Traction motor technology for challenging EV applications

Dr. Sab Safi, C.Eng. SDT Drive looks at the role that traction motors could have in the shift towards electric vehicles.

Climate change is one of the biggest challenges facing the world and decarbonising our energy system is a major part of responding to this. Clean transportation has a critical role in the fight against climate change since fossil-fuel-powered motor vehicles, trains and planes account for approximately one-quarter of the world’s energy consumption – and about the same proportion of global carbon-dioxide emissions. These emissions are harmful to both the environment and human health.

Electric vehicles (EVs) can help decarbonise both transport and electricity supply. This is both via reduced tailpipe emissions and due to the flexibility that EV batteries can offer to the electricity system. They offer a source of untapped flexibility that can provide significant benefits to energy systems.

The electrification of road transport is facilitating an unprecedented level of innovation in innovative electric traction Drive technologies. Novel architectures, better materials and advanced manufacturing processes are all being explored in response to the diverse and demanding requirements of the automotive sector. Looking further ahead, it is expected that EVs will start to displace conventional propulsion systems in the next 5-10 years, assuming that they become cost competitive on a total-cost-of-ownership basis.

Since the introduction of electric vehicles, various innovative electric traction Drive technologies have been implemented in commercially available electric vehicles to increase efficiency and power density. It is expected that power density and performance of the traction motor drive will have to be improved significantly for future electric vehicles in order to increase the user space in the vehicle, extend the range and increase market adoption.

Vehicle electric traction Drives, which include electric traction motors and power electronics, are a key enabling technology for advanced vehicle propulsion systems. 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 ultraefficient machines. Additionally, to penetrate the market, these electric traction drive technologies must enable vehicle solutions that are economically viable.

SDT is addressing these challenges head on and is leading the way in developing light weight and energy-efficient motors to ensure every product they develop delivers a more cost effective, higher performing and greener solution for all their target sectors.

Technology Trends in Electric Traction Drives:

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 (Figure 1). 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. An example diagram of an electric drive for an electrified vehicle is shown in Figure 2. 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).

Pathways to achieve these trends include Motor Topology advancements, Advance Power Electronics, Advanced Materials, Winding Design Approach, Design Optimization and Robust Design Method and Modular and Scalable Designs.These have the potential to significantly reduce a motor’s costs or radically improve performance. A review of these strategies and solutions for traction motors is presented below:

Motor Topology advancements: 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.

The induction motor has a long history of successful operation in industrial applications and has proved to be a rugged and reliable machine. Electric motor manufacturers have a great deal of knowledge and expertise in the design and fabrication of induction machines, which can be readily applied to a traction motor design.

SR motors offer a lower cost option that can be easy to manufacture. They also have a rugged structure that can tolerate high temperatures and speeds. Among the key barriers for high volume and low cost SR motors are torque ripple, availability of suitable power convertors and poor noise, vibration and harshness. All of these barriers point at limits for both power electronics and the available topologies. Also, SR motors are less efficient than other motor types, and require additional sensors and complex motor controllers that increase the overall cost of the electric drive system. However, anticipated improvements in the area of power electronic systems and their control will provide step changes.

PM motors are the most popular choice to maximize both efficiency and compactness. Energy efficiency is important for EVs to optimize the operational energy consumptions and maximize the travel ranges. For these reasons, the first class of motor one should consider for vehicle drives should be a PM motor using high performance magnets. With energetically ‘free’ excitation, low fundamental reactance and the ability to have high pole count, PM machines can be extremely light weight and highly efficient.

The segment of medium performance and low power traction motors shows opportunities for alternative (non-PM) topologies. SR and hybrid PM/ Synchronous Reluctance topologies constitute the greatest potential for future step changes in technology. Both of these are however, dependent on advancements in control and electronics at an acceptable cost. In addition, PM motors have a key place in this power range as well. Development of automotive specification and lighter induction motors could also be beneficial for city type vehicles predominantly due to the anticipated cost.

Most of the PM motors used in EV traction are Internal Permanent Magnet (IPM) rotors (Figure 4). 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.

Advance Power Electronics: 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.

