Charged EVs | A better take a look at brushed AC motors in EVs

Every thing previous is new once more: BMW’s fifth-generation eDrive contains a wound-field synchronous AC motor

Brushed DC (and AC/DC common) motors are a lot maligned for a variety of causes, however one of many chief—if, maybe, considerably exaggerated—complaints is that the graphite brushes, and the segmented copper commutator they trip on, put on out over time, all of the whereas producing an extremely high quality—and probably harmful—conductive carbon and copper mud within the course of. So, BMW’s selection of a brushed motor for its Fifth-generation eDrive know-how (which debuted in autos just like the 2022 BMW iX M6) would appear to be a step again in technological progress—and in some methods it’s. However it is a brushed AC motor, extra formally often called a wound-rotor (or discipline) synchronous AC motor, and right here the brushes and slip rings (not a commutator—extra on that beneath) have a a lot simpler life than of their brushed DC counterparts. This has an amazing influence on the anticipated service lifetime of the motor, nevertheless it additionally permits for a lot extra management of the motor’s pace and torque in all 4 quadrants of operation (i.e. motoring and regenerating in each ahead and reverse). The truth is, the wound-field synchronous AC motor is a direct analog of the separately-excited DC motor, and so they behave very equally from each the angle of the load and the top-level management scheme, regardless of their starkly completely different bodily constructions.

The wound-rotor synchronous motor (WRSM) makes use of a radial array of electromagnet coils within the rotor for its discipline, fairly than the everlasting magnets which can be both positioned on the floor of, or embedded into, the rotor of the everlasting magnet synchronous motor (floor and inside PMSM sorts, respectively). The PMSM is by far the most well-liked kind of traction motor at the moment utilized in EVs, so a assessment of its traits will likely be useful to raised perceive why an OEM like BMW would possibly select to go together with the WRSM as a substitute.

Each kinds of PMSM—inside and floor—invariably use uncommon earth magnets for the rotor discipline, as a result of they maximize two qualities which can be vital on this utility: a excessive discipline energy (usually within the vary of 0.9 to 1.2 Teslas), and a excessive coercive drive, or resistance to demagnetization. Since torque is proportional to the magnetic discipline energy produced by the motor’s discipline, stronger magnets are most well-liked (more often than not, anyway), and because the discipline/rotor magnets in a motor are subjected to opposing fields from the armature/stator, resistance to demagnetization is completely vital. Pretty much as good as uncommon earth magnets are, nevertheless, they aren’t excellent by any means, and one of many chief downsides to them is they’re costly (the phrase “uncommon” form of provides it away). One other main shortcoming of uncommon earth magnets is the truth that their resistance to demagnetization begins dropping at a painfully low temperature—as little as 80° C for the neodymium kind, actually—which may unduly limit the quantity of steady energy {that a} given motor can ship.

Operationally, a PM discipline is each a blessing and a curse (however primarily the latter). On the plus facet, having the complete discipline energy obtainable always makes it simpler for the PMSM to ship a predictable most torque beginning at 0 RPM, particularly in comparison with an induction motor. On the minus facet, again EMF—or the voltage generated by the motor which opposes its provide—is proportional to discipline energy and RPM, so the highest pace is severely restricted in a PMSM with out both using discipline weakening or a ridiculously excessive battery voltage. Sadly, discipline weakening in a PMSM requires actively suppressing the sphere from the PMs, which dangers demagnetizing them (particularly at elevated temperatures). Moreover, a catastrophic failure mode known as uncontrolled era can happen if the sphere weakening abruptly ceases whereas the motor continues to be spinning at a excessive RPM (from, say, the inverter faulting off, or a lack of rotor place suggestions, and so on.). Ought to this happen, the BEMF will all of the sudden shoot as much as a a lot greater worth, which might vastly exceed the battery voltage, if it weren’t for the truth that the battery will clamp that voltage exhausting. So, as a substitute, an enormous quantity of present will circulation from the PMSM again to the battery by the anti-parallel diodes throughout every bridge change within the inverter, destroying them. Failure modes apart, the inside PMSM can often tolerate extra discipline weakening than the floor kind as a result of burying the PMs within the rotor partially shields them from demagnetization, and in addition permits for greater rotational speeds with out having to fret concerning the magnets all of the sudden ungluing themselves to punch a gap by the stator. That mentioned, the rotor within the IPMSM is much costlier to fabricate—these magnets don’t bury themselves, in any case.

The opposite motor mostly utilized in EVs (although much less so today) is the AC induction kind, or ACIM. In some respects it’s extra just like the WRSM than is the PMSM, in that the sphere will be managed (albeit not directly), and there’s no threat of uncontrolled era. That mentioned, the cynic in me suspects that the actual cause the ACIM is (or was) so standard is as a result of it is among the least expensive sorts to fabricate, because it’s not terribly well-suited to traction purposes. It’s because it requires very computationally-intensive management schemes to ship excessive peak torque at 0 to low RPMs, and even then, it’s tough to get a peak torque of greater than about 3x the nominal ranking, no matter what kind of black magic the inverter algorithm would possibly make use of. Additionally, the quantity of torque relies on poorly managed and/or difficult-to-estimate parameters equivalent to rotor bar resistance and inductance. Nonetheless, the ACIM is among the most bodily strong motor constructions that may tolerate plenty of environmental (or operational) abuse, which is certainly some extent in its favor for automotive use, even when it isn’t as power-dense because the PMSM, or pretty much as good at producing gobs of torque at a useless cease, just like the collection DC motor.

