Gaining traction in EV applications

Traction drives, including electric traction motors and power electronics, are key to advanced propulsion systems, says Dr Sab Safi of SDT Drive Technology.

Traction motor design is facing challenges in terms of meeting very demanding metrics for power densities and conversion efficiencies, thereby motivating the exploration of advanced materials and manufacturing for next generation of lightweight ultra-efficient machines. Additionally, to penetrate the market, these electric traction drive technologies must enable vehicle solutions that are economically viable.

Basic electric traction configurations: the automotive industry has pursued a wide variety of electric traction configurations to meet various performance requirements. Most common vehicle configurations are single motor via mechanical transmission and independently controlled in-wheel drives (direct-drive or via reduction gear). Historically they focused on hybrid electric vehicles (HEVs), but the focus has shifted to full EVs). The electric traction of an electrified vehicle primarily encompasses the power electronics (inverter, boost converter, and on-board charger), and traction motor used to propel the vehicle. The battery connects to the power electronics (inverter) that then feeds the motor connected to the wheels of the vehicle.

The design and manufacturing of electric traction motors must undergo a permanent development and improvement process to enable large market penetration of electric vehicles.

Technology trends include improving electric motors, with a particular focus on reducing the use of rare earth materials inside the rotor magnet since the magnets account for the largest portion of the motor costs but are essential to meeting the performance targets. Furthermore, the main trends are reducing cost, volume, and weight while maintaining or increasing performance (higher efficiency at higher power) and reliability (a 15 year, 300K mile lifetime).

Many types of electric motors have been used by EV manufacturers, and each has their advantages and disadvantages regarding compactness, efficiency, speed, range and reliability. Three main types of motors are currently used by major EV manufacturers: induction motors, switched reluctance (SR) motors and Permanent Magnet (PM) motors.

Most of the PM motors used in EV traction are Internal Permanent Magnet (IPM) rotors. IPM rotors have magnetic saliency that produces an additional reluctance torque and further increases the power density.

The main disadvantage of using PM motors is the cost of the heavy rare-earth materials used in the PMs. The amount of magnet material that is required for a given power rating is a key cost consideration. The cost of magnet material is high compared with the cost of the other materials used in electric motors and design attributes that minimizes or eliminates the required amount of magnet material are important considerations in motor selection.

Vehicle power electronics primarily process and control the flow of electrical energy in EV. Power electronics are critical for several functions, but perhaps most critical of all is the main inverter, which converts the DC battery into three-phase AC to drive the electric traction motor. At the core of power electronics devices is power-switch technologies (transistors), which have already seen more than five generations of advancement. Development activities in power electronics primarily focuses on improving inverters as they are the key component and have the biggest impact on power electronic targets. These development activities are aiming to reduce inverter volume, reduce part count by integrating functionality, and reduce cost. Today, silicon insulated-gate bipolar transistors (Si IGBTs) dominate the medium power range, including electric vehicle inverters. We are now transitioning to a sixth-generation, with wide-bandgap (WBG) semiconductor materials taking over: The two most commonly used WBG materials are silicon carbide (SiC) for high voltage/power applications and gallium nitride (GaN) for lower-voltage and power.

Mass and size, efficiency and reliability of electric machines are determined and ensured by the properties of the materials used in them. The elements, such as the electrical steels, coils or magnet and insulations can contribute to a significant proportion of the overall mass. Electrical steel (silicon steel, or lamination steel or soft magnetic materials) is a key material in the manufacture of electric machines. They form the cores in electric machines and serve to concentrate and direct the alternating magnetic field which produces iron losses (hysteresis and eddy current losses).

As mentioned previously, the introduction of WBG semiconductors is allowing power conversion electronics and motor controllers to operate at much higher frequencies. This reduces the size requirements for passive components (inductors and capacitors) in power electronics and enables more efficient, high rotational speed electrical machines. However, none of the soft magnetic materials available today are able to unlock the full potential of WBG based devices. This leads material specialists to look for other materials to meet the needs of newer high frequency devices.

The stator carries windings (coils of copper wire) which are inserted into the slots and connected to form the winding so as to form magnetic poles when energized with current. Winding methods including distributed, concentrated and hairpin offer the potential to maximise performance and reduce machine losses. However, each approach must dissipate heat effectively while delivering increased material fill factors.

The stators for traction machines are wound with concentrated windings or distributed windings. The distributed windings can be random wound with strands or bar wound in the hairpin fashion. In recent production vehicles, the hairpin winding design was used, and it is becoming the popular trend. This winding design enhanced slot fill factor more than 80%, reduced end turn length, improved thermal performance and lending itself to a highly automated manufacturing process, compared to the random wound.

To reach the long

term performance and cost targets, winding design methods and manufacturing innovations are permitting the use of higher operating temperatures, better dissipation of heat losses, more effective utilisation of existing materials, and higher space factors (ratio of copper section to total cross section of windings).

Utilising better modelling capability, simulation and design tools will improve performance (higher efficiency at higher power), performance/ weight ratio and could improve existing technologies and accelerate the uptake of new ones. Multi-physics modelling, for example, enables multiple phenomena such as thermal, electrical and mechanical characteristics to be simulated together which is crucial in accelerating the development of individual powertrain technologies.

The temperature rise and thermal design is critical in traction motor applications. Very accurate estimations of the temperature rise of critical parts of the motor are crucial to its reliability. Understanding the key hotspots in a machine allows more targeted thermal management and also using advanced calculation design tools and accurate electric machine datasets (materials, electrical, mechanical thermal etc). With such criteria in mind, we can determine the optimal motor design. By coupling analytical electrical design software, Finite Element Method (FEM) electrical design software and high fidelity 3D thermal modelling it is possible to simulate precisely the temperature distribution in the machine. The primary benefit is the ability to define operating losses with the respective impact on component sizing, temperature, and thermal management at the component.