Wide band gap (WBG) semiconductors are becoming more popular as they are more efficient, can reduce part count, can withstand higher temperatures than silicon components, and the reduction in switching losses enable higher switching frequency to be considered which could improve the size and efficiency of systems. The ability to operate at higher temperatures can decrease system costs by reducing the requirements for complex thermal management systems. WBGs thus offer a significant opportunity for future cost effective and high performance EV power electronics.

Converters are used to increase (boost) or decrease (buck) battery voltages (typically 200 V to 450 V) to accommodate the voltage needs of motors and other vehicle systems. If the vehicle electric motor design requires higher voltage, such as traction motor, it will require a boost DC/DC converter. If a component requires lower voltage, such as lighting, infotainment, it will require a buck DC/DC converter that reduces the voltage to the 12V or 42V level. Additionally, integrating both the traction motor and power electronics into a single unit is one potential route for OEMs to reduce costs and packaging (weight of external cables) requirements. Research is currently being carried out to improve converters by developing next generation topologies which are more efficient, reduce part count, and enable modular, scalable devices.

Advanced Materials: 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). Since the 1800s, when iron was the only soft magnetic material available, metallurgists, materials scientists, and others have been periodically introducing improved materials. The invention of silicon steel in 1900 was a notable event for soft magnetic materials. Such materials provide excellent manufacturability and adequate magnetic performance. Silicon steel still dominates the global market of soft magnets and is the material of choice for large scale electrical machines. However, its low electrical resistance makes it subject to large losses from eddy currents, particularly as operating frequencies are increased.

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.

Improvements to existing materials are being developed by some, while others are looking at dramatically new approaches. Potentially useful areas include the cost effective introduction of additional materials such as silicon, cobalt, manganese, vanadium or chromium into electrical steels, and tailoring the properties to deliver performance in specific areas in order to more effectively manage flux paths. There is substantial research work being conducted into improved formulations of amorphous metals, with the major focus on increasing the saturation flux density of these materials.

High-efficiency motors are built with high-grade silicon steel, which typically reduces hysteresis and eddy current losses. Reducing the lamination thickness also lowers eddy current losses. The thickness is typically in the range of 0.20 mm to 0.65 mm. Thickness is limited by the cost of manufacturing and the complexity of handling for core building. Development is underway using thinner laminations (0.1mm-0.2mm) and optimising the crystal structure of grain-orientated steels, to improve electrical steel properties.

Silicon steels are available with a variety of surface coatings. These include an inorganic finish with natural oxides, inorganic insulation formed by a varnish applied to the surface of the material, inorganic insulation formed by a chemical treatment applied to the material, and inorganic/organic insulation applied to the surface of the material. Better electrical steel coatings are recognised as a promising area of development and can help in minimising losses and mechanical stress.

Insulation Materials: The insulation materials need to be carefully selected by the designer to prevent undesirable electrical flow. To function properly, the individual copper conductors in stator windings must be completely insulated from neighbouring conductors in the same coil and from the surrounding iron (referred to as ground). Not only must the material withstand the applied voltage, it must also withstand the high temperature of conductors, mechanical and electromechanical vibration forces, moisture, chemicals, and abrasion by dust and dirt.

Insulation must prevent electrical breakdown between components, but it must also be used as sparingly as possible. Less insulation means more space available for copper and better heat dissipation. More copper and better heat dissipation improves maximum output from the motor. So there is currently considerable motivation to make insulation as thin as possible, but that leads to another problem, which is that the fast switching of voltage by the inverter (high instantaneous rate of voltage change (dV/dt) occurs in inverters), that needs to be as fast as possible to minimize switching losses—hastens insulation breakdown and causes capacitively-coupled currents to flow across the bearings. This has led to the development of so-called “inverter-rated” wire for motors, which was originally just a heavier application of the insulating coating.

The two key criteria for evaluating whether a new motor winding insulation is better are its dielectric strength—or the voltage per unit thickness the insulation can withstand—and its dielectric loss coefficient—or how much heating is caused by the flow of AC across it. Improvements in one or both qualities must not compromise the maximum allowed operating temperature or the flexibility of the coating.

