AC and DC Traction Motors in Diesel Electric Locomotives

diesel locomotive with electric motors with vacuum circuit breaker

Diesel locomotives use electric traction motors to generate motive power. These traction motors come in two different types: alternating current (AC) and direct current (DC). AC traction motors are powered by a three-phase rectified supply, which is a form of electrical power that is more efficient than single-phase or DC power. DC traction motors, however, are powered directly by the DC supply from the locomotive battery or auxiliary generator.

Traction Motors in Diesel Electric Locomotives

In the blog post, we will review the components of both AC and DC motors and then briefly review how they actually work. Then at the end, we will discuss how diesel-electric and fully electric locomotives work.

AC traction motors are typically used on newer diesel locomotives because their smoother power delivery reduces wheel slip and provides greater torque at low speeds compared to DC traction motors. Moreover, AC traction motors have fewer parts than DC traction motors and require less maintenance. However, they cost more than DC traction motors and can be difficult to repair in remote locations due to their reliance on complex electronics.

DC traction motors have been used for many years in railway applications and provide reliable service over long distances. These traction motors tend to be ruggedly constructed with simple electronics and relatively few moving parts, making them easier to maintain than AC models. In addition, DC traction motors are less expensive than AC models and can be serviced in remote locations. However, they have limited torque at low speeds compared to AC units.

Both AC and DC traction motors provide reliable operation and are key components of modern diesel locomotives. When selecting a type of traction motor for a particular application, engineers must consider factors such as cost, power requirements, maintenance needs, and serviceability in remote areas. Ultimately, the choice between AC and DC will depend on the specific characteristics of the locomotive system.

Despite their differences, both AC and DC traction motors are essential components of modern diesel locomotives that enable them to operate reliably over long distances. As technology advances, these types of motors are likely to become more sophisticated and capable of providing even greater performance and efficiency.

Components of AC traction motor

three phase induction motors in electric locomotive

A traction motor is the main component of an AC electric drive system used for locomotive and industrial applications. It is an electric motor that converts electrical energy into mechanical energy, providing the necessary torque to move the vehicle or machine forward. The components of an AC traction motor include:


Rotors consist of laminated steel laminations with windings located inside them, which spins when current is applied.


Stators are static and surround the rotor. It has three sets of windings (each set consisting of two phases); these are connected in a wye configuration so that they produce a rotating magnetic field.


The commutator device helps keep the current flowing in the right direction by collecting electricity from the brushes and passing it to the windings.


Brushes are pieces of metal that rub against the commutator to conduct electricity from the power source into the motor’s windings.

Cooling System:

This helps prevent overheating of the motor by circulating air or liquid around its components.


This part attaches the motor to a vehicle or machine. It also helps absorb vibration, reducing noise levels.

The AC traction motors used for locomotives and industrial applications are designed for high reliability and efficiency in order to ensure safe operation. These components help convert electrical energy into mechanical energy, providing the necessary torque to move the vehicle or machine forward.

How an AC traction motor work

An AC traction motor works by converting electrical energy into mechanical shaft rotation. It consists of an outer stator wound with copper wire to create a magnetic field and an inner rotor with permanent magnets or electromagnets attached. As the electric current flows through the copper wires, it creates a rotating magnetic field that interacts with the rotor creating torque and causing it to rotate. Gears and other components then harness this rotating force to turn wheels, allowing for greater control and speed when compared to DC motors. The AC motor also allows for quicker acceleration as its high starting torque can compensate for the lack of low-end torque seen in DC motors. Additionally, AC motors are more efficient than their DC counterparts due to their ability to regulate power input. This makes them ideal for use in electric vehicle applications as they require less energy and help reduce operating costs.

Components of DC traction motor

DC Traction Motors are the motive power unit for electric vehicles and play a crucial role in their performance. DC traction motors are made up of several components, including the armature coil, field magnet, commutator, collector ring, brushes, shafts, and bearings.

Armature coil

The armature coil is part of the motor that interacts with the magnetic field to generate torque. It consists of copper wire wound around a steel core to form multiple loops. The number of loops determines the strength of the current generated when exposed to a magnetic field.

Field magnet

Field Magnets generate a strong magnetic field that interacts with the armature coils to create movement in the motor’s rotor. Depending on the type of motor used, these can be either permanent magnets or electromagnets.

The commutator

is an electrical component that helps control the direction of current in the motor’s armature coils. It consists of two copper plates mounted to the shaft and connected by a copper slip ring. The brushes are made from carbon and make contact with the commutator segments to direct current.

Collector rings

provide power to the DC traction motor when they rotate through a set of stationary contacts known as the collector shoes. These allow for multiple windings in a single coil, allowing for higher torque output from smaller motors.


are responsible for transferring power from the electric motor to whatever mechanical system it is powering, such as wheels or axles. Bearings support the shaft and reduce friction, allowing for smoother operation.

DC traction motors are essential components in electric vehicles, providing them with the power to move. The above components play an important role in their performance, from generating torque to transferring power. When all these elements come together, they form a reliable source of power that allows electric vehicles to run efficiently and effectively.

Overall, DC traction motors are reliable and powerful components of electric vehicles. They combine several components to create a motor that can generate torque and provide power for various applications. Today, these motors are essential for modern electric vehicles, making them an important part of their functioning.

