The move to electric mobility is well past passenger cars.
From Bangladesh to Bangor, Maine, silent, zippy, electric-powered transit buses fill the streets. In recent years, we have seen the transition to electric emergency vehicles, sanitation trucks, and mass transportation as well as first and last mile delivery services.
To make this technology more universal, there’s a dire need for more efficient electric motors. Traditional designs positioned the electric motors similarly to internal combustion engines, connecting the vehicle's wheels through a driveshaft. However, electric wheel hub motors are positioned on the rotational axis of the wheels (or, for bicycles, in the center sprocket), allowing for
- Higher torque
- Reduced size & weight
- Improved efficiency
Thanks to advances in soft magnetic composite (SMC) materials, brushless wheel hub motors are allowing engineers to reshape electric motor design.
What Makes Electric Wheel Hub Motors Viable in Vehicle Design?
There are several reasons why wheel hub motors are becoming a fascinating option for transportation applications, from e-bikes to supercars.
Stability & Safety
From the engineer's point of view, a hub motor offers greater flexibility than most current drivetrain designs. In many traditional designs, a motor transfers torque to an axle that ultimately connects to and rotates all wheels. In a four-wheel drive electric vehicle, you would need two main drive units functioning in unison.
For example, if a driver makes a turn, the outside tire needs to move faster than the inside. In traditional designs, this is accomplished by disengaging the inner wheel drive system through a limited slip differential. However, with four independent wheel hub motors, the exact speed requirements of each wheel can be precisely determined, thus maintaining the safety and stability of all wheel drive (AWD).
Wheel hub motors can drive each wheel individually, resulting in
- Greater stability
- Easier handling
- Smoother acceleration
- Shorter turning radius
- Enhanced traction-control system
Compact Design
Thanks to the relatively compact size of a small wheel hub motor, designers in industries from automaking to lawn care robotics have room to add more components like the aforementioned mounting features—or perhaps generators, fuel cells, or battery packs. If a designer doesn’t need additional components, they could simply roll out a smaller, lighter vehicle with greater battery capacity.
By eliminating the primary “power plant,” an electric hub motor doesn’t bog down the EV with a heavy components like
- Transmission
- Axles
- Differential
This reduction of weight and parasitic losses makes the EV more energy efficient while also reducing noise, vibration, harshness, and cost.
An Almost Perfect E-Mobility Solution...
While electric wheel hub motors appear to check all the boxes that EV designers need to satisfy buyers and government regulations, it’s impossible to ignore some of their drawbacks.
The critical challenge that faces hub motor applications is unsprung weight. This term refers to the mass of all components unsupported by the vehicle’s chassis, including
- Brakes
- Tires & rims
- Wheel hub motor
- Drive gearing (if any)
- Miscellaneous safety & control features
This unsprung weight is what travels up and down over bumps, debris, and potholes as a vehicle tries to follow the road's contours. These components are directly in contact with the road and are not cushioned from road irregularities by the suspension.
An obvious design solution is to reduce the weight of hub-based electric motors—a problem engineers have been grappling with for more than 50 years. However, with soft magnetic composite technology, engineers now have a cost-effective solution for emerging topologies such as the yokeless axial flux design.
SMCs: A Game Changer for Electric Wheel Hub Motors
The electrification of transportation and other recent trends have increased demand for new and improved motor designs. Yokeless axial flux motors have emerged as one of the go-to designs for wheel hub motors. However, utilizing electrical steel laminations in this design is very problematic. Enter SMCs, a technology that enables cost-effective manufacturability of this geometrical design. These iron-based powders are coated with a uniform layer of electrically insulating film, allowing for higher magnetic performance.
The trend of SMCs has continued to grow in AC designs that require high electromagnetic performance and compact shape-making capability. In wheel hub motors with SMC stator materials, the escape of heat energy is mitigated. Because the motor produces less heat, engineers can reduce the size and number of fans necessary to cool the motors. With less energy diverted to cooling fans, more energy runs the motors, creating a more efficient system overall. Further, reducing the number or size of fans reduces the overall weight of the vehicle.
In addition to reducing core losses and vehicle weight, SMCs offer the following benefits:
- Ability to create unique motor shapes such as 3D flux paths
- Greater manufacturing efficiencies like easier assembly and less waste
- Lower copper loading by using a simplified winding scheme for reduced copper use
The Copper Effect
The energy required to produce an EV is more than double that of a similar sized internal combustion engine vehicle. According to the US Department of Energy, the average fully electric vehicle has about 200 lbs (90kg) of copper per vehicle. By choosing a new topology such as the axial flux or yokeless axial flux, manufacturers could reduce total copper usage by a minimum of 25%! When considering the energy required for copper production, from mining to smelting and wire drawing, this savings can significantly reduce the overall energy used to build an EV.
Why SMC Wheel Hub Motors Are the Future of E-Mobility
Due to their adaptability, wheel hub motors are an efficient alternative to traditional motors for compactness and high torque. We’re starting to see the fruits in shop floors everywhere. Wheel hub motors are being used in several types of electric vehicles, including
- Cars
- Skateboards
- Motorcycles
- Scooters
- Bicycles
Until now, wheel hub motors have remained an elusive goal. However, SMCs offer numerous benefits that improve performance and make electric mobility more affordable to all.
To get more free education on designing a more efficient electric motor, head to our resource page.
