The use of unmanned aerial vehicles (UAVs), also known as drones, is rapidly increasing across a range of mission-critical defense applications. From battlefield information, surveillance, and reconnaissance, to targeting, package delivery, and attack, drones provide the functional advantage of remote operation with minimum supervision and provide access to hard to reach locations or those that create vulnerable positioning for human forces. For example, a drone can be effectively deployed to maneuver through urban landscapes and buildings to monitor enemy movements in real-time surveillance operations without any troop insertion.

With the expanding possibilities drones provide for battlefield advantages, modern armed forces will require technologies to make UAVs faster, more agile, reliable, and robust. Yet drones are unique products that combine several functional challenges: a good vision platform to support the end-user functionality, a strong propulsion system to support the required flight-times and distances, and efficient AI platforms to enable automated operation for maximized precision and productivity.

As metallurgy directly applies to drone motors, Carpenter Technology, a company that specializes in high-performance specialty alloy materials, modeled motor performance with an advanced material solution to investigate improved UAV functionality with the goal of maximizing power and reducing weight. The results showed that a drop-in solution for stator production could translate into tactical, operational, and strategic battlefield advantages.

Drone Design

Figure 1. Drone hardware components

The standard hardware components of common drones are highlighted in Figure 1. Of the three challenges presented by drone designs – vision platforms, AI operation platforms, and propulsion systems – motor technology was identified as the system with the most viable opportunity for improvement. Ever-improving camera technology and smart software platforms address the vision and automation challenges of drone design, so Carpenter Technology’s material experts investigated the propulsion system of UAVs, including the motor, propeller blades, and power management systems. Energy-dense battery technology can optimize performance for faster acceleration and longer flight times while providing the endurance to cover more distance.

Figure 2. Outer rotor permanent magnet synchronous motor

The outer rotor permanent magnet synchronous motor (OR-PMSM) design utilized in UAVs, as depicted in Figure 2, is useful for direct-drive applications. Carpenter Technology modeled the motor for a desired UAV flight cycle to compare performance using a drop-in replacement material in the stator core through finite element analysis.

Stator Core Modeling

As its name suggests, silicon steel is steel with silicon added to increase electrical resistance and reduce its hysteresis loss. Silicon steel is widely used for most drone motor applications these days. The Hiperco® family of iron-cobalt alloys were developed by Carpenter Technology to optimize magnetic induction, exhibit high permeability, and lower core losses. The Hiperco 50 family offers tailored variations for magnetic and mechanical properties to fit an application's requirements; for this model, Hiperco® 50A was chosen because of its superior magnetic properties with the highest DC permeability and lowest AC losses to provide more efficiency to the motor.

Figure 3. The flight cycle used for motor modeling. The Drone motor ran for 60 minutes within a thermal envelope.

For the comparison, stator cores of both silicon steel and Hiperco 50A were utilized, each with a lamination thickness of 0.25 mm. Because the use of soft magnetic materials in the stators does not improve performance enough to be cost-effective, both models’ rotor cores consisted of silicon steel laminations, and NdFeB magnets with 42 MGOe were used as the permanent magnets. The varying stator cores were interchanged, and the motor was modeled between the two materials. The motor modeling was completed with a flight profile to ensure the thermal boundary conditions of the operation. The motor accelerated from 0 to 2500 rpm in five seconds and was then run at a constant speed for 60 minutes (Figure 3). The 60-minute flight time provides a thermal envelope to the motor operation, making the model a realistic drone flight simulation.

Key Properties to Look for in Soft Magnetic Materials

Electrical resistivity: A measure of how easily electrical current can pass through an alloy, the higher the resistivity, the lower the eddy current losses in alternating magnetic field applications. Reducing eddy current losses lowers wasted energy.

Permeability: A ratio of the magnetic induction output (B) to the magnetic field strength input (H) that produced the induction. The higher the permeability, the better the magnetic performance.

Magnetic induction: High values allow the design of electromechanical devices that will function with greater force and efficiency.