On condition that the WRSM makes use of electromagnets for its discipline as a substitute of PMs, it’s clear that the 2 large downsides to the PMSM talked about above will likely be rendered moot proper off the bat. The value paid is that these discipline electromagnets have to be provided with energy, a process which falls upon a pair of brushes and slip rings (although, after all, on the inverter itself, finally). Whereas slip rings and commutators are each examples of a freely-rotating electrical connection, and each use carbon brushes within the motor housing to ship energy to the rotor, the similarities finish there. A commutator is so named as a result of it energizes every coil within the armature (of a DC machine) in succession because the shaft rotates. This can be a exhausting life for each brushes and commutator for a number of causes: (1) the armature is the place the overwhelming majority of the facility in a motor is dealt with, so the brushes and commutator must cope with excessive currents; (2) the inductance of every armature coil shops power (proportional to 0.5LI²) which causes an arc each time its pair of commutator segments are disconnected from the brushes; (3) the commutator segments must be insulated from one another and the ensuing gaps and insulating materials can topic the brushes to influence hundreds if the commutator will not be resurfaced periodically; (4) to deal with excessive currents the commutator segments have to be made out of copper, however copper is a comparatively tender steel, so it wears poorly. The slip rings in a WRSM, nevertheless, are supplying the comparatively low energy discipline with DC, so not one of the above 4 points apply. The truth is, the common-or-garden automotive alternator is a sort of WRSM and when one fails, it’s virtually all the time the digital elements (rectifier bridge or discipline regulator module) which can be at fault, not the slip ring meeting.

If the sphere in a WRSM is provided with a relentless present, then it would behave precisely like a PMSM (minus the chance of catastrophic demagnetization, after all). This can be a fairly unsophisticated management scheme, although, and the standard strategy is to differ the sphere present with the torque demand beneath synchronous pace, then scale back its most worth proportionally with RPM above synchronous pace (i.e. the field-weakened area of operation). A fairly under-appreciated good thing about matching the sphere excitation to the torque demand is that the WRSM will current as a unity energy issue load to the inverter. This eliminates the reactive present that will in any other case slosh forwards and backwards between the inductance of the motor windings and the DC hyperlink capacitance, doing no helpful work within the course of, however nonetheless heating up the bridge switches and anti-parallel diodes. In distinction, the ACIM all the time presents a lagging (inductive) PF, resulting in excessive switching losses within the bridge switches if IGBTs are used (as a consequence of their gradual turn-off), whereas the PMSM often presents a number one (capacitive) PF (nevertheless, see beneath) resulting in excessive switching losses if MOSFETs are used (as a consequence of power saved within the drain-source capacitance). Since MOSFETs have all however supplanted IGBTs in EV inverters, the usually-leading PF of the PMSM is a slight drawback over the WRSM, then.

One other potential benefit of the WRSM over the PMSM is that electromagnets can obtain the next discipline flux depth than even the strongest uncommon earth PMs (relying primarily on the saturation restrict of the actual grade {of electrical} metal used to assemble the rotor and stator) which might truly scale back the dimensions of the motor for a given energy output. For instance, pure iron can face up to about 2.3 T earlier than saturating, whereas the standard grades of silicon metal utilized in transformers and motors can take 2 T, each of which comfortably exceeds the 1.4 T or in order that the strongest Nd magnets can produce, a lot much less the 1.2 T max for Nd magnets with a excessive sufficient most working temperature ranking to be usable in a motor. Nevertheless, the AC losses of pure iron make it a poor selection for the stator, and whereas the rotor doesn’t expertise such losses, it does have to be fairly sturdy to carry itself collectively, each from the torsional forces of torque manufacturing in addition to centripetal drive at excessive RPMs, and pure iron has too low a tensile energy. Additionally notice that the house required by the copper windings for the electromagnets might partially and even utterly nullify the benefit of the 40 to 90% or so greater saturation restrict.

In conclusion, the WRSM doesn’t use costly and environmentally-unfriendly uncommon earth magnets, so it may survive greater temperatures, and is resistant to demagnetization. In comparison with its PMSM counterpart, its torque and pace are extra controllable, it may’t flip into an uncontrolled generator (so long as the sphere provide turns off within the occasion of an inverter fault), and it may be operated at unity energy issue. That’s a fairly compelling record of pluses, so maybe we’ll be seeing extra of them in EVs sooner or later.  

This text appeared in Concern 60: April-June 2022 – Subscribe now.