Motors are essential components in several applications, powering equipment in many industries. In simple terms, a motor is a device that converts electrical energy into mechanical energy, generating motion or providing rotational force. Different types of motors have variations in design, each with specific features such as power, speed, torque, size, and control precision.
A traction motor is an electric motor optimized for drive or propulsion, where high torque and low speed are required. These motors are often used in electric vehicles (EVs), locomotives, and other equipment.
Traction motors can be designed as dc or ac motors, including synchronous, asynchronous (induction), and multiphase ac motors. Modern motor-control electronics tend to favor multiphase ac drives. However, ac induction motors and permanent-magnet synchronous motors (PMSM) are most commonly used in EVs. Each manufacturer has engineering and market reasons for choosing a specific motor arrangement.
The type should be carefully assessed to ideally fit an application based on design, performance, reliability, and cost. Additionally, the motor’s physical structure should be evaluated, including its lamination layout, windings arrangement, and the amount of copper wiring and iron used. The electrical schematic of a traction-optimized motor and another type might be the same on paper, but the physical details differ.
Do your due diligence
Motor design is continually evolving, driven by demands for increased efficiency, improved performance, and reduced costs and environmental impacts. Advances in motor-related control algorithms and power-switching devices mean that previous limitations and conventional wisdom may no longer apply.
A motor that was once an ideal choice for an application might be outdated, so design engineers must do their due diligence.
For example, the classic brushed motor is generally only useful for low-end toys or products requiring little motion control. This type of motor is typically avoided in advanced applications. However, a recent design of a medical infusion pump shows that a modern brushed motor is an ideal choice to meet its performance objectives.
Given today’s advances, it’s no longer appropriate to associate one type of motor with a specific application. Motors come with deceptively simple electrical schematics that rarely exemplify their full features and capabilities. Unfortunately, motor family trees are unreliable as shown in Figure 1 and Figure 2 because they only focus on the motor’s design and fail to include its applications.
Ensuring the ideal choice for an application means considering the motor’s features (including its efficiency, capacity, durability, and control capabilities) and its compatibility with other system components (such as the size, weight, and torque requirements). Additional factors such as maintenance needs, cost-effectiveness, and regulatory or environmental considerations should be evaluated before deciding on a motor.
Figure 1. This family tree is focused primarily on ac motors and shows the many variations in use. (Image: ibiblio.org).
Figure 2. This family tree shows less detail but gives comparable attention to ac and dc-powered motors (Image: Monolithic Power Systems).
Choosing a traction motor
Traction motors differ significantly from industrial motors, including those with similar power ratings. Whereas industrial motors usually power equipment or machinery in enclosed spaces and within a rated or limited range of operating conditions, traction motors are designed for mobile applications.
Traction motors are critical for converting electrical energy into mechanical energy to propel an application (such as an EV) forward.
For this reason, they require efficient power conversion, including:
- High torque for start-up acceleration
- Low speed for the general operation
- Low torque for high-speed cruising and frequent starts/stops
- The capacity and reliability for a range of operations, particularly in terms of speed (Figure 3)
Figure 3. Traction motors can deliver high torque at low speeds for start-up acceleration, along with low torque at high speeds. (Image: Everything PE).
Overall, traction motors must withstand a high rate of acceleration and deceleration, with variations in speed and torque. In most installations, traction motors also lead a “hard life” with respect to shock and vibration, temperature extremes, exposure to dirt and debris, and start/stop operation. They can span tens to several thousand horsepower (roughly ten to several thousand kilowatts). The lowest value might be for an electric forklift truck, which only operates at low speeds and where acceleration is not an issue.
An electric vehicle and a full-power railway locomotive require greater power. A typical EV traction motor is rated at around 200 kW. Of course, these higher power levels necessitate heavy electrical conductors, strong cable portions, and connectors that can endure.
Common applications
As traction motors are used to provide propulsion, they’re commonly used in railway locomotives, including diesel-electric and overhead catenary-powered all-electric locomotives, urban light-rail vehicles (LRVs or trams), and suburban LRVs.
