Charged EVs | Options to everlasting magnet motors in EV traction functions

In my very first article for Charged over 10 years in the past, I opined that the Switched Reluctance Motor, or SRM, would finally come to dominate the EV traction market, if for no different motive than the truth that it’s nearly as low cost as dust to fabricate. I received’t sprain my shoulder patting myself on the again for a prediction that took 10 years to (partially) come true, but it surely does appear that the times of the Everlasting Magnet Synchronous Motor, or PMSM, and the AC Induction Motor, or ACIM, dominating EV traction functions are coming to an in depth, with each Toyota and even stalwart ACIM fan Tesla introducing reluctance motors into the combination (albeit not ones that purely make use of magnetic reluctance, however extra on that beneath).

Although the PMSM and ACIM sorts dominate the EV traction market, neither know-how is especially well-suited to the job, primarily due to their restricted overload and beginning torque capabilities. Torque is a operate of the magnetic area energy of the motor’s area construction, and that’s clearly fastened in a PMSM, as a result of its area is being equipped by literal magnets, however it is usually restricted within the ACIM, as a result of its area is induced into its rotor by way of transformer motion, and like every electromagnetic construction using a high-permeability core materials, it’s topic to saturation, or the purpose at which an extra improve in present fails to extend the magnetic area energy thus developed. Since traction functions require most torque at 0 rpm (to beat each the inertia and static friction, or stiction, of the load), having a tough and quick torque restrict is unquestionably a draw back. Moreover, the PMs invariably employed within the PMSM are uncommon earth sorts—which, because the identify implies, are typically costly—and most flavors of those magnets—notably the neodymium kind—have a really low higher temperature restrict earlier than they begin to lose their potential to withstand demagnetization (in some instances as little as 80° C), an particularly unlucky weak spot, as a result of the one technique to lengthen the fixed energy pace vary of a PMSM is with field-weakening, during which an opposing (i.e. demagnetizing) area is utilized to the sphere magnets to cut back the again EMF, or the voltage produced by the rotation of the rotor, to beneath that of the battery voltage.

That brings up one other potential draw back to the PMSM: since BEMF is proportional to rpm, if the rotor is spinning quick sufficient to require field-weakening and the inverter shuts down (or misjudges the timing of the stator currents) for any motive, then it (and presumably the motor) will nearly definitely be destroyed from the massive soar in BEMF inflicting an equally enormous soar in present from the now rapidly-braking motor (a scenario that’s considerably euphemistically termed uncontrolled era). This failure mode can’t happen with the ACIM, in fact, as a result of the rotor’s area can solely exist by way of the motion of the stator currents (and a distinction within the relative rpm of each), however the ACIM is far much less power-dense than the PMSM (particularly the inside PM model), so reaching a given energy output requires a a lot bigger motor, and that’s a big drawbacks itself in an EV traction software. With all these downsides of each applied sciences, it’s honest to marvel how the PMSM and ACIM ever got here for use in EVs within the first place—a lot much less how they turned the preferred selections—and a very good chunk of the reason being that they each use the identical inverter {hardware} that has been used to regulate the pace and torque of commercial motors for many years, with solely minor variations within the software program and rotor place suggestions to raised adapt them to traction service.

The least-radical various motor kind—and one which has already been utilized by BMW, in reality—is the wound-rotor synchronous motor, or WRSM. There’s some irony on this, each as a result of supplying present to the rotor requires brushes and a slip ring, which is nearly anathema to these EV purists who shun the king of all traction motors, the sequence area DC kind, due to its brushes and commutator (to be honest, the latter wears out much more shortly), but in addition as a result of it’s downright historical know-how, predating even the induction kind. Mainly, the PMs of the PMSM are changed by electromagnet coils within the rotor, which suggests the energy of the sphere will be instantly managed, one thing that isn’t doable with fixed-strength everlasting magnets, in fact. This makes field-weakening to realize excessive rpms a non-event, moderately than one thing akin to juggling chainsaws, and makes minimizing the quantity of reactive present that might in any other case slosh forwards and backwards between the inverter and motor doable (this present does no helpful work, however does expend a few of the RMS present score of the inverter’s semiconductor switches and trigger heating from I²R loss).

The downsides to the WRSM are a considerably decrease energy density and a better meeting value for the rotor (a decrease materials value, nonetheless, since copper is more likely to stay a lot cheaper than any of the uncommon earths utilized in high-strength magnets). Additionally, in fact, the sphere wants a separate energy stage to provide it, and the controller software program is a little more advanced, however that is well-established know-how, and the WRSM is eminently appropriate for 4-quadrant traction functions (i.e. motoring and braking in ahead and reverse).

