Engineers who are tasked with designing and specifying electric motors for new and updated applications are now facing three interdependent challenges: optimizing operating efficiency, simplifying the manufacturing processes to reduce cost, and addressing the emerging concern of recyclability and sustainability.
Traditionally, the radial flux motor design has been the go-to solution for almost all applications. However, recently, axial flux motors (AFMs), trapezoidal radial flux motors (TRFMs), and transverse flux motors (TFMs) are gaining traction in many new electric motor designs. Given the demands of these non-traditional motor types, standard lamination assemblies are often limiting. Powder metallurgy—and specifically soft magnetic composites (SMCs)—enable these new motor topologies while solving new age design challenges.
Radial Flux Motor
The radial flux motor utilizes magnetic flux in the radial direction of the motor. The copper windings are wrapped around slots, and the corresponding flux is generated perpendicular to the axis of rotation.
As an example, traditional radial flux motors can be either induction motors or rotor-mounted permanent magnet motors. In either case, the stator has “teeth” that are wrapped with copper wire in the length direction of the motor, thus generating the field and the corresponding motor torque. The current in the stator’s copper wire is alternating, thus shifting the magnetic polarity of the teeth to create rotational energy.
Trapezoidal Radial Flux Motor
Known for its unique design and efficiency, the trapezoidal topology offers a new spin on the traditional radial flux motor. Unlike traditional motors, which rely on sinusoidal magnetic flux, this motor uses a trapezoidal waveform for its magnetic flux distribution. The radial flux design means that the magnetic field radiates outwards from the rotor, which is the rotating part of the motor, toward the stator, the stationary part. Low-cost manufacturability of this stator and rotor design is possible only with soft magnetic composite (SMC) materials and the powder metallurgy process.
This motor topology is ideal for high torque and efficiency in EVs, marine applications, drones, and industrial automation. The trapezoidal flux distribution optimizes magnetic field use, reducing energy loss and boosting performance while enabling a compact, lightweight design that’s perfect for tight spaces.
Image courtesy of EVR Motors
Axial Flux Motor
The AFM differs from the RFM in that the magnetic flux is generated parallel to the axis of rotation. (The right hand still applies, specifically, the resulting torque vector is still tangent to the rotor). Although this type of motor is far from new, its use has been limited due to its difficult manufacturability and high costs when utilizing lamination steels. SMCs allow the designer to fully exploit the axial topology, which will drive more widespread utilization of this design.
FUN FACT
When exploring the axial flux motor design, Nikola Tesla knew manufacturability would be the biggest hurdle to overcome—and envisioned the design being made from an iron powdered material! 100+ years later, SMC materials are making Tesla’s vision a reality.
Compared to RFMs, AFMs offer higher power density, simplified stator winding with shorter magnetic paths, and higher power density per unit weight of the motor. As with RFMs, the AFM utilizes commutation of the stator current to create the rotational movement of the rotor, thus enabling torque. The inherent shorter path length of AFMs enables greater utilization of the copper windings and can simplify windings using pre-wound bobbins.
Transverse Flux Motor
The TFM utilizes a copper winding that radially wraps the copper wire around each stator tooth, as shown in the figure below. This design enables a 3D flow of magnetic flux through the stator—both circumferentially through the rotor and radially through the gaps. This design enables an increase in low-speed torque and efficiency. The TFM design is uniquely suited for SMCs, which reduce its cost by simplifying its manufacturability.
Image courtesy of Linear Labs
Energy Efficiency
The vast majority of RFMs are less than 5 HP. Traditionally, these motors have operating efficiencies in the 80–88% range. As we noted in an earlier article, increasing this energy efficiency by 3 to 5% in household and commercial applications alone can result in a 1–2% reduction in total energy usage across the US. Switching to either an AFM or TFM design enables this increased efficiency while simultaneously increasing manufacturability of these non-traditional low-horsepower electric motors.
Manufacturability
With traditional RFMs, the magnetic flux is carried radially in the motor. This is the preferred orientation of laminations. Thus, traditional RFMs are well suited to a laminated steel design. Stamping of the stator and rotor can be accomplished in the same progressive stamping die setup while maximizing material utilization. This aspect of RFM manufacturing facilitates low cost and ease of manufacturing. However, material usage in this process is not as high as with SMCs, which allow for 98% material utilization.
The AFM, TRFM, and TFM designs, on the other hand, result in a magnetic flux path that is not conducive to the radial flux motor stamping practice. Unless the designer is willing to sacrifice magnetic path efficiency or significantly increase manufacturing complexity, then an alternative approach is needed. SMCs allow engineers to create the complex 3D shapes these machines demand without compromising the magnetic flux paths or creating a manufacturing quagmire that sends costs soaring.
Recyclability & Sustainability
RFMs that utilize laminations are very difficult to effectively recycle (see Figure 1). The copper is integral with the stator, making it very difficult to remove in many cases. Often, lower horsepower RFMs are simply discarded without any attempt to recycle them. Because of the high cost of steel lamination, however, higher horsepower machines (>10 HP) are often re-wound. This means that in a large percentage of low-horsepower RFMs, both the copper and steel wind up in the trash. Because high copper levels in steel result in hot shortness—a significant duration in workability of the steel ingots—recycling motors in conventional steel production is not a preferred alternative.
The loss of copper has significant impacts. Although copper is seemingly readily abundant, the process of producing it from ore is quite expensive and environmentally unfriendly. Copper is often strip mined and then transported long distances for refining. If you want to get a picture of the cost of this process, simply look at the London Metal Exchange pricing of copper; it might shock you.
How can this situation change? Currently, only 30–35% of all virgin copper is recycled. Although the EV demand has plateaued for the moment, the eventual widespread transition to EVs is inevitable. However, this comes with some potentially unforeseen implications. Currently, the average internal combustion vehicle contains around 40 pounds of copper, whereas a pure EV can contain up to 200 pounds of copper. One estimate suggests that the amount of copper required to maintain current trends—let alone fulfill a 100% transition to EVs—would exceed all the copper mined throughout human history. Not only could this create a worldwide shortage, but the environmental impact could far exceed that of the internal combustion engines they replace.
It’s obvious that copper recycling must increase substantially. Thankfully, SMCs create an opportunity to easily recycle copper windings and steel stators using a simplified hammer mill grinding of the SMC motor. The inherent friability of SMCs facilitates nearly 100% recovery of the copper, steel, and magnet material without the intrinsic high labor intensity of accomplishing the same for laminated steel motors.
Additionally, the SMC approach to producing the stator and rotor offers greater manufacturability and increased material utilization, resulting in reduced manufacturing costs. The cost savings of an SMC design is further augmented by the lower operational costs achieved with greater efficiency. In these ways, SMCs help to reduce waste, increase recycling opportunities, and lower long-term operating costs—a win-win-win!
