66 November/December 2023 | E-Mobility Engineering An electric, hydraulic and mechanical powertrain provider notes that AF and RF motors have a lot in common in these areas. “If high efficiency is the target, an AF architecture enables smart design solutions, especially for the stator, to reduce the losses,” the expert says. “These solutions also support high power density, as the power of electric motors is usually assigned to a certain time like 30 seconds peak power and at least 30 minutes continuous power.” Minimising losses All motor engineers strive to maximise efficiency by minimising energy losses. Among the most important things to look at are direct cooling of the rotor, addressing eddy current losses in stator cores and rotor back-iron. An academic in the area points to yokeless stator design, which reduces both weight and iron losses. It is also important to consider the size of stator winding heads. “This can come in the form of winding layout and length, minimising harmonic losses, working to optimise the relationship between motor and inverter, or direct oil cooling,” says an expert from an AF motor developer that pioneered direct oil cooling. “There is a move to use higher performance materials to aid with rotor speed and therefore power density, and our AF motors have very short windings and short, direct flux paths, further helping to minimise losses.” Among the most important issues to address when seeking to minimise losses is AC copper loss. This results from I2R power loss, skin and proximity effects – particularly for concentrated windings with relatively wide ribbon conductors and open slots – according to the first motor developer. In the skin effect, magnetic fields generated by high-frequency AC induce eddy currents that flow near the outer edges of the conductor so that current is unevenly distributed through the cross-section, increasing resistance. The proximity effect also alters the distribution of the current within conductors in a way that increases resistance. In this case, when multiple conductors carrying AC are in close proximity, their electromagnetic fields interact so that in each conductor the current density is higher on the side facing away from its neighbours. Techniques to mitigate the skin effect include the use of litz wire, which consists of individually insulated strands that are twisted or woven into cables and provide multiple paths for the current. Conductor shaping can also help, with flat or hollow wires that are less susceptible to skin and proximity effects than round conductors. The use of flat wire also contributes to higher power density, a wire bending specialist notes. He adds that the technology used to bend the wires is important to avoid imperfections such as thickening at the corners, and that optimising the wire cross-section helps reduce losses from the causes mentioned above, with a focus on greater width-to-height ratios. Eddy current loss in the permanent magnets can also arise in electrical machines, including AF motors, for various reasons. These include space harmonics, particularly sub-harmonics from concentrated windings, time harmonics from the inverter switching, and high ripple currents caused by low winding inductance, the first motor developer explains. Inverter-driven motors are provided with AC voltage as short pulses rather than a continuous sinusoidal waveform, which introduces harmonic components. These can cause current and magnetic flux distortion that in turn can affect the operation of the motor and bring additional losses. One developer notes that the company’s own AF motors don’t require magnet segmentation or rotor back-iron lamination, because the flux density distribution from its distributed winding is highly sinusoidal. Thermal design is also important to ensure adequate heat rejection from the windings and magnets. In aerospace applications, minimising losses is crucial to the power density, low mass and high efficiency that aircraft demand. “If you were to look at the small changes we have made during the design of our AF motor, it could be elements like the pole bar in the copper we use, and whether there is a noticeable impact from changing it by half a millimetre in height or changing the thickness of the copper,” the aircraft motor specialist comments. He reiterates the importance of looking at the system as a whole, rather than the motor alone. “For example, one way to improve your efficiency is to increase the amount of copper, as copper doesn’t get as hot,” he explains. “But there’s a point of diminishing Focus | Axial flux motors Copper coil for an AF motor made from flat wire bent through its width rather than its thickness to form the turns around the core space (Courtesy of Wafios)
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