Electric Vehicle Efficiency: Beyond Battery Capacity, Speed is a Key Factor
Automakers are engaged in a competitive push to improve electric vehicle (EV) performance, focusing not only on extending range per charge but also on addressing a less-discussed factor: motor speed. Increasingly, manufacturers are developing motors capable of exceeding 30,000 revolutions per minute (RPM), a trend that, while boosting power density, introduces new challenges related to energy consumption and durability.
The focus on motor speed comes as regulators are also taking steps to improve EV efficiency. China, for example, implemented stricter consumption standards on , based on vehicle weight and category, with maximum limits set for energy use. For a compact 2-ton electric SUV, the proposed limit is 15.1 kWh/100km, calculated using the CLTC standard, which is more lenient than the European WLTP and North American EPA standards. These new targets, representing an eleven percent increase in stringency compared to previous recommendations, aim to incentivize manufacturers to improve efficiency through battery technology and design rather than simply increasing battery size.
A key aspect of this regulatory push is controlling the rotational speed of electric motors. Beyond environmental concerns, high-speed acceleration and sustained high velocities pose safety risks, as not all drivers are equipped to handle the substantial power output of these vehicles. Energy consumption increases significantly at higher speeds and during intense acceleration. Authorities in several countries are exploring the legal framework for implementing technical limitations.
This principle of limiting speed for efficiency and safety isn’t new; internal combustion engine (ICE) vehicles typically have their top speeds electronically limited to 250 km/h, a practice largely based on a tacit agreement among European manufacturers.
The Trade-offs of High Motor Speeds
While increasing motor speed can enhance performance, it’s not without its drawbacks. In an ICE vehicle, a higher maximum RPM generally translates to greater power output, allowing drivers to maintain speed before shifting gears. Power is a function of both RPM and torque. However, the dynamics differ significantly in EVs.
Electric motors already operate at significantly higher RPMs than their gasoline counterparts. While a typical gasoline engine peaks around 5,000-6,000 RPM, average electric motors commonly operate between 10,000 and 15,000 RPM, covering the entire vehicle speed range without the need for multiple gears. This inherent characteristic has driven manufacturers to further increase motor speeds to achieve greater power density and compactness.
Tesla and Lucid were early adopters, launching the Model S Plaid and Air in with motors capable of reaching approximately 20,000 RPM. By , companies like BYD, Xiaomi, and GAC announced plans for even higher-speed electric motors, exceeding 30,000 RPM.
The pursuit of higher RPMs is facilitated by advancements in compact, high-power motor designs. If a motor delivers the same power output in a smaller package, material costs decrease, and the reduced size and weight free up space within the vehicle for passengers, cargo, or other powertrain components, such as hybrid systems.
According to data from IDTechEx, increasing the maximum RPM of radial flux permanent magnet (PM) machines from 10,000 to 20,000 RPM increased power density by 69 percent. Further increasing it to 30,000 RPM resulted in another 41 percent increase.
Generally, increasing the maximum RPM also increases the motor’s power density.
However, these gains come with challenges:
- Increased Energy Consumption: Higher RPMs inherently lead to greater energy consumption.
- AC Losses: Electric motors are typically powered by three-phase alternating current (AC) in the stator windings. Faster motor speeds increase the frequency of the AC current, leading to increased parasitic losses in both the stator windings (copper AC losses) and the laminations (iron AC losses).
- Rotor Structure: At higher speeds, the centrifugal forces acting on the rotor increase, posing challenges to its structural integrity.
- Cooling: Managing heat becomes more difficult as components become more compact.
- Gearing: Higher motor speeds necessitate greater gear reduction to achieve the desired wheel speed. Adding more reduction stages increases cost, complexity, and weight.
- Bearing Stress: Bearings are subjected to greater stress and frictional heat, and any imbalance in the rotor directly translates into dynamic forces on the bearings.
Potential Solutions
Several approaches are being explored to mitigate these challenges:
- AC Losses: Reducing the number of poles can lower the required frequency. Using thinner laminations or amorphous materials can also help.
- Rotor Structure: Reducing rotor diameter allows for higher speeds, reducing centrifugal forces. Greater engineering effort is being devoted to rotor structural design and magnet geometry. Some manufacturers are even exploring carbon fiber coating for the rotor to enhance its strength.
- Cooling: Direct oil cooling, bringing the coolant closer to heat-generating components, is becoming increasingly common. While this adds complexity, it can eliminate the need for water jackets used in previous designs.
- Gearing: Minimizing the need for additional reduction stages requires very small pinion gears in the first stage.
- Bearings: Increased engineering focus is being placed on bearings, with ceramic (or hybrid ceramic) bearings becoming a more frequent solution.
a balance must be struck between the potential performance and cost benefits of smaller, higher-speed motors and the added complexity and cost of addressing the associated challenges. A significant portion of the EV market is likely to retain moderately-speed motors, while the adoption of high-speed motors, electric drive units, and eAxles will continue to grow.
IDTechEx forecasts that more than 140 million electric motors will be needed in the EV market by , spanning segments such as cars, trucks, buses, two-wheelers, three-wheelers, microcars, and light commercial vehicles.