Traction motors are also finding use as electric drives in heavy construction equipment. To power such heavy-duty equipment, these motors are similar to a diesel-electric locomotive with an onboard fuel tank and a fixed-speed diesel engine — driving a generator to power the various motors (Figure 4).
Figure 4. Electric-based drivelines using traction motors are now being used in some heavy equipment, with a diesel-driven generator as the primary electrical power source (Image: ABB).
Most notably, traction motors have become a popular choice in hybrid and electric vehicles. Traction motors offer EVs several advantages. Since they’re designed for high efficiency, they can optimize the conversion of electrical energy from a vehicle’s battery into mechanical energy, maximizing its range and performance.
This has spurred R&D in mid-range traction motors and their driver circuits and components. For instance, traction motors can integrate with a vehicle’s control system (such as its sensors, motor controllers, and software algorithms) to ensure precise speed, torque, and power output control.
What’s more: traction motors can incorporate regenerative braking capabilities, allowing them to act as a generator during deceleration. This feature lets the motor recover and convert a portion of a vehicle’s kinetic energy back to electrical energy, which can be stored in the battery for later use.
Dc versus ac motors
In the early 20th century, when electric traction was first developed, dc and ac motors were tried and tested. Dc motors were favored because they provided the necessary torque characteristic for railway operation and were easier to control then.
For example, the first electric motor was the brushed dc motor. The brushes (spring-loaded contacts) press against an armature extension called the commutator (Figure 5). As the magnetic fields of the stator and commutator fields interact, the commutator rotates, and the brushes “switch” the current direction so the field reverses and continues to push the rotor.
Figure 5. The classic dc-driven motor requires no electronics as it self-communicates the current via the conductive brushes, which alternate the current’s direction and thus reverse the magnetic field. (Image: The Railway Technical Website).
The high current results in strong magnetic fields and high starting torque (turning force), so it’s well-suited for starting a heavy object like a train. Controlling the speed and torque over a wide range, however, is difficult. It was done by manually switching resistors in and out, in series and parallel, to match the applied current and current to the load, speed, and torque objectives.
By the ’80s, power electronics had significantly progressed, and 3-phase ac motors became a more efficient alternative in most cases. They’re far simpler to construct, require no mechanical contacts (brushes) to wear or fail, and are lighter than dc motors of the same power output.
Today, ac motors can be processor-controlled with sophisticated algorithms that improve performance, control adhesion and slippage, and offer several operational advantages. They’re also more reliable and easier to maintain than dc motors. For this reason, most new systems use ac-driven motors.
Although ac motors became the preferred choice for decades, advances in electronics have provided a greater freedom of choice. Ac was once easier to step up and down from a primary power source, such as a generator at the power plant to the desired voltage for the transmission line. But the availability of high-performance solid-state devices, such as insulated bipolar gate transistors (IGBTs, which act as fast switching devices) and thyristors, makes it possible to step up and down dc effectively. For some applications, such as long-transmission lines, dc has advantages.
Understanding ac-driven motors
There are two types of ac motors: synchronous and asynchronous (induction) motors. The synchronous motor rotates by alternating the ac current applied to its windings. It rotates at the same speed as its stator rotating magnetic field.
This is not the case with asynchronous motors, which operate at a speed slightly below the synchronous speed. This speed difference, known as slip, is necessary for the motor to generate an induced voltage and rotor current — creating the torque required for motor operation.
An advantage of the ac motor is that it requires zero brushes since there’s no electrical connection between the armature and the fields. The armature can also be made of steel laminations instead of the numerous windings required in other motors. These features make it more reliable and cost-effective than a dc-based commutator motor.
Ac motors can be single or 3-phase, with 3-phase motors used in bulk or higher power-conversion applications. The 3-phase traction motor is controlled by feeding in three ac currents, which cause a machine to turn. The phases are most efficiently provided by an inverter that supplies the three variable-voltage, variable-frequency (VVVF) inputs with electronic and optimized voltage and frequency variations.
The frequency of its supply determines the speed of a 3-phase ac motor. At the same time, the power must be varied to match the load and torque requirements. Modern electronics, such as the IGBT, make the asynchronous ac drive practical for applications, likeEVs (Figure 6).
