(Source: Volodymyr Herasymov / stock.adobe.com)
Traction electric motors are widely used in railway locomotives, automobiles (fully electric or hybrid), and other vehicles that need to be mobile. The general requirements of traction motors are significantly different from motors for industrial motors, even of similar power ratings. Traction applications often require frequent stops and starts under a full load as well as high-speed operation.
These requirements translate to high torque at low speed for start-up acceleration, low torque for high-speed cruising, frequent starts and stops, and a very wide speed range of operation (Figure 1). In contrast, industrial motors are generally optimized for a more limited range of operating conditions, operate in enclosed spaces (even if they are in a harsh environment), and are immobile.
Figure 1: Traction motors can deliver high torque at low speed for start-up acceleration and low torque at high speeds. (Source: Hayrettin Gökozan;[1] redrawn by author)
Traction motors are not a unique type of motor architecture. In fact, a traction motor can be built as almost any type of AC or DC motor. It is not the motor type that makes a motor into a traction motor (although some types are better suited for traction than others) but the way the motor is built. The electrical schematic and basic mechanical diagram of a traction-optimized and a non-traction motor may be the same "on paper," but the physical details of their construction are very different concerning wire gauge, coil windings, and other build attributes.
Traction motors are used in railway locomotives (e.g., diesel-electric and overhead catenary-powered all-electric), urban light rail vehicles (LRVs), and suburban LRVs. They are also used in automotive electric vehicles (EVs), an application that has spurred advances and innovation in mid-range traction motors, their driver circuits, and related components.
The power levels of traction motors span tens of horsepower to several thousand horsepower (roughly ten to several thousand kilowatts). Traction motors can be DC series-wound motors, single-phase AC series-wound motors, or multiphase AC drive motors. The choice is a function of many factors, but technical advances and the advantages of modern motor-control electronics have pushed most new designs to multiphase AC drive motors.
With few exceptions, the traction motor is mounted on (or as part of) the axle of the wheel assembly it is driving as a direct-drive system with minimal or no intervening gears or linkages. This minimizes size and weight while reducing the parts count and thus enhancing reliability.
The operating environment for traction motors is challenging. In most installations, these motors lead a hard life with respect to shock and vibration, temperature extremes, exposure to dirt and debris, and start/stop operation. Further, their relatively higher power levels necessitate heavy electrical conductors, cable portions, and connectors.
In the early days of electric traction systems—the first decades of the 20th century—operators tried both DC and AC motors. At that time, the technology (obviously very crude by our standards) favored the DC motor, as it provided the needed torque characteristic for tram and railway operation and was amenable to control, even if it was not simple to implement. The first electric motor was the brushed DC motor, and large and small versions are still in use. The brushes—spring-loaded contacts—press against an extension of the armature called the commutator. 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 thus continues to push the rotor. The high current results in strong magnetic fields and high starting torque (turning force), which is well-suited for starting a heavy object like a train. However, controlling the speed and torque over a wide range is difficult and was done by manually switching resistors in and out, in both series and parallel, to match the applied current and current to the load, speed, and torque objectives.
By the 1970s and early 1980s, power electronics had progressed to the stage where three-phase AC motors had become a more efficient alternative to DC motors because:
In general, most new systems do not use DC brushed traction motors due to control and maintenance issues and the high performance that can now be achieved using AC-driven motors.
There are two types of AC motors: synchronous and asynchronous (induction). The synchronous motor is "encouraged" to rotate by the alternating AC current applied to its windings. The AC motor has no brushes since there is no electrical connection between the armature and the fields; the armature can be made of steel laminations instead of the large number of windings required in other motors. These features make building an AC motor more robust and cheaper than a DC-based commutator motor.
Modern electronics, especially the insulated bipolar gate transistor (IGBT) for higher-power installations, make the asynchronous AC drive practical. The speed of a three-phase AC motor is determined by the frequency of its supply, but the power must be varied to match the load and torque requirements. A three-phase traction motor is controlled by feeding in three AC currents, which cause the rotor to turn. The three phases are most easily provided by an inverter that supplies the three variable-voltage, variable-frequency (VVVF) inputs, with voltage and frequency variations electronically controlled and optimized.
Traction motors can use either AC or DC as a higher-voltage primary-line power source. Of course, this high-level line voltage source needs to be stepped down and re-converted to the voltage/frequency required by the traction motor, regardless of type.
For many years, AC power delivery (distinct from the type of motor) was preferred for the primary-side source because it was much easier to step up/down from the power source (i.e., a generator at a power plant) to the desired voltage for the transmission line. However, the availability of high-performance solid-state devices such as IGBTs and thyristors makes it possible to effectively step up/down DC via inverters and conversion subsystems, and there are long-transmission line advantages to using DC.
In cars, traction motors use DC power as the primary source. Again, this must be inverted, stepped up/down, and transformed to AC for the motors. EVs, in their many forms, provide a huge market for traction motors. Each EV vendor has technical, market, and cost reasons to choose a specific motor arrangement. For example, Tesla uses a combination of motor types and power ratings in their Model Y.[2]
Power sources and controllers for traction motors can be standard, off-the-shelf units or custom-designed units. For applications such as automobiles, they are custom-designed for the required performance and other specifics, as the production volumes are high enough to absorb the engineering cost while achieving optimal performance across multiple parameters (e.g., size, weight, power, form factor, packaging, connectors).
Traction motors are an important class of electric motors. Among their attributes, they are optimized to provide high torque at start-up and low speeds and can be implemented using different basic motor arrangements. They power small mobile vehicles such as forklift trucks, medium products such as EVs, and large systems such as all-electric and diesel-electric locomotives.
With the increased development of consumer electric vehicles (both pure battery and hybrid), they are also seeing many advances in performance and form factors. Modern electronics have made the older brushed DC motor less attractive and supplanted, in many cases, with various AC-based motor types using IGBTs and MOSFETs for power switching under processor control.
Sources
[1] https://doi.org/10.31590/ejosat.699699 [2] https://www.tesla.com/ownersmanual/modely/en_cn/Owners_Manual.pdf
Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.
At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.
Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.
He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.