Dual-rotor axial machines also have thermal issues, as the windings are located deep within the stator and between the two rotor discs. One major challenge with axial machines is maintaining a uniform air gap between the rotor and the stator, as the magnetic forces are much higher than in radial machines. Radial motors are well-understood, and manufacturing methodologies and machinery are readily available. However, axial flux motors pose some serious design and production challenges that have made them far more costly than their radial counterparts, despite the technological advantages. The design of the coil windings is a key area where suppliers can differentiate between themselves. Segmented stator radial machines introduce additional loses owing to breakage of the flux path in the stator, but that is not a problem for axial machines. Designs have improved winding methods, as the coil overhang results in additional weight, cost, electrical resistance and more heat wasted.Īxial flux machines have far fewer coil overhangs, and some designs use concentrated or segmented windings that are fully active. Radial flux motors have traditionally used distributed windings, where as much as half the winding are not active as they overhang the magnets. The steel makes it easier for the flux to pass through, and this results in the efficiency gain. It also has to follow a two-dimensional path.īecause the flux path in axial flux machines is one-dimensional, grainoriented electrical steel can be used. That means the flux path is much shorter compared to that in radial flux machines, allowing the motor to be smaller for the same power, and have a higher power density and efficiency.īy contrast, in radial motors the flux moves through the first tooth and then via the stator back to the next tooth to the magnets. In a dual-rotor axial flux topology, the flux loop starts at the first magnet, passes axially through a stator tooth, and immediately arrives at the second magnet. It then passes through a second tooth to arrive at the second magnet on the rotor. Here, the magnetic flux loop starts at a permanent magnet on the rotor, passes through the first tooth on the stator, then flows radially along the stator. Most permanent magnet motors these days work with a radial flux topology. There are two principal topologies of axial flux motor – dual-rotor singlestator, sometimes called torus-style machines, and single-rotor dual-stator. Both these factors change the options for electric vehicle EV platform designers. The motors are shorter, typically by five to eight times, and can be two to five times lighter. That comes from the shorter, one-dimensional flux path, which is comparable to or better than the very best 2D radial flux motors on the market. The slim and lightweight structure results in machines with a higher power and torque density than a comparable radial machine, without the need to resort to very high speed operation.Īxial flux motors can also be highly efficient, with efficiencies typically over 96%. That makes axial flux motors much more compact the axial length of the machine is much shorter compared to radial machines, a factor that is often crucial for an application such as an inwheel motor. That can only go so far, however, so moving to a completely different machine type such as axial flux might be a good alternative.Īxial flux permanent magnet machines typically provide more torque for a given volume of motor than a radial motor, as the active magnetic surface area is the face of the motor’s rotor rather than the outside diameter. Traditional radial flux motors, which use permanent magnets or induction motors in an electric field, are undergoing extensive development aimed at optimising their weight and cost. For many years they have been used in stationary applications such as elevators and agricultural machinery, but over the past decade a number of developers have been working on improving the technology to make it suitable for applications such as electric motorcycles, airport pods, delivery trucks, electric cars and even aircraft. Axial flux motors can change the way a powertrain is designed, moving it from the axle to inside the wheelĪxial flux motors have many EV design advantages over their radial flux counterparts, as Nick Flaherty explains.Ī new type of motor is gaining traction in the e-mobility sector – the axial flux type.
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