Figure 6. The modern 3-phase ac motor is controlled by three variable-voltage, variable-frequency (VVVF) inputs. Although fairly complicated, it offers many operational benefits. (Image: The Railway Technical Website).
Adding permanent magnets
Permanent magnet (PM) motors are electric motors that use magnets in the rotor instead of electromagnets. The interaction between the PMs and the electromagnetic field produced by the stator windings generates the required torque for motor operation.
The PM motor is a 3-phase ac synchronous motor. The typical squirrel cage construction is replaced by magnets fixed in the motor. It requires a complex control system, but it can be up to 25% smaller than conventional 3-phase motors with the same power rating. The PM design provides lower operating temperatures, so cooling requirements are simplified.
Some traction motors use PMs. For example, Tesla uses a combination of motors, including PMs, because the vehicle space is limited (Figure 7). A few of Europe’s 25 AGV highspeed train sets and LRV trams also rely on PM motors, such as those in France and Prague. The reduced size is particularly attractive in low-floor LRVs where hub motors can be combined in a compact bogie.
Figure 7. The choice of traction motor involves balancing many factors and tradeoffs, and a single vehicle may use different motor types for its front and rear axles. (Image: Tesla Motors).
Mounting the motor
With few exceptions, a traction motor is mounted on or in part of the wheel axle that it drives. It’s typically a direct-drive system with minimal or no intervening gearing. This means a reduced parts count in an application. A traction motor’s low weight and small size are also advantages.
In most locomotives, there’s only one motor per axle on the train bogie (the bogie or “truck” is the chassis or framework that carries a wheelset). One of the challenges in railroad design involves offsetting the weight of the motor on the un-sprung wheel axles and placing it on the sprung part of the bogie for better balancing and handling (Figure 8). For lighter-service engines, only one axle is powered.
Figure 8. The traction motor is typically designed to mount on or immediately adjacent to the axle it drives. (Image: The Railway Technical Website).
In other applications, one motor powers both axles of the bogie, referred to as a mono-motor bogie. The ideal design depends on the vehicle size, weight restrictions, required speed, and other factors.
At the controls
Controllers or inverters play a critical role in controlling the operation of a traction motor, which is responsible for driving a vehicle’s wheels or propulsion system. Most are custom-designed to meet application characteristics and ensure optimal performance.
Take an EV, for instance, where a percentage point or two in efficiency is critical due to cooling needs and range objectives.
However, certain standard power and controller units exist for traction motors, depending on the make. Curtiss-Wright recently introduced 100-800 Vdc input/420-kW (at 700 Vdc) inverters for single-motor (CWTI-S420) or dual-motor (CWTI-D420) applications in electric busses, hybrid vehicles, and diesel-electric off-road vehicles (Figure 9).
Figure 9. This power inverter (dimensions shown are in millimeters) can deliver up to 420-kW from 700 Vdc inverters electric busses, hybrid vehicles, and diesel electric off-road vehicles. It embeds advanced algorithms to optimally drive different traction-motor types with high-efficiency and safety. (Image: Curtiss-Wright Corp.).
For flexibility, both models can feed power to various motor technologies, including ac induction, PM motors, and interior permanent-magnet (IPM) types. IPMs incorporate permanent magnets within the rotor core, which creates a magnetic field that interacts with the stator windings to generate torque for motor operation.
According to Curtiss-Wright, its advanced motor-control software runs adaptive tuning to get two percent higher levels of efficiency between the inverter and motor. These traction inverters offer short-circuit and fault protection, as current and temperature are measured directly on the IGBTs. The inverters also use vehicle-grade components certified to AEC Q-100, 101, and 200 standards, adhering to ISO 26262 — an international safety standard for developing vehicle electrical and electronic systems.
Conclusion
Traction motors are an important class of electric motors optimized for high torque at start-up and low speed. They can power small movers such as forklifts and large systems such as all-electric locomotives and electric vehicles.
With the increased development of consumer EVs, several advances have been made in motor performance and design. Modern electronics have made the older brushed dc motor less attractive, replacing them with ac-based motors using IGBTs for power switching under processor control. The ideal choice depends on the application space, design, power requirements, and overall project budget.
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