Thought Leadership: Magnetic propulsion – A Game Changer for eVTOL Efficiency and Reliability

Efficiency and reliability remain two of the biggest challenges for the development of viable eVTOL aircraft.

Gary Rodgers, CEO of Magnomatics, explains how scalable magnetic motor systems address these challenges while providing manufacturers with versatility and cost-effectiveness.

A novel approach to propulsion

With urban roadways unable to cope with growing traffic volumes, eVTOL aircraft promise an effective way to combat congestion while providing transport that is quick, quiet, clean and green.

For that promise to be realised, the development of these aircraft requires detailed planning and meticulous research. During this process, countless prototypes are created and analysed to ensure that an eVTOL’s design is safe, reliable and energy-efficient and that it complies with regulations and meets the needs of potential buyers.

With regards to propulsion, focusing on energy-efficient alternatives is vital for delivering aircraft that offer manufacturing, operational and maintenance cost savings. Compared to standard systems, magnetically geared motors are substantially more compact, lightweight and energy efficient.

These attributes not only offer energy-saving propulsion during flight, but they also enable manufacturers to build highly efficient eVTOL aircraft. Moreover, being scalable, they offer exceptional versatility for eVTOL development as they can be used in various applications, including small-scale eVTOL projects, like drone deliveries, personal air taxis and emergency services.

For developing commercially viable aircraft, magnetic motors offer important benefits for manufacturers. They reduce manufacturing costs due to their easy installation, compatibility with standard electronics and the need for fewer components; maintenance costs are lowered because of built-in safety and reliability features; and operating costs are minimised by the motor’s excellent energy efficiency.

This efficiency, together with their quiet operation and the need for fewer raw materials during manufacture, means magnetically geared motors are intrinsically more sustainable than traditional systems.

Issues with permanent magnet motors

Permanent magnet (PM) motors are the conventional way to generate the required power density in electric propulsion. However, when utilised in eVTOL applications, the PM motor output speed, which can exceed 10,000rpm, has to be reduced to 1/7 of the original speed.

While mechanical gearboxes are the traditional way to reduce speed, there is the potential that an undetected single-point failure can cause rotor jamming and result in a disastrous loss of propulsion power. This means mechanical gearboxes are completely unsuitable for eVTOL applications where safety is critical, especially for passenger flights and transport over urban areas. As a result, both passenger and non-passenger-carrying eVTOLs require safer, more reliable propulsion systems.

Maintenance is another concern for mechanical gears. Prone to physical wear and failure, regular physical inspections will be a necessity, and worn parts will need replacing more frequently. This will result in higher maintenance costs and increased downtime, making eVTOL aircraft that use mechanical gears less marketable to potential buyers.

A final issue with mechanical gears is that they are noisy. This noise not only affects passenger comfort but can also result in noise pollution, especially when the aircraft are taking off, landing and flying at relatively low altitudes over urban areas. There is the
potential that noise levels could result in aircraft failing to meet current or future regulations, resulting in noise certification or licences being refused.

Noise regulations for eVTOLs are still evolving and the European certification regulator, EASA, has drawn up the first
proposals for assessing noise generated by eVTOL aircraft.

An innovative approach for eVTOL design

Given the issues with PM motors, magnetic motor systems offer a feasible alternative that eradicates the need for mechanical gears and provides a range of other advantages for eVTOL aircraft.

An innovative design features a concentric combination of a magnetic gear and a permanent magnetic motor. With the magnetic gear mounted inside the stator and the gear’s outer magnetics attached to the stator’s inner bore, this enables the stator’s copper windings to drive the magnetic gear’s inner rotor.

This design is ideal for eVTOL applications that demand high torque and low speed, as the system’s topology maximises the propulsion system’s torque capability. Surpassing the limitations of traditional direct-drive electrical machines, the magnetic gear delivers the same uplift in output torque as a single-stage mechanical gear.

In addition, with a fully integrated central gear element, the dual use of the inner rotor, combined with the slight increase in
mass from the outer magnets and pole-piece rotor array, results in a lightweight, compact design.

Crucially, the design also improves safety and reliability. The lack of magnetic gears means it is less susceptible to failure than an equivalent mechanical gear and full load torque can be maintained following a phase-bank failure.

Additionally, the thermal, electrical, mechanical and magnetic isolation of phases and banks delivers full duplex redundancy, averting fire and limiting the impact of electrical shocks, while the fault-tolerant motor acts as a passively resettable torque fuse, removing the need for a clutch or shear pins if jamming occurs.

When developing a reliable, energy-efficient and lightweight electric motor, effective thermal management is vital. Heat transfer paths can vary considerably, depending on the motor’s engineering parameters, thermal interfaces and component materials. Rather than an electric-powered fan, a more appropriate alternative for eVTOL aircraft is Totally Enclosed Forced Ventilation (TEFV).

With an eVTOL’s rotor load being approximately proportional to the square of the rotor speed, a lighter and more compact centrifugal fan can be mounted to the motor shaft, utilising its high speed. This approach enables relatively high heat transfer without recourse
to internal airflow, mitigating concerns linked to external fluid cooling systems, such as complexity, additional mass and reliability.

Thermal management is also enhanced through advanced winding technology. Precision wire placement of round conductors, combined with modular stator build methods, results in a higher packing factor that increases winding thermal conductivity and minimises eddy current losses. As a consequence, heat rejection requirements are minimised.

Moreover, there is a substantial decrease in copper volume as the magnetic gearing reduces the Minimum Marketable Feature (MMF) requirements. Winding temperatures are thus lowered due to the lower losses, high thermal conductivity and smaller winding cross-section.

For eVTOL manufacturers seeking a scalable and versatile solution suitable for a wide range of applications, magnetically geared motors offer the reliability, cost-effectiveness and energy efficiency critical for developing commercially viable aircraft.