Having the ability to use the wealthy ecosystem of current digital parts, software program and data behind each industrial variable frequency drive, or VFD, is a large benefit that can not be overstated, and that is little doubt one of many main causes that the axial-flux variation of the PMSM has obtained any consideration, as a result of it’s positively a contender for the “most costly technique to assemble a motor” award. Because the identify implies, the axial-flux PMSM orients its area magnets parallel to the output shaft on a disc that’s sandwiched between the stator part windings (and the “again iron” to finish the magnetic path, if mandatory). This naturally leads to a motor with a comparatively giant diameter-to-length ratio, and since torque is proportional to the sq. of rotor diameter, however solely instantly proportional to rotor size, a wide-in-diameter however short-in-length motor—a disc form—is perfect for high-torque functions, corresponding to EV traction. Securing the sphere magnets in a disc-shaped rotor requires some type of banding on the circumference of the disc, so it’s arguably about as vulnerable to disintegration at excessive rpm because the floor PM development, however it’s undeniably much less efficient than burying the magnets, as within the IPM development. Moreover, the push and pull of the magnetic fields from the stator poles exert torsional, or wave-like twisting, forces on the axial-flux rotor, and resisting these forces is just not the sturdy go well with of a disc form. Consequently, the components of the rotor that maintain the sphere magnets in place should have a really excessive strength-to-weight ratio, which principally means utilizing unique (learn: costly!) supplies like woven carbon fiber composite. The mix of unique development supplies and uncommon earth magnets within the rotor is the explanation that the axial-flux configuration is likely one of the most costly sorts being thought-about for EV use, and there doesn’t appear to be a lot that may be carried out to decrease its value sooner or later. For a extra in-depth rationalization of the axial-flux PMSM development, see my article within the Could/June 2020 problem of Charged. For these questioning if there’s an axial-flux analog to the ACIM; properly, it’s theoretically doable, however doesn’t actually supply any compelling benefits over its typical radial-flux counterpart.

One other various motor development that leverages the present VFD ecosystem is the Inside Everlasting Magnet-Synchronous Reluctance Motor (IPM-SynRM), which mixes the buried magnets of the Inside PM Synchronous Motor (aka IPM-SM) with the rotor construction of the reluctance motor. Mainly, the IPM-SynRM behaves just about just like the IPM-SM so far as the inverter is anxious, however with a big contribution from reluctance torque (extra on what that’s beneath). In each the IPM-SynRM and IPM-SM, burying the rotor magnets additionally partially shields them from warmth in addition to demagnetization from the stator (throughout overloads and area weakening) and in addition eliminates the chance of being flung off the rotor at excessive rpm, as is the invariable case for the floor PM development (that, to be honest, isn’t utilized in EVs nowadays). Each Toyota and Tesla have reportedly began utilizing one of these motor, albeit not solely.

Transferring alongside the continuum from the pure synchronous motor to the IPM-SM, then the IPM-SynRM, we lastly come to the pure reluctance motor, which operates solely on the attraction between electromagnets within the stator and a magnetically-soft iron or metal rotor. Reluctance is the magnetic analog to resistance, so a magnetic circuit with minimal reluctance happens when magnetic area strains can freely movement from the North to South poles. This situation happens in a reluctance motor when a diametrically-opposed pair of electromagnets on the stator align with an identical pair of projecting tooth on the rotor (most reluctance happens when the rotor tooth are between two pairs of stator poles). By setting up the rotor with fewer tooth (or salient poles) than the stator (e.g. 4 rotor poles to six stator poles), it may be ensured that when some salient poles are aligned with the stator electromagnets, some can be in between them. Sequentially energizing the stator electromagnets—switching from one pole pair to the subsequent, therefore the extra frequent identify of Switched Reluctance Motor—will trigger the rotor to rotate in a lot the identical style as in a PMSM or ACIM. One essential distinction within the SRM, nonetheless, is that the stator electromagnets are equipped with a trapezoidal however unipolar (DC) present waveform, moderately than a bipolar (AC) one. This implies there aren’t any iron losses within the rotor, which is a large plus, however the torque output has quite a lot of ripple, and this topics the rotor to immense rigidity and compression, which might then trigger vibration and noise. It additionally implies that the traditional half-bridge energy stage used for every part in an inverter/VFD isn’t your best option right here; moderately, a two-switch, two-diode energy stage is most popular, as it might probably quickly improve the winding present, and simply as quickly lower it (whereas recycling any vitality nonetheless saved within the winding inductance again to the battery).

One other problem that has hampered the adoption of reluctance motors till lately is the truth that the present by way of the stator electromagnets should be ramped up quickly and with exact timing as a given rotor pole pair comes into alignment with a stator pole pair, then held at an outlined degree for a number of levels of rotation, after which quickly ramped down once more as that rotor pole pair rotates out of alignment. This requires very correct data of the rotor place, which typically means a comparatively costly (and comparatively delicate) absolute encoder, and as if that weren’t sufficient, the inductance of the stator electromagnets will increase as a rotor pole pair comes into alignment (as a result of the reluctance drops, and inductance is inversely proportional to reluctance), which makes rising the present that rather more troublesome, particularly given the truth that the dI/dt (i.e. charge of change in present over time) should be fastidiously managed for optimum torque manufacturing. This requires quite a lot of computing energy, however happily, trendy microcontrollers and Discipline-Programmable Gate Arrays (FPGAs) have greater than ample grunt for the duty. Consequently, the SRM is sure to be the dominant EV traction motor of the longer term… Any day now, I’m positive…  

This text appeared in Subject 63: Jan-Mar 2023 – Subscribe now.