The Diverse World of Electric Motors: Exploring Torque and Power Delivery
Any car enthusiast can attest to the distinct characteristics of their favorite engine. From the torque-peakiness of an Eighties turbo car to the rev-hungry climb of a naturally aspirated Honda, or the gut-wrenching low-end torque of a Detroit V8, combustion engines have a variety of styles when it comes to delivering power. This diversity is one of the reasons why the automotive world is so captivating. However, with the rise of electric vehicles (EVs), there is a common concern that this variety will be lost. After all, isn’t an electric motor just an electric motor?
It turns out that electric motors, like many other components in EVs, are not all created equal. The way they are constructed can significantly influence their torque and power delivery characteristics. While auto journalists may not yet be extolling the virtues of electric motors like they do turbochargers or V-8s, that may change in the future as motor technology continues to advance. The motors of tomorrow promise to be lighter, more powerful, and even more thrilling to drive.
To grasp how electric motors can differ, it’s important to understand their general principle of operation. The primary goal of an electric motor is to convert electric power into mechanical energy. While there are numerous ways to achieve this, most electric motors rely on two core components: a stator and a rotor. The stator is a stationary magnet, while the rotor is a rotating assembly that is magnetically attracted and repulsed by the field of the stator. By manipulating the magnetic field inside the motor, the rotor is set into motion. The rotation of the rotor is what propels electric motors, including those used in cars.
The earliest electric vehicles, predating the Tesla Roadster and Nissan Leaf, used a simple type of electric motor known as brushed (direct-current) motors. These motors featured a wire coil directly on the rotor and magnets on the stator. The rotor’s rotation was achieved by reversing the electrical current to the wire coil, altering its electromagnetic field to continuously oppose the magnets on the stator. Reversing the current was facilitated by brushes, which made contact with a component called the commutator on the stator. The commutator reversed the current direction, flipping the electromagnetic field and allowing the rotor to keep spinning. Brushed motors operated using direct current (DC), which aligns with the native energy delivery of EV battery packs.
According to electric motor manufacturer Parvalux, brushed motors offer high initial torque and are relatively inexpensive due to their simplicity. However, they have drawbacks such as the physical contact between the brushes and the motor, leading to friction and reduced efficiency. This friction causes the brushes to wear out and require replacement. Additionally, the heat generated limits the top sustained speeds that brushed motors can achieve. These factors led to brushed motors falling out of favor for automotive applications until BMW introduced its eDrive drivetrain, which revived interest in brushed motors. BMW’s AC brushed motor design uses alternating current and offers improved efficiency and power-to-weight ratio compared to traditional brushed motors. In their latest iX M60 model, twin brushed motors deliver an impressive output of 610 horsepower and 811 pound-feet of torque.
The other major type of electric motors is brushless motors. These motors do not rely on brushes or physical contacts to switch the direction of the current. Instead, they shift the magnetic field without contact. Brushless motors can be categorized into direct-current (DC) brushless motors and alternating-current (AC) brushless motors. While older hybrid vehicles employed DC brushless motors, most modern electric cars utilize AC brushless motor designs.
AC brushless motors, such as permanent magnet synchronous motors (PSM) and asynchronous induction motors, operate by rotating the magnetic field of the stator using AC power. Both the stator and the rotor of an induction motor consist of wire windings. AC power supplied to the stator creates a rotating magnetic field, inducing current in the rotor. The current in the rotor generates its magnetic field, which opposes the stator’s field, resulting in rotation. The magnetic field shift of the stator always outpaces the rotation of the rotor, explaining the term “asynchronous.” Induction motors are well-proven designs, dating back to Nikola Tesla’s early work. They are cost-effective and straightforward to manufacture. However, they have lower peak efficiency compared to other motor types and tend to be bulky and heavy due to the wire windings required.
Induction motors have found widespread application in Tesla vehicles, including the original Roadster and the front axle of their latest all-wheel-drive Model Y. While induction motors are still utilized, the automotive industry has mostly shifted towards permanent magnet synchronous motors (PMSM) for electric cars. PMSMs use permanent magnets for the rotor and supply current solely to the stator. The rotation of the magnetic field is synchronized with the rotor’s spin, giving them the “synchronous” designation. Permanent magnet motors offer excellent torque from the start, weigh less, and are more compact than induction motors. This is why nearly all recent electric cars, including Tesla’s lineup, have adopted permanent magnet motors. The main challenge with permanent magnet motors lies in procuring the rare-earth metals required for their construction, as these metals are expensive and primarily sourced from China.
Looking ahead, the next major development in electric motors is likely to be the transition from radial flux motors, which are used in most designs today, to axial flux motors. Radial flux motors generate a magnetic field that moves radially, whereas axial flux motors position the conductors radially, with the field moving along the rotor’s axis. In axial flux motors, the rotor is no longer within the stator; instead, the two components function as parallel disks. Although the concept of axial flux motors has existed for centuries, the strength of the magnets necessary for industrial-scale implementation was not achievable until recently.
As magnet technology improves, axial flux motors have become feasible, even though they have not yet become mainstream. Koenigsegg’s Gemera model utilizes a hybrid radial/axial flux motor called the “Quark” drive unit, which boasts remarkable power density. YASA, an electric motor manufacturer now owned by Mercedes-Benz, claims that their motors offer four times more torque and double the power densities of radial motors while weighing half as much and being 20% shallower. These advancements are exemplified by Rhys Millen’s Pikes Peak win in 2015, where YASA motors powered the victorious vehicle. Axial flux motors are not only more efficient but also require significantly fewer rare-earth metals for manufacturing than their radial counterparts. As costs decrease, more manufacturers may start incorporating axial flux motors into consumer-grade EV drivetrains, potentially leading to the displacement of radial-flux motors and inspiring nostalgia for their classic design.
In conclusion, the coming wave of electric vehicles does not spell the end of diversity in torque and power delivery. Electric motors, despite being an electric motor at their core, can be vastly different based on their construction. Brushed motors, which fell out of favor for decades, are making a comeback with advancements in AC brushless technology. Permanent magnet synchronous motors have become the dominant choice for most automakers due to their excellent torque and compact size. Looking forward, the industry may witness a shift towards axial flux motors, which promise even greater efficiency and reduced dependence on rare-earth metals. The world of electric motors is evolving rapidly, and it’s an exciting time for both enthusiasts and automotive manufacturers as they explore new possibilities in electric powertrains.