Charged EVs | How to improve EV traction motor efficiency

A have a look at enhancing motor effectivity in two primary classes: electrical and mechanical.

Common readers of Charged don’t should be advised that the everyday EV drivetrain is far more environment friendly from battery to wheels than its ICE-based equal, however it’s maybe much less well-known that the approximate drivetrain effectivity ranges from 75-90% for the EV to a fairly much less spectacular 20-35% for the ICE automobile. The overwhelming majority of losses in an ICE automobile happen within the conversion of chemical power to mechanical—the so-called Carnot Effectivity—which could strategy 40% in one of the best of instances. In distinction, the principle drivetrain parts in an EV—battery, inverter, motor, and equipment discount and/or transmission—all have efficiencies within the mid-to-high 90-percent vary. Nonetheless, out of these particular person drivetrain parts, a lot of the losses come from the motor (tied, maybe, with losses from the gear discount/transmission), therefore it’s the most promising goal for enhancing general effectivity in an EV. That stated, it’s value mentioning that enhancing the effectivity of the traction motor from, say, 94% to 97%, would require reducing losses in half. And to succeed in the identical 99% effectivity as the everyday lithium-ion battery (outlined by the ratio of cost in vs. cost out, or the coulometric effectivity), would require a downright heroic discount in losses of over 83%!

In gaming out the methods to enhance motor effectivity, there are two primary classes of losses to think about: electrical and mechanical. Electrical losses embrace the I²R, or typical ohmic resistance, of the windings (together with the shorting bars within the rotor of an AC induction motor), the frequency-dependent adjustments in these resistances as a consequence of pores and skin and proximity results, and the assorted “iron” losses incurred within the magnetic circuit between stator and rotor, comparable to magnetic hysteresis—whose losses improve with frequency—and flux leakage—whose losses are extra the results of the bodily development of the motor. 

Mechanical losses embrace friction within the bearings (a linear perform of RPM), displacement of grease within the bearings (a cubic[!] perform of RPM), and displacement of air (or, worse, a liquid coolant comparable to glycol antifreeze, oil, and so forth) by the rotor meeting (aka windage, which can be a cubic perform of RPM). Frictional losses needs to be such a small a part of the whole losses in an EV traction motor as to represent a rounding error—if not, you’ll virtually definitely be made conscious of the problem by the horrendous squealing that sad bearings inevitably produce. It’s value mentioning, nonetheless, that the most important driver of losses is, properly, the driving force—or, extra particularly, the place on the effectivity map the motor spends most of its time working (see Fig. 1).

Whereas the particular effectivity values will fluctuate from motor to motor, and with software, after all, the overall distribution of these values will have a tendency to use to all instances, and the important thing takeaway right here is that working on the extremes of the torque or pace functionality of any given motor leads to drastically decrease effectivity (plummeting to zero, in reality, at max torque and 0 RPM—that’s, stalled—or any RPM and 0 torque—that’s, unloaded).

Minimizing electrical losses requires a multi-pronged strategy, and that often implies that there might be some tradeoffs concerned. For instance, torque is proportional to magnetic flux depth, which itself is proportional to the product of present and the variety of turns in every armature winding, so doubling the variety of turns cuts the required present to provide the identical torque in half. It will lead to a winding with twice the resistance, however since losses are proportional to present squared, the web impact is a 50% discount in losses. So why not go along with 10 instances the turns and 1/tenth the present—or nonetheless a lot additional you wish to take this argument? Effectively, the again EMF produced by a motor is proportional to the variety of turns, too, so the battery voltage essential to function at a helpful RPM would quickly exceed what’s sensible.

One other strategy is to make use of silver wire fairly than copper (or aluminum) for the windings, netting a discount in resistance of about 7% (or 39% for aluminum), albeit at a tough-to-swallow 100x improve in price. Nonetheless, when extrapolated to a theoretical helpful working lifetime of, say, 10,000 hours, at a mean energy of 20 kW and a value per kWh of $0.20, that might save round $2,800 in electrical energy, making a reasonably compelling argument for silver windings proper there. In fact, one of the best winding materials could be one with no resistance in any respect—that’s, a superconductor—however the not too long ago damaged promise of LK-99 exhibits us that that purpose remains to be some methods off sooner or later, and even when it had been out there in the present day, there are different sensible concerns, comparable to whether or not the superconducting materials will be fashioned right into a wire with an enamel insulating coating, as is required for the windings in motors (and transformers, inductors and different electromagnetic parts).

Resistive losses aren’t only a drawback at excessive torque (i.e. excessive present) ranges—they’ll sap effectivity at excessive RPM too, because the efficient resistance of a wire quickly will increase above a sure frequency due, primarily, to the phenomenon colloquially generally known as “pores and skin impact.” What principally occurs is that an alternating present induces small loops, or “eddies,” of present in its personal conductor. These eddy currents oppose the stream within the heart of the wire and add to it within the periphery, therefore it seems that the present is constrained to the outermost portion, or pores and skin, of the wire. The efficient depth that present will use in a conductor is inversely proportional to frequency, and since motor RPM is instantly proportional to frequency, this units an higher restrict both on wire diameter (and subsequently present) or RPM. The standard answer to minimizing pores and skin impact is to interrupt up a single wire into many individually insulated smaller wires—one thing which could should be achieved, anyway, simply to make winding the motor (or transformer, and so forth) sensible. Observe, nonetheless, that this will increase the share of the winding space that’s taken up by insulation, fairly than copper (or silver, and so forth), so there’s positively a regulation of diminishing returns right here.

