Everything old is new again: BMW’s fifth-generation eDrive is equipped with a synchronous AC motor with a winding
Brushed DC motors (and universal AC/DC motors) are often maligned for a number of reasons, but one of the main – if perhaps somewhat overstated – complaints is that the graphite brushes and segmented copper commutator they ride wear out with over time, all the while producing incredibly fine — and potentially dangerous — conductive carbon and copper dust in the process. So BMW’s choice of a matte engine for its 5th-generation eDrive technology (which debuted in cars like the 2022 BMW iX M6) would seem like a step backwards in technological progress—and in a way, it is. But this is a brushed AC motor, more formally known as a wound-rotor (or field) AC synchronous motor, and here the brushes and slip rings (not the commutator – more on that below) have a much easier life than their matte DC counterparts . This has a big impact on engine life expectancy, but it also allows much more control over engine speed and torque in all four quadrants of operation (ie both forward and reverse motion and regeneration). In fact, a wound-field AC synchronous motor is the direct counterpart of a separately excited DC motor, and they behave very similarly in terms of both the load and the top-level control scheme, despite their dramatically different physical designs.
A wound-rotor synchronous motor (WRSM) uses a radial array of electromagnetic coils in the rotor for its field, rather than permanent magnets placed on the surface or built into the rotor of a permanent magnet synchronous motor (surface and internal PMSM types, respectively). The PMSM is by far the most popular type of traction motor currently used in electric vehicles, so an overview of its characteristics will be useful to better understand why an OEM such as BMW might choose a WRSM.
Both styles of PMSM—internal and surface—invariably use rare-earth magnets for the rotor field because they maximize two qualities important in this application: high field strength (typically in the 0.9 to 1.2 Tesla range) and high coercive force. or demagnetization resistance. Since torque is proportional to the strength of the magnetic field produced by the motor field, stronger magnets are preferred (most of the time), and since the field/rotor magnets in the motor are exposed to opposing fields from the armature/stator, demagnetization resistance is absolutely critical. While rare earth magnets are good, they are by no means perfect, and one of their main drawbacks is that they are expensive (the word “rare” gives it away). Another major disadvantage of rare-earth magnets is the fact that their resistance to demagnetization begins to drop at very low temperature – as little as 80°C for the neodymium type in fact – which can severely limit the amount of continuous power a given motor can deliver.
Operationally, the PM field is both a blessing and a curse (but mostly the latter). On the plus side, having full field strength at all times allows the PMSM to deliver predictable maximum torque starting at 0 rpm, especially when compared to an induction motor. On the other hand, back emf – or the voltage created by the motor that opposes its supply – is proportional to field strength and rpm, so top speed is strictly limited in a PMSM without the use of field weakening or ridiculously high battery voltage. Unfortunately, field weakening in PMSMs requires active field suppression from PMs, which threatens their demagnetization (especially at elevated temperatures). Additionally, a catastrophic failure mode called runaway can occur when the field weakening suddenly stops while the motor is still rotating at high RPM (e.g. due to inverter failure or loss of rotor position feedback, etc.) . If this were to happen, the BEMF would suddenly rise to a much larger value, well in excess of the battery voltage, if it weren’t for the fact that the battery would be squeezing that voltage hard. So instead, a huge amount of current will flow from the PMSM back to the battery through the anti-parallel diodes on each bridge switch in the inverter, destroying them. In addition to failure modes, an internal PMSM can generally withstand more field weakening than a surface type because the placement of the PMs in the rotor partially shields them from demagnetization and also allows for higher rotational speeds without the worry of the magnets suddenly coming off to punch a cam. hole through the stator. However, the rotor in IPMSM is much more expensive to manufacture – after all, these magnets don’t bury themselves.
Another motor that is most commonly used in electric vehicles (although less so these days) is the AC induction motor, or ACIM. In some respects it is more like WRSM than PMSM in that the field can be controlled (albeit indirectly) and there is no risk of uncontrolled generation. However, the cynic in me suspects that the real reason ACIM is (or was) so popular is that it is one of the cheapest types to manufacture, as it is not very well suited to traction applications. This is because high peak torque at 0-low rpm requires very computationally intensive control circuits, and even then it’s hard to get peak torque more than 3x rated, no matter what kind black magic. the inverter algorithm can use. In addition, the magnitude of the torque depends on poorly controlled and/or difficult-to-estimate parameters such as the resistance and inductance of the rotor core. However, the ACIM is one of the more physically robust engine designs that can take a lot of environmental (or operational) abuse, which is definitely a point in its favor for automotive use, even if it’s not as power-hungry as PMSM, or as good at producing dead-end torque as a series DC motor.