How a DC traction motor work

freight trains use electric locomotion for power output

The motor works by using direct current (DC) electricity to create rotation in its armature, which is then transmitted through gearing to propel the vehicle forward. A DC traction motor can be powered by various sources such as batteries, fuel cells, or generators.

As stated, the construction of a DC traction motor consists of several components, including an armature winding, stator core (the body), brushes, bearings, commutator rings, and carbon segment plates. The armature winding is made up of insulated copper wire that wraps around the steel laminations of the core; this produces an electromotive force when charged with an electrical current. The stator core is the body of the motor and provides a stiff, rigid platform for all other components to attach. Brushes are held against the commutator rings, which consist of segments separated by an insulating material; these conduct electricity from the stationary field coils to rotating parts of the armature. Bearings support rotation between moving parts so that friction does not slow down performance.

When an electric current passes through the armature windings, it creates strong magnetic fields which interact with those created in the field coils (stator windings). This interaction produces a torque reaction force between both sets of magnets, causing them to rotate in opposite directions while continuing to attract each other. This rotation transmits power to the external gearing, which is then used to propel the vehicle forward.

The speed of a DC traction motor is controlled by varying the amount of current that passes through its armature winding; reducing or increasing this current will decrease or increase torque, respectively. This allows for smooth and efficient acceleration in electric-powered vehicles. Additionally, regenerative braking can be used to recover energy normally lost during braking, providing further efficiency gains.

DC traction motors are an integral part of modern electric propulsion systems due to their efficient operation and ability to provide power in both directions (forward and reverse). By controlling the flow of electricity through the armature windings, they also allow for precise regulation of speed and torque output. As such, DC traction motors will continue to be a crucial part of electric-powered vehicles for years to come.

How diesel-electric locomotives work

diesel electric technology used in diesel powered locomotives

Diesel-electric locomotives use diesel engines to turn a generator. The generator produces electricity, which is then sent to the traction motors connected directly to the locomotive’s wheels. This provides power for acceleration, braking and changing direction.

The diesel engine is usually located at the front end of the locomotive and is connected to the generator in a single unit. The traction motors are usually mounted on the axles of the locomotive, although some designs have them mounted inside the locomotive’s frame.

The generator produces a three-phase power which is then sent to the traction motors and controls their speed. This power can be increased or decreased depending on how fast the locomotive needs to go. The power can also be reversed, allowing the locomotive to change direction.

When the locomotive is decelerating, some of the energy generated by braking is fed back into the generator and stored as electricity. This allows for shorter braking distances and better control of acceleration and deceleration.

The diesel electric locomotive is one of the most efficient and reliable types of locomotives available. Its simple design makes it easy to maintain and operate, and its energy efficiency helps reduce emissions. The combination of diesel and electric power also allows for greater flexibility in routes and smoother running on curves and hills. It’s no wonder that diesel electric locomotives are still the most common type of locomotive used today.

How do electric trains work

modern ac electric locomotive have powered and unpowered axles

Electric locomotives are powered by electricity and use electric motors to convert mechanical energy into motion. They typically use two main types of power sources: either an onboard transformer or a catenary system. In both cases, the electrical power is converted from alternating current (AC) to direct current (DC), which powers the traction motor that turns the wheels and propels the locomotive forward.

The transformer onboard an electric locomotive is a device that steps down the voltage from overhead lines or from an external power source to a predetermined level. This energy is then converted into DC and sent to the traction motors, which convert it into mechanical energy that moves the wheels. An electrical control system monitors and regulates the speed of the locomotive, as well as its direction.

Catenary systems use a network of wires and poles to feed electric power to the locomotive via a pantograph device. The electricity is transmitted in AC, but once it reaches the locomotive, it is converted into DC and sent to the traction motors. This type of power transmission is used by many high-speed trains since it reduces disruption from external sources of electricity.

The motors on an electric locomotive are also controlled by a computer system that regulates the speed, direction, and throttle settings to ensure optimal operation of the vehicle. The system also monitors temperature levels in the traction motor and other components, makes sure the locomotive is not overloaded, and can be used to detect fault signals.

In general, electric locomotives are more reliable and efficient than diesel locomotives because of their ability to generate torque directly from electrical power sources. This makes them ideal for long-distance travel, as well as for high-speed passenger services. They are also quieter and more environmentally friendly than diesel locomotives since they don’t produce any emissions. Electric locomotives are now a common sight on railway networks worldwide, and their use is set to continue to grow in the future.

Battery Electric Locomotive

Battery locomotive is designed to operate on electrified railway lines and use traction battery cell as an energy source. These locomotives typically have large-capacity rechargeable batteries (often lithium-ion or lead acid) that can be recharged from the pantograph during operation or when stationary at a charging station. Battery electric locomotives do not require an overhead line and are typically used in short- and medium-distance applications or for shunting operations.

Battery electric locomotives are a relatively new development in the railway industry but have already seen some successful implementations. The advantages of these locomotives include their environmental friendliness (no emissions), low operating costs (due to no need for fuel), and low noise levels. Additionally, their relatively small size and maneuverability make them ideal for use in urban areas. On the downside, battery electric locomotives have a limited range and require regular recharging to remain operational.

Overall, battery electric locomotives are an attractive option for many railway applications. They offer a clean, quiet transportation option that is cost-effective and can be used in many different environments. While their range may be limited, they can reduce emissions and cut down on operational costs compared to traditional diesel locomotives. As battery technology continues to improve, these locomotives are likely to become more popular in the future.

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