLY doubt that. even if the railroad supervisors had the ability to increase the HP there would never be one rated at 4000 HP if nore was available. I would believe that GE had the software necessary to uprate the units should the railroad decide to. By the way... as a forum newbie, what does IIRC stand for? IIIRC=If I recall correctly.........The information about the NS units has been widely reported on various forums and publications including postings by NS engine service employees. The keys are in the possession of Road Foremen who are under standing orders to leave the locomotives at the lower ratings(to control fuel consumption) unless the higher HP rating is needed in specific service....... CSX is derating all of its GE DC locomotives to 4000hp.

AC traction motor question Date: 01/08/01 18:51 Author: Zook6753 Hello. Today in my electronics class at school, we we're talking about the difference between AC and DC motors. But I was wondering, why do the GE AC traction motors make a whining sound while the GE DC traction motors do not make one nearly as loud? Thanks in advance. Date: 01/08/01 19:03 Author: DRLOCO I work for CSX, and everyone at work thought I was crazy for noticing this. They seem to whine a little louder, but the standard DC motors get loud between 46-49mph...i don't know if it's some kinda harmonic balancing thing or what...~~DRLOCO (insane in the brain!) Date: 01/08/01 21:27 Author: powerbraker1 Got to be the harmonics. Ever notice the whoom...whoom...whoom sound at speed? Author: InsideObserver The whining you're hearing is in all probability gear noise. Normally diesel locomotve traction motors use non-resonant gearing in order to attenuate this as much as possible, but recent gearing seems to be more noisy than gears made 15-20 years ago. The difference in tone you hear between DC and AC motors is attributable to differences in gear sizes, relative armature speeds, wheel diameters, gear ratio, and gear wear. Gear wear is most significant: the noisier the gears, the more worn they are. A DC traction motor also has a whine produced by the commutator bars passing under the brushes, but the frequency of this rises beyond the upper limits of human hearing at speeds over about 1-3 mph. Date: 01/09/01 07:16 Author: jc The difference is in the way the power is delivered to the windings of the motor A/c has a carrier frequency that makes a whine at all speeds, and when the motor torques up, the pulses going to the windings in a three phase configuration makes a little different whine that resonates at different speeds...This is called a variable frequency drive where the frequency determines the speed. D/c drives systems are somewhat different. The diesel motor turns the alternator just like the a/c drive. D/c drives have a chopper circuit that makes a different pulse width power output to the motor and the duty cycle of the pulse width determines the voltage/current output to the motor. The speed of this motor is controlled by changing the pulse width duty cycle which appllies a more or less continous wave form to the dc motor.. This is a simplified discussion typical to the control of motor speed in both a/c and dc depending on the drive manufacturer.... jc Date: 01/09/01 21:39

Author: PMPete Are you sure about the chopper in the alternator output ciruit. I thought the alternator output was rectified and current/voltage was controled by alternator field excitation. Date: 01/10/01 22:20 RE: AC traction motor question Author: InsideObserver The excitation of the alternator is used to control output voltage, which is fed to banks of rectifiers. The rectified AC is a bit "dirty" in that it consists of a series of overlapping 1/2 cycle AC waves. This is sometimes cleaned up by feeding it through a smoothing reactor (a large induction coil) which averages out the peaks and trough between the successive AC waves. When a chopper is used to convert AC to DC, or to regulate DC for things like rapid transit cars, it is usually in the form of a GTO ("Gate Turn-Off thyristor"). Control volages are appled to the gate to vary the width of DC pusles which are rectified from the AC. This output is then sent to the smoothing reactor, which averages out the pulses to an appropriate DC voltage. Essentially, the wider the pusles, the higher the DC voltage. That annoying hum you hear when BART trains start moving is the chopper doing it's thing. It also operates at an odd frequency (37 cycles per second I believe) so that there will be no problems with inductive and harmonic interference from commercial 60 cycle AC power. An AC traction motor uses variable frequency AC to regulate "armature" speeds. At low speeds, 0-5 mph, the frequency is something like 10-50 cycles; at higher speeds the frequency is greater. At low speeds the AC hum is noticeable, at higher speeds its gets drowned out.