E-Mobility Engineering | September/October 2023 61 Magnets A key issue of course is the magnets used in the motor design. The material they’re made from, whether a samarium cobalt alloy or neodymium, determines how much magnetic field can be induced when magnetised to create a permanent magnet. This then determines the magnetic field that the magnet can produce for a given weight, which determines the size and performance of the motor. Arnold says it has the world’s strongest samarium cobalt magnet, the Recoma 35e, which has a flux density of more than 250 kJ/m3. The choice of magnet material is based on a number of factors, although generally samarium cobalt is used where there are the highest temperature variations, and this is usually with airframe designs with one or two engines. If there are a lot of smaller motors on an aircraft such as an air taxi then neodymium magnets are more often used. Design Researchers at MIT in the US have developed a 1 MW (1300 bhp), aircooled, outer-rotor Halbach array permanent magnet motor with a specific power of 17 kW/kg for aircraft propulsion as part of a contract with NASA. Experiments have validated the highest-risk elements of the design, including stator core loss, structural stability, winding insulation and permanent magnet field strength. Two manufacturing processes were developed for the stator cores using a combination of iron, cobalt and vanadium, and compared through core loss and B-H curve measurements. A conventional lamination bonding process increases core losses by 20%, so the researchers have developed and tested a new approach to modelling Halbach array rotors. The resulting modular singlephase winding pattern improved the robustness of the motor, enabling single-phase inverter drives. The design is being tested on two prototype machines connected through a shaft at 12500 rpm, with one acting as a motor and the other as a generator. All the major components, such as the stator cores, rotors and heat exchangers, have been successfully manufactured. Remaining items such as machining the superstructure and winding the stator cores are in progress. During the manufacturing process of the 1 MW demonstrator, the highestrisk aspects of the design have been validated through stator core loss experiments, winding and insulation tests, rotor spin-pit testing at full operational speed and temperature, and rotor magnetic field measurements. The experiments show that the demonstrator will meet the design specifications and achieve full power. The motor winding consists of 10 independent three-phase pole pairs. Modular winding schemes are increasing in popularity for electric machines designed for aviation, owing to their better reliability than non-modular ones. A pole pair contains three singlephase concentrated windings driven by three single-phase inverters, as singlephase inverters have been shown to improve power density and robustness. Rectangular Type 8 (AWG 24) Litz wire bundles are used to minimise the losses in the AC windings and maximise the copper fill factor. Kapton tape and Nomex slot liners are placed between the phase windings in the slots and the end turns to improve phase-to-phase insulation. The slots are vacuum pressure impregnated (VPI) with an epoxy resin to improve thermal conductivity and electrical insulation. The resin has a favourable dynamic viscosity, improving the penetration of the varnish during the VPI process. A mock-up of a single pole-pair section of the stator was manufactured to verify the winding process. The stator section withstood the full operational voltage and current without insulation failure and successfully demonstrated the winding pattern, insulation and thermal sensor installation. The stator core loss estimate has been validated through measurements on toroidal samples and full-size stator laminations, and shows that the design reduces the losses in the stator by 20%. One of the spin-outs of the project is a new technique for modelling the stator. It is a computational algorithm that is easy to use and adapt to different machine topologies rather than using finite element analysis (FEA). The technique was used to design the rotor with four tangential segments per pole. It uses the Halbach array approach, producing a magnetic field scan of the rotor that closely matches the analytical model produced Calnetix has developed a 100 kW motor that weighs 2.3 kg (Courtesy of Calnetix)
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