60 September/October 2023 | E-Mobility Engineering power as possible with open and shortcircuits and all the things that can happen in an electrical system. It comes back to understanding the system in context, which might be the motor, the inverter and cooling; the integration of the motor and the controller is really important as well.” Certification also covers the development of software for the motor controller and the rest of the powertrain, which can be difficult if the motor is developed in isolation as a single component. Instead there is a shift to developing the motor as part of the whole powertrain so that elements of the motor can be optimised with the inverter and the energy source. Developing the software to control these elements is also a key challenge to ensure that the software can be certified as safe. “The means of developing the software for the controller has to be according to a very specific set of plans for performance and safety,” Sercombe says. “This means you need an oversight and an insight into every element of the system rather than optimising the motor on its own. “There are very interesting things on the horizon, such as the ability to have an influence over developing an energy storage system that operates at altitude. “We are looking at type certificates in 2025 or 2026, but that entails working with our customers and integrators as there are a number of permutations for the motors. We have a number of projects that are public, and the initial applications for certification are fixedwing retrofits such as the 650 kW motor with the Beaver, then we will see what comes next.” Another supplier, Calnetix, has been developing electric machine and corresponding drive technologies for various aerospace applications. These include electric propulsion, power generation for hybrid electric aircraft, and starter/generators for auxiliary power units. “Calnetix’s technology minimises the torque required, and consequently reduces the size and weight of the electrical machine for equivalent output power,” says Co Huynh, co-founder and business unit leader for aerospace at Calnetix Technologies. “Further weight optimisation of the design is achieved by using the latest electrical and magnetic materials and implementing efficient thermal management configurations customised for each application. Calnetix has a suite of motor, bearing and drive technologies that optimise system for power density and efficiency,” he says. The company has developed a motor that is rated at 100 kW and weighs 7.7 kg in a 2.3 litre volume, equivalent to a power density of 13 kW/kg and 43 kW/litre. This was achieved by optimising the electromagnetic design, including the magnetisation topologies and optimising the magnetic circuit, as well as the machine’s L/D (length over diameter) aspect ratio. The design process also looked closely at the selection of materials, including the grade of magnets, the stator laminations and their thickness, the magnet retention sleeve and other materials to achieve that 13 kW/kg target. “When efficiency and weight are primary concerns, a machine with a permanent magnet [PM] rotor is superior in most applications,” Co Huynh says. “Some other advantages of using PM motors include zero excitation power, efficiency of 95% or higher, smooth rotor, large air gap, high resistivity and very low permeability of the rotor, and reduced inverter size and losses.” Venky Krishnan, executive vicepresident and business unit leader for Calnetix, says, “From starting with a few 100 kW, 50,000 rpm motors, we have developed multi-megawatt machines as well as mission-critical applications for the International Space Station. “There have been recent advances in magnet strength and temperature capabilities, as well as improvements in the capabilities of switching devices, such as IGBTs and SiC devices,” he adds. Deep insight | Electric motors for aircraft Carbon fibre wound rotor assemblies are lighter and have lower inductive losses than aluminium or Inconel (Courtesy of Arnold)
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