![]() ![]() The bigger the difference in speed, or slip, the stronger the current induced into the rotor will be, and the more torque will be available…up until a point, that is, beyond which the motor stalls and the rotor quickly overheats (the rotor in an induction machine is essentially a short circuit on the secondary of a very high step-down ratio transformer). This is because current can only be induced into the rotor if there is a relative difference in the speed of its rotation with respect to the apparently rotating magnetic field from the stator/armature. The latter approach is the basis of the AC induction motor, or ACIM, and it is also the exception mentioned above to the field always being stationary in space. The field in an AC motor must be located in the rotor, then, and it can either be directly supplied by permanent magnets (or even an electromagnet fed by brushes and slip rings), or indirectly supplied by the stator coils through transformer action, or induction. What many find maddening is that this relationship is pretty much always reversed in the AC motor: the stator consists of an array of coils that are sequentially energized by phase-shifting the currents flowing through each of them (via the inverter, in an EV), rather than mechanically switching from one coil to the next with a commutator. At any rate, the terms stator or field and rotor or armature were interchangeable when DC motors were the only game in town, because the respective functions and locations within the motor were inextricably linked: the field was in the stator and the armature was in the rotor because the latter needed a commutator and brushes to sequentially energize its coils, thereby producing a rotating magnetic field. ![]() The inrunner configuration is far more common with conventional radial motors, but one common application of the outrunner design is in ceiling fans. The last bit of mechanical classification is whether the motor is an inrunner, with the rotor in the center, or an outrunner, which flips the motor inside out, putting the stator in the center and making what would be the motor housing the rotor. Additional terms often used interchangeably with the stator, and not always accurately, are yoke, motor housing and back iron, though the latter more specifically refers to the part of the motor housing that conducts magnetic flux on the back side of the stator-said function is typically performed by the housing (or yoke) in radial flux AC machines. Similarly, the rotor is the part of a motor that rotates while the stator is the part that stays put. Motors produce torque from the interaction of two magnetic fields: one is stationary (or nearly so-more on this exception in a bit) and is produced by the field, succinctly enough, while the other appears to rotate in space and is produced by the armature. If you already know that the rotor isn’t always the field and the stator isn’t always the armature (and what each of these terms mean, of course) then you could skip this paragraph, but don’t blame me if your head starts spinning like a yokeless axial flux outrunner later on. There’s a bewildering number of ways to construct a motor, so a brief review of the nomenclature might be helpful. The axial flux design is actually one of the oldest ways of constructing a motor-it’s just that, up until relatively recently, it was relegated to niche applications in which the primary requirement was maintaining a low profile. Consequently, axial flux motors may go from being a niche use and historical curiosity to an important (and, perhaps, dominant) part of the EV traction market. What really changed the situation for axial flux motors were advancements in both permanent magnets and composite materials. ![]() ![]() Despite being characterized as an advanced construction technique, the axial flux design is actually one of the oldest ways of constructing a motor-it’s just that up until relatively recently it was relegated to niche applications in which the primary requirement was maintaining a low profile, rather than high power, efficiency, etc. In Issue 49, we reviewed some of the more promising advancements in materials and construction techniques for EV traction motors, one of which-the axial flux design-will be the focus of this article. Posted by Jeffrey Jenkins & filed under Features, Fleets and Infrastructure Features, Tech Features. ![]()
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