Given that WRSM uses electromagnets for its field instead of PM, it’s clear that the two big drawbacks of PMSM mentioned above will immediately become moot. The price paid is that these field electromagnets must be energized, a task that falls to a pair of brushes and slip rings (though of course the inverter itself, ultimately). While slip rings and commutators are examples of a free-rotating electrical connection, and both use carbon brushes in the motor housing to supply power to the rotor, the similarities end there. The commutator is so named because it charges each coil in the armature (DC machine) in series as the shaft rotates. This is a tough life for both the brushes and the commutator for several reasons: (1) the armature is where the vast majority of the motor’s power is handled, so the brushes and commutator must deal with large currents; (2) the inductance of each armature coil stores energy (proportional to 0.5 LI2), which causes an arc whenever its pair of commutator segments is disconnected from the brushes; (3) the commutator segments must be insulated from each other, and the resulting gaps and insulating material may subject the brushes to shock loads if the commutator surface is not periodically resurfaced; (4) to handle large currents, the commutator segments must be made of copper, but copper is a relatively soft metal, so it wears badly. However, the slip rings in the WRSM provide a relatively low power DC field, so none of the above four problems apply. In fact, the humble car alternator is a type of WRSM, and when one fails, the electronic components (bridge rectifier or field regulator module) are almost always to blame, not the slip assembly.
If the field in the WRSM is supplied with direct current, then it will behave exactly like a PMSM (without the risk of catastrophic demagnetization, of course). However, this is a fairly simple control scheme and the usual approach is to vary the field current with a torque requirement below synchronous speed, then reduce its maximum value in proportion to the rpm above synchronous speed (ie in the weak field region). A rather underappreciated advantage of matching the field excitation to the torque demand is that the WRSM will represent a unity power factor load to the inverter. This eliminates the reactive current that otherwise wanders back and forth between the inductance of the motor windings and the DC link capacitance, doing no useful work in the process but still heating the bridge switches and anti-parallel diodes. In contrast, ACIM always shows a lagging (inductive) PF, leading to large switching losses in the bridge switches when IGBTs are used (due to their slow turn-off), while PMSM usually presents a leading (capacitive) PF (however, see . below) resulting in large switching losses when MOSFETs are used (due to energy stored in the drain-drain capacitance). Since MOSFETs have virtually supplanted IGBTs in EV inverters, the typically leading PF PMSM is at a slight disadvantage compared to WRSM.
Another potential advantage of WRSM over PMSM is that electromagnets can achieve higher field flux intensities than even the strongest rare-earth PMs (depending mainly on the saturation limit of the particular grade of electrical steel used to construct the rotor and stator), which can actually reduce the size of the motor for a given power output. For example, pure iron can withstand about 2.3 T to saturation, while typical grades of silicon steel used in transformers and motors can withstand 2 T, both of which comfortably exceed 1.4 T, which can produce the strongest Nd -magnets, much less. 1.2 T max. However, pure iron’s AC losses make it a poor choice for a stator, and although the rotor does not experience such losses, it must be strong enough to hold itself together, both against the rotational forces created by the torque and the centrifugal force at high revolutions, and pure iron has too low tensile strength. Also note that the space required by the copper windings for the electromagnets can partially or even completely negate the advantage of a higher saturation limit of 40 to 90% or so.
In conclusion, the WRSM does not use expensive and environmentally harmful rare-earth magnets, so it can withstand higher temperatures and is immune to demagnetization. Compared to its PMSM counterpart, its torque and speed are more controllable, it cannot become an uncontrolled generator (as long as the field supply is disconnected in case of inverter failure), and it can operate at unity power factor. That’s a pretty compelling list of pros, so maybe we’ll see more of them in electric cars in the future.
Charged electric cars | A closer look at frosted AC motors in electric vehicles
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