Flux density: Also known as saturation induction, high flux density allows the development of a strong magnetic field that enables the designer to maximize the force that can be applied in an electromechanical part.

Performance Evaluation

Figure 4. Magnetic Induction shown in the magnetization curve.

Hiperco has a significantly higher magnetic flux density than silicon steel. Both alloys' magnetization curve shows Hiperco 50A exhibiting a 12% improvement in saturation magnetic flux density. The permeability of Hiperco alloys is also two to four times higher than standard silicon steel alloys. The flux distribution in the stator core delivers a maximum flux density in the silicon steel core in the 1.9 T range. In contrast, the Hiperco 50A core reaches the 2.25 T range (Figure 4). The increase in magnetic flux density helps increase torque. This increase in torque can enable reduced machine sizes, allowing, for example, for smaller drones or more payload capacity.

Figure 5. The continuous power and torque of the motor in the drone operation, which increased by 36% and 25% respectively.

The modeling showed a significant increase in the motor's performance using the Hiperco 50A stator core, with improvement in both continuous power and torque. The continuous power of the Hiperco 50A stator improved 36% compared to the silicon steel core. The continuous torque also increased by 25% at 2500 rpm speed (Figure 5). Both performance improvement metrics enable drones with lower core losses for higher system efficiency with increased agility.

Figure 6. Overall efficiency improvement of the propulsion system from motor efficiency improvement and lower stator current requirement.

The peak torque of the motor also increases significantly with the Hiperco 50A stator core. Influencing how quickly the motor varies the speed of the propeller for responsiveness is key and the higher magnetic flux density, or induction, from the Hiperco 50A magnetic core enables a 16% peak torque increase over the silicon steel stator core. As a drop-in replacement for silicon steel, Hiperco 50A offers more torque for more agile drones with faster acceleration. The Hiperco 50A core-based motor's power density is 16% higher than the silicon steel core model, resulting from the increase in peak power because of the higher magnetic induction.

The power-dense motor with the Hiperco 50A core is also more efficient than the silicon steel core motor. A 2% increase in overall power efficiency and a stator phase current 11% lower was measured utilizing Hiperco 50A cores compared to silicon steel. The torque of a motor is a product of the magnetic flux injected into the circuit. Motors with more magnetic force require less current to produce the same or increased torque. Thus, motors operating at lower currents can benefit from proportional power saving to the battery while the drone is operational, making high thrust, low current drawing motors ideal for drone applications. The overall efficiency of the propulsion system increases by 13%, translating to an increased flight time by eight minutes (for the simulated 60 min flight time).

Conclusion

The modeling analysis found that the Hiperco 50A core-based motor performed substantially better than the silicon steel core stator-core motor, providing the following benefits:

  • 16% increase in power density: Power-dense motors make drones more functional, providing the designer the flexibility to tailor options to add more value through items such as more sensors or improved cameras to meet users’ functionality requirements.
  • 16% improvement in peak torque: Increasing the peak torque of UAVs enables them to be more agile in operation with faster responses.
  • 36% increase in continuous power: Higher continuous power provides drones with more mobility and reliability. The acceleration and speed of drones will continue to increase because of optimization gains in peak and continuous torque.
  • 8-min increased flight time (on a 60-min flight drone): Overall efficiency of propulsion systems increases as motors operating at lower currents save battery power, allowing for more flight time.

The spectrum of performance improvement benefits realized through the drop-in replacement of silicon steel with a Hiperco 50A stator translates perfectly to the warfighter's needs in drone applications. Hiperco 50A equipped drones provide an advantage in speed, range, agility, endurance, and mission payload. Hiperco 50A stators deliver targeted benefits spelled out by the Army's Future Vertical Lift (FVL) plan. Combat Commanders can translate these capabilities into advantages at the tactical, operational, and strategic level that provide the margin for victory on the battlefield.

This article was written by Md Mehedi, Ph.D., Sr. Application Development Engineer, Carpenter Technology (Philadelphia, PA). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the May, 2021 issue of Aerospace & Defense Technology Magazine.

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