Trystan Casey
Diesel-electric locomotives are a type of locomotive that uses a diesel engine to generate electricity, which is then used to power the locomotive's traction motors. These locomotives can use either direct current (DC) or alternating current (AC) motors, with earlier models predominantly using DC motors and more modern locomotives using AC motors. The use of AC or DC power in diesel-electric locomotives depends on various factors, including the available infrastructure and the specific requirements of the railway system.
| Characteristics | Values |
|---|---|
| Type of current used | Direct Current (DC) and Alternating Current (AC) |
| Power source | Diesel engine coupled with a generator |
| Functionality | The diesel engine powers the generator, which produces electricity for the traction motors on each axle that propel the train |
| Components | Diesel engine, generator/alternator-rectifier, traction motors, and a control system |
| Control system | Consists of the engine governor and electrical/electronic components like switchgear and rectifiers |
| Modern improvements | Traction inverters capable of delivering higher voltages (1,200 volts) compared to earlier traction generators |
| Braking system | Dynamic (rheostatic) braking uses the rotation of traction motor armatures and a separate field winding to act as a generator for braking |
| Torque | Highest torque produced when the locomotive is at or near standstill, helping to overcome inertia |
| Transition | Issue with DC locomotives where wheelspin can occur until the driver adjusts the power setting |
| Dual-mode | Some diesel trains can switch to electric power when overhead lines or third rails are available |
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What You'll Learn
Early diesel-electric locomotives used DC traction motors
The history of diesel-electric locomotives can be traced back to the 1920s, but it wasn't until the 1940s that large locomotives were manufactured, replacing steam power. Early diesel-electric locomotives in the United States used direct current (DC) traction motors. These DC traction motors played a crucial role in the development of diesel-electric propulsion technology.
DC traction motors in diesel-electric locomotives are powered by a diesel engine, which drives a DC generator. This generator produces electricity, which is then supplied to the traction motors mounted on the axles of the locomotive. The traction motors are responsible for converting electrical energy back into mechanical energy, driving the wheels and propelling the train forward.
One of the key advantages of DC traction motors in early diesel-electric locomotives was their ability to deliver high torque, particularly at low speeds or when starting from a standstill. This torque output helped overcome the inertia of a stationary train, facilitating smooth starts and efficient acceleration. Additionally, DC motors were relatively simple to control compared to their AC counterparts, making them a practical choice for early diesel-electric propulsion systems.
However, DC traction motors also had their limitations. As locomotive power requirements increased, the limitations of DC motors became more apparent. DC motors were generally less powerful than AC motors, and they required more maintenance due to the presence of brushes and commutators, which were prone to wear and tear. Additionally, DC motors could experience issues with arcing and spark generation, especially in dusty or corrosive environments, leading to safety concerns and increased maintenance needs.
In the 1970s, advancements in technology led to the development of AC (alternating current) traction motors for diesel-electric locomotives. The German DE2500 diesel-electric locomotive, built by Henschel-BBC in 1971, was the first to successfully employ an AC three-phase asynchronous traction motor drive system. This marked a significant shift towards AC technology in diesel-electric propulsion.
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AC traction motors came into use in the 1990s
In the 1990s, high-speed trains began to use lighter, lower-maintenance three-phase AC induction motors. The N700 Shinkansen, for example, uses a three-level converter to convert 25 kV single-phase AC to 1,520 V AC, then to 3 kV DC, and finally to a maximum of 2,300 V three-phase AC to run the motors. AC induction motors are simple and low-maintenance, but until the advent of power semiconductors, they were challenging to apply for traction motors due to their fixed speed characteristic.
The development of power semiconductors has made it possible to fit a variable-frequency drive on a locomotive, allowing a wide range of speeds, AC power transmission, and the use of rugged induction motors that do not have wearing parts like brushes and commutators. This technology has been used in road vehicles (cars, buses, and trucks) with electrical transmission systems, which began to be developed in the latter part of the 20th century.
In the 1980s, EMD (then the Electro-Motive Division of General Motors) partnered with Siemens to develop six-axle diesel-electric locomotives with AC drive and high axle loads for heavy-haul freight trains on US Class I railroads. In 1989, EMD produced the first US diesel-electric locomotive to use inverters with GTO thyristors and evaporative cooling. Freight locomotive AC traction gained momentum in 1991-1992 with the EMD SD60MAC, and in late 1992, these units demonstrated high adhesion qualities during testing. In March 1993, Burlington Northern (BN) placed an order with EMD for 350 SD70MAC locomotives, which was later expanded to 450. The first unit entered service in December 1993, and by 1994, 130 were in service.
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DC generators are generally less than 3000 hp net output
Diesel-electric locomotives are a type of diesel locomotive that uses a diesel generator (or "prime mover") to generate electricity, which is then routed to electric traction motors on the wheels to propel the train. Some diesel-electric locomotives haul passenger trains and have a second, smaller generator to supply power to the passenger cars.
