70 November/December 2023 | E-Mobility Engineering in the stator is one of the main reasons why we can achieve higher continuous power than a radial motor.” He adds that a really well designed radial motor for propulsion might deliver only 65-75% of peak power continuously owing to thermal limitations, while one of his company’s yokeless AF motors has a much higher continuous power rating, at above 90%, thanks to improved high thermal-contact cooling. Oil cooling also offers extra insulation between electrical components, offering another layer of protection. Integration issues The differences in the shape and torque characteristics of AF motors bring opportunities as well as challenges for integration into vehicle drivetrains. Benefits include more freedom in design for a range of applications such as e-axles and wheel hub drives, while leaving more space for batteries and passengers, but managing the production of numerous variants can be challenging, the academic cautions. Because the required wheel torque is delivered at lower motor speed, fewer reduction stages are needed. That places less of a burden on transmission components in terms of lubrication, potentially leading to longer service lives. However, a parallel shaft, single-speed transmission might impose a diameter restriction on an AF motor in order to allow the vehicle’s driveshaft to clear the motor housing. One expert notes that AF motors are often more suitable than RF motors for near-wheel or in-wheel solutions in applications such as skateboard platforms for last-mile delivery vehicles designed to maximise the width of the load bed, for example. The shorter axial length of a yokeless and segmented armature AF machine, combined with an outer diameter similar to typical RF machines, provides real benefits in packaging, the second AF motor developer says. Ferrari’s hybrids using such motors provide an example here. “An RF motor would simply not fit in the back of a V8 without increasing the length of the entire vehicle,” one AF motor developer’s expert points out. “The much wider low-speed continuous torque band of an AF motor changes the requirement for gearboxes. Where an RF motor typically runs at higher rpm, there is not sufficient torque at lower rpm, which could mean having to use a two-speed gearbox.” The pancake shape of an AF motor makes it a good fit for aircraft. “If it has a gearbox, usually it is a comparable diameter, which allows you to stack one or more motors very easily without causing too big of an impact on the aircraft design,” the aviation specialist says. “That gives you great benefits, and if we are talking about eVTOL applications, it’s even more critical where you have to fit the motor between firewalls or structural members – shorter is better.” He also argues that the speed limitations of AF motors are not as restrictive as might be assumed. “We are currently using our axial flux machines far beneath their capable mechanical speed. We have lots of potential left to find.” Omni’s M27 axial flux motor mated with a VE34 planetary gearbox is an example of a typical off-highway wheel drive application (Courtesy of Omni Powertrain Technologies) Saietta three-in-one e-drive combining an axial flux motor with an integrated and cooled controller and a single-speed transmission (Courtesy of Saietta)
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