As electric traction drives increase in voltage, better insulators that meet the automotive durability and cost requirements are also needed. Improved enamels and varnish systems are required to assist the motor survive 300,000 vehicle miles due to high dV/dt from advanced inverter switching. Clearly, the insulation system need to be designed to meet the reliability and life requirements encountered in automotive environment requirements.

Similarly, an electric motor’s performance can be further optimised through conductive materials. Conventional motor packaging materials (epoxies, fillers, winding insulation, slot liners) can often pose a significant resistance to heat removal from the motor. There is a need to increase the thermal conductivity and reduce contact resistances of several elements in the motor packaging stack-up—thermally conductive epoxies, fillers, as well as winding insulation materials to help with increasing the power density, reducing footprint and cost of the motor while maintaining good reliability.

Winding Methods: 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. Typical examples of recent vehicle stators are shown in Figure 5. 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.

The concentrated windings have shorter end windings leading to lower copper loss and volume than the distributed windings, with the latter typically having longer end turns and consequently higher Joule losses. However, because of the rich harmonic content of its magneto motive force (MMF), a concentrated winding will generate more stator and rotor iron losses as well as more eddy current losses in the PMs.

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).

Improved Modelling, Simulation and Robust Designs Tools- 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.

Electrical and thermal improvements of 30%-50% could be achieved through analytical understanding, more accurate modelling and optimization of motors enabled by high-performance computing. By using advanced simulation models and individuals with extensive experience, the results are quickly available, and the performance is known before the testing starts which shortens lead time and lowers cost.

Modular and Scalable Designs - Scalable components in high volume and at low cost are missing. Standardization of architectures or other means to increase production volumes of fewer component designs. The ultimate reliability of electrification components and systems has far reaching impacts, including on the need for replacement parts and how stocking them might be handled.

To maximize scalability, the range of modular traction motors enables the swap or exchange of component models. This approach focuses on component-based design which can then be selected based on specific client requirements, allowing for diverse and tailored solutions. Traditionally each motor can be custom made to accommodate the individual technical specifications to provide flexible customized and construction and to fit a specific vehicle. This inevitably results in long lead times to accommodate the design, engineering specific components, product-specific supply chain logistics, quality assurance and the creation of new production line facilities.

SDT’s primary focus is incorporating its advanced technology into products aimed at existing commercial markets for electrically propelled vehicles. SDT has developed a modular, high performance, high efficiency, multi-voltage and scalable electric motors with built-in flexibility so that customer-specific requirements can be met using a single modular design. Their scalable approach enables them to deliver premium specification products rapidly and cost effectively that have been optimised to support from low through to high-volume production.

The scalable approach employed focuses on component based design which can then be selected based on specific client requirements, allowing for diverse and tailored solutions to deliver premium specification products rapidly and cost effectively that provide: (i) performance optimisation (ii) to help reduce the number of components (iii) to upgrade the major components of the motor to fulfill their needs concerning space and performance and to meet the requirements for environmental robustness and durability, without significant re-engineering.

SDT’s current and continuously expanding technology portfolio provide solutions for electric vehicles. Designing electric motor architectures that reduce magnet use and using machine topologies that do not use permanent magnets (i.e. induction and switched reluctance SR and hybrid PM/ synchronous reluctance topologies) have also been identified as promising routes for future step changes in technology to reducing the automotive sector’s use of rare earths.

Additionally, assess and develop the knowledge base to support advanced technological solutions (such as modular construction) by improving the power density and efficiency of these machines, thereby giving rise to innovative designs and improvements of cost effective-machine topologies.

Currently SDT is working to extend traction motor products to serve a range of transport needs. The focus is on developing the knowledge and technology (objective quality information) necessary to develop traction motor drive system that enables future load flexibility. Efforts ongoing to further standardize the structure, increase the efficiency and reliability. For example, induction motors hold the promise of being as efficient as the permanent magnet motor, even if the single point efficiency of the induction motor is lower than the single point efficiency of the permanent magnet motor.

SDT aims to strengthen the advantages and minimize the disadvantages with its future advanced traction motor technology products. The current engineering constraints are likely to be pushed upwards enabling higher ratings to emerge in future.