Turning now to the iron losses in a motor, a generally used rule of thumb for any electromagnetic gadget which handles alternating present is that the iron and copper losses needs to be roughly equal. As this rule of thumb implies, there are mutually unique tradeoffs between the 2—for instance, growing the cross-sectional space to cut back flux density reduces iron losses however requires longer windings, which will increase resistance (it additionally makes the motor considerably heavier). The overwhelming majority of motors in the present day use metal that’s alloyed with silicon to assemble the magnetic circuit—that’s, the components of the motor that conduct loops of magnetic area—because it combines a comparatively excessive saturation flux density, good formability, excessive electrical resistance, and comparatively low price, however on the expense of less-than-impressive hysteresis losses (roughly equal to frequency-dependent losses in wires). Worse nonetheless is that the losses in a magnetic materials have a tendency to extend at an exponential fee with frequency and flux swing (usually to the 1.5-2.5 energy, relying on materials), which means {that a} comparatively small improve in inverter basic frequency (which determines RPM) or section present (which determines torque) may end up in an outsized improve in losses. 

There are quite a few magnetic supplies with a lot decrease losses, however few of them are appropriate to be used in motors. For instance, the assorted ferrites generally utilized in high-frequency magnetic parts have a decrease saturation flux density (within the vary of 0.3-0.35 Tesla, in comparison with 1.8-2.0 Tesla for silicon metal) and are extraordinarily brittle, so that they’re tough to type, and aren’t your best option for a motor that might be bouncing alongside the street. Rather more promising supplies make use of slight variations within the processing and/or alloying parts of silicon metal, leading to both a particularly fantastic grain construction (nanocrystalline) or no grain construction in any respect (amorphous). Amorphous metals have been employed in transformers and motors for many years (and for these of a sure age, additionally within the heads in tape decks) and whereas they’ll obtain a powerful as much as 70% discount in losses (from round 1.4 W/kg to 0.4 W/kg, relying on frequency, flux swing, and so forth), they’re dearer to fabricate and extra brittle (although not almost as brittle as ferrite). Between the 2 variants, the nanocrystalline model has the sting in saturation flux density and mechanical properties, whereas amorphous is cheaper to provide and a extra established materials. Both method, decreasing iron losses by as much as 70% will get you nearer to enhancing general motor effectivity to that legendary 99% determine than the comparatively meager contribution from silver wire, and presumably at a decrease price.

As talked about earlier, frictional losses within the shaft bearings needs to be a tiny fraction of general losses, so this isn’t actually an space the place additional optimization is feasible, though we must always observe that one insidious failure mode of motors equipped by a variable-frequency drive is spark erosion of the interior bearing surfaces brought on by capacitively-coupled currents produced by the quickly switching voltages from the inverter (i.e. from excessive dV/dt), therefore the rising use of ceramics for the bearing balls and races. In any other case, shifting away from total-immersion liquid cooling—particularly if the coolant is oil, which has a a lot greater viscosity than aqueous coolants like glycol and water—can considerably scale back windage loss. One other key issue just isn’t working at extraordinarily excessive RPMs, as, once more, windage loss scales with the dice of RPM, so a modest lack of 100 W at, say, 3,000 RPM turns right into a far-less-tolerable 2.7 kW of loss at 9,000 RPM.

On a associated observe, the usage of multi-speed transmissions (both mechanical or the fairly novel digital strategy mentioned under) fairly than the single-speed gear discount mostly utilized in EVs in the present day can assist maintain the traction motor in that candy spot of reasonable torque and reasonable RPM for a better proportion of time (whereas enhancing acceleration and prime pace). With a mechanical transmission there might be some improve in frictional/windage losses (notably for gears bathed in oil), however the typical 97-98% effectivity of a gear practice will at all times examine favorably to working within the sub-70% area of the motor’s effectivity map. For instance, each Porsche and Audi have used a two-speed gear transmission of their EVs, however Bosch is currently making the case for its CVT4EV, which is a constant-velocity transmission that’s optimized for EVs by reducing its pace discount vary in half to double its most torque functionality (as in comparison with its ICE counterpart). 

Alternatively, it’s doable to simulate a multi-speed transmission electronically by breaking apart the section winding pairs into a number of units that may be rewired on the fly by the inverter to vary both the obvious variety of poles or the variety of phases. Every of those options successfully adjustments the rotational angle that the magnetic area produced by the stator has to behave upon—appearing over a shorter arc trades a better torque for a decrease synchronous pace and vice versa—however a caveat is that this may require very completely different inverter {hardware} and software program, so don’t anticipate to see these options carried out in an OEM EV anytime quickly. That stated, historical past exhibits that at any time when a mechanical answer will be changed by an digital one, it’s all however positive to be adopted…finally.  

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This text appeared in Issue 65: July-September 2023 – Subscribe now.