Traction motors are gaining increased relevance due to the significant growth in electric and hybrid vehicles. While a standard electric motor and a traction motor share the same underlying principles of operation, traction motors are specifically designed to drive a variety of vehicle types – from light-duty carts to locomotives and airplanes. Some EVs may even use two, three, or four traction motors.

Such applications require a motor capable of delivering high torque and low speeds, lower torque and high speeds, and handling frequent stop-and-go cycles. These demanding duty cycles require a motor that is, frankly, built differently.

Traction motors: Tuned for torque

Traction motors operate on the same basic principles as standard AC or DC electric motors, which are among the most common types used in industrial and consumer applications. Electrical charge is delivered to copper windings in the motor stator, creating an electromagnetic field that induces rotation in the motor’s rotor.

However, there are notable differences in how traction motors are designed and constructed compared to typical electric motors. Since traction motors are tasked with initiating vehicle movement from a standstill, their design emphasizes torque across a range of RPMs. Increasing the number of windings creates additional poles in the motor, allowing manufacturers to generate a stronger magnetic field and lower RPMs. Torque can also be optimized by material selection – stronger permanent magnets integrated into the rotor will create a more powerful reaction to the magnetic field.

Engineers, particularly those designing traction motors for EVs, where power density is a key concern, also focus on increasing the efficiency of their circuit designs to ensure the maximum amount of electricity delivered to the motor is converted into kinetic energy. One technique for doing this is embedding magnets between windings, which redirects potential magnetic flux back to the rotor, thereby increasing rotational energy without additional current or voltage.

After the vehicle has started moving, the traction motor's duty cycle more closely resembles that of a standard electric motor, requiring higher RPM but lower torque.

Differences in applications, differences in design

Since traction motors are used in cars, trains, planes, and more, their designs must account for operational challenges like dirt and debris. Totally enclosed designs can keep out common contaminants but must also consider thermal management, which can be achieved through fan cooling or, in applications like locomotives, by integrating a heat exchanger into the enclosure. Vented enclosures may be suitable for light-duty applications where the vehicle isn’t expected to encounter harsh outdoor conditions.

Like standard electric motors, traction motors are often rated for continuous or peak usage. Standard electric motors are typically classified according to NEMA ratings, which govern the permissible temperature rise between the ambient temperature and the average temperature of the windings under full operating load. Traction motors often have continuous ratings or, depending on the application and industrial standards, one-hour ratings. These ratings also dictate the maximum temperature rise, and in the case of one-hour ratings, a peak, temporary rating should the motor need to exert more torque.

Cold temperatures can also affect motor performance, particularly by decreasing the viscosity of lubricants in bearing housings. Cold temperatures can also make certain plastic or elastomer components brittle, such as seals, wire insulation, or fans. Selecting resistant materials can help, though these components should still be regularly inspected.

Shock and vibration also require discrete engineering, although these techniques are similar to those used for standard electric motors. Vibration-dampening materials may help isolate the motor from the vehicle chassis. Using high-reliability interconnects and surface-mount components can help prevent electrical issues caused by rattle or shock. Regular maintenance and inspections can help identify unbalanced motors.

Motor mass is another critical consideration – each additional pound of weight reduces range, and each additional inch of cubic space reduces payload or increases vehicle size. As a result, motor power density becomes more significant, as it involves delivering the optimal torque for the size and weight of the motor.

Regenerative braking and traction motors

Traction motors are often part of a regenerative braking system, where the stator’s electric fields are reversed, and the current is induced into the windings and fed back into a battery management system. These systems are less common in standard and economy EVs but may become more prominent in the future. Regenerative braking also extends the life of wear parts like brakes.

Summary

Standard electric motors cannot meet the demands of heavy loads, high torque, stop-and-go cycles, and the efficiency required for transportation applications. As a result, traction motors are becoming increasingly important in the electrified transportation market, where their specialized design meets the unique challenges of modern vehicles.