64 November/December 2023 | E-Mobility Engineering The flat pack Axial flux (AF) motors are attracting a great deal of attention and engineering effort in the e-mobility arena, and for good reasons. Different motor topologies – such as radial and axial flux – are best suited to different applications, and each comes with its own set of pros, cons and development challenges. The main distinguishing feature of an AF motor is that the magnetic flux that is central to generating torque moves in a path that is parallel to the motor’s axis of rotation. With radial flux (RF) motors, the flux follows a path that is radial or perpendicular to the axis of rotation. The rotor of an AF motor, therefore, spins beside the stator (or above or below it, depending on the motor’s orientation) rather than in the centre of or around the outside of the stator, as in RF motors. We approached a number of leading motor and component developers and other experts to get a feel for the kind of decisions confronting engineers when choosing a motor architecture for their application. The available packaging space is one of the first factors to be considered. Pancake-shaped AF motors are a better fit where axial (lengthwise) space is at a premium, while more sausage-shaped RF motors are better suited to spaces in which there may be a diameter restriction. A further consideration is whether the application needs a motor optimised for torque density or power density. Typically, AF motors are the better choice for the first, while RF motors are more suitable for the second because of their ability to run at higher rpm. Need for speed Speed also has to be thought about in terms of the relationship between the maximum speed and the base speed/ torque knee point. That is the point on any motor’s performance curve at which the motor’s ability to produce torque begins to decrease as the rotational speed increases. At speeds below the knee point, the motor can produce relatively high levels of torque, making it suitable for applications that require high starting torque, strong acceleration or the ability to handle heavy loads. Above the knee point, the torque begins to fall but it can operate more efficiently, making it suitable for applications that prioritise speed over torque. AF motors are better than RF ones at spinning slowly and generating high levels of torque; however permanent magnet AF motors in particular have high magnetic loading with low saliency, so they are limited in their fieldweakening capability compared with RF Peter Donaldson canvasses expert opinions on the decisions to be made about whether to choose an axial flux motor for a given application The 42 kg AFT140i axial flux motor is offered in 48 and 96 V versions delivering up to 21 and 45 kW respectively for light three and four-wheel EVs (Courtesy of Saietta)
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