A DC generator is an electrical machine that converts mechanical energy into electricity. The external structure of the DC generator is known as a yoke. The DC generator can be classified into two main categories: separately excited and self-excited. In a separately excited generator, the field coils are energised by an independent external DC source, while in a self-excited generator, the field coils are energised by the generated current within the generator.
The output of a DC generator depends on various factors, including the speed at which it is spun and the number of coils or windings. For example, a vintage automobile DC generator, when spun at 3000 RPM, can output +35 volts DC. Similarly, a 3000-watt DC generator can supply 24 volts of DC power continuously when mated with batteries. However, it is important to note that the input power to a DC generator is not fully transformed into output power, and some losses occur during the energy conversion process.
DC generators are generally rated at less than 3000 hp net output. This is because, at higher power levels, the iron core of the generator can become saturated, leading to reduced efficiency and potential issues with insulation breakdown or commutator/brush action. To achieve higher output voltages, manufacturers often design alternator frames that can accommodate twice as many turns of half-size wire, allowing for a switch to higher voltages, such as 60-volt electrical systems.
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AC alternator-rectifiers generally have 3000 hp net output or more
Diesel-electric locomotives are a type of locomotive that uses a diesel engine coupled with a generator to produce electricity, which is then supplied to traction motors that drive the wheels of the train. These locomotives can operate on either direct current (DC) or alternating current (AC).
AC alternator-rectifiers in diesel-electric locomotives typically have a net output of 3000 hp or more. The high power output of these alternator-rectifiers is due to the use of advanced technologies such as three-phase full-bridge rectifiers with multiple diodes, which improve the efficiency of the locomotive's power generation system.
The use of AC power in diesel-electric locomotives offers several advantages over DC power. AC traction motors, for example, are often more powerful and efficient than their DC counterparts, allowing for higher train speeds and improved acceleration. AC systems also tend to have lower maintenance costs due to their simpler design and reduced number of moving parts.
Additionally, AC locomotives can recover energy during braking, which is then stored in the form of electrical energy and used to power auxiliary systems or assist in acceleration. This regenerative braking system not only improves the energy efficiency of the locomotive but also reduces wear and tear on mechanical brake components.
The net output of 3000 hp or more from AC alternator-rectifiers enables diesel-electric locomotives to haul heavy loads over long distances, making them suitable for freight transport and long-haul passenger services. This high power output also provides a level of redundancy, ensuring that the locomotive can continue to operate even in the event of a failure of one or more components.
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Modern locomotives use AC to DC traction inverters
Diesel-electric trains are equipped with a powerful diesel "prime mover" that generates electrical current for use by electric traction motors to turn the train's axles. Depending on the design of the train, it can either produce AC or DC current using a generator powered by the diesel engine. Modern locomotives use AC to DC traction inverters, which offer several advantages over older DC systems.
The primary advantages of AC traction are adhesion levels up to 100% greater than DC, significantly higher reliability, and reduced maintenance requirements for AC traction motors. The improved adhesion levels of AC traction units allow modern lightweight AC locomotives to provide the same or more tractive effort than older, heavier DC units. For example, a modern AC locomotive like the RX500 can provide as much or more tractive effort than an old-style DC unit like the SW1200, which weighs 60% more.
AC locomotives can control to a specific motor torque level, allowing the tractive effort to remain essentially constant at the higher range of available adhesion. This fast-acting wheel slip control can counteract any wheel slip, allowing the torque level to be set close to the upper limits. Additionally, AC traction provides improved adhesion through weight transfer compensation when a locomotive is pulling a load, and the weight transfers from the front axle to the rear axle of each truck.
The choice between AC and DC traction systems is usually a trade-off between cost, efficiency, and performance. While AC systems offer improved performance and efficiency over long distances, DC systems are still very popular due to their lower manufacturing costs. DC diesel-electric trains are widely used for both freight and passenger services and are particularly suitable for routes with existing infrastructure that supports DC power transmission.
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Frequently asked questions
A diesel-electric locomotive is a type of locomotive that uses a diesel engine to generate electricity, which is then used to power the train's traction motors.
Diesel-electric locomotives can use either AC (alternating current) or DC (direct current) motors. Early diesel-electric locomotives used DC motors, but AC motors became more common in the 1990s.
In a diesel-electric locomotive, the diesel engine drives either a DC generator or an AC alternator-rectifier. The output of which provides power to the traction motors that drive the locomotive. There is no mechanical connection between the diesel engine and the wheels.
AC motors offer several advantages over DC motors, including higher power output, improved efficiency, and reduced maintenance. AC motors can also provide regenerative braking, which allows the train to recover energy that would otherwise be lost as heat during braking.
One disadvantage of using AC motors in diesel-electric locomotives is the increased complexity of the electrical system. AC motors require more advanced control systems and components than DC motors, which can increase the cost and complexity